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DIPTERA, Tachinidae (Robineau-Desvoidy 1863 -- <Images>
& <Juveniles> Please refer also to the following links for details on this
group: Tachinidae = Link 1 Description & Statistics
In 1938 Thompson noted that there were few dipterous parasitoids
of Diptera in contrast to the frequent attack by Hymenoptera on Diptera. Among the Tachinidae, Admontia, Siphona and Trichoparia have been reared from
tipulid larvae, and several species of the first genus are common on that
host in certain areas. Other records
of attack on Diptera must be questioned; and considering their large size and
exposed position, it is unusual that the larvae of Syrphidae are not subject
to attack by tachinids (Clausen 1940/62). Insect orders with aquatic larvae are also almost immune to
attack by Tachinidae; but the few aquatic Lepidoptera are readily
parasitized, and the species responsible show pronounced adaptations for life
in water. In many of these orders,
the larvae are always immersed in water, often at a considerable depth. However, some of them leaf it for pupation
in the mud or sand near the water's edge, at which time they would be
vulnerable to attack, particularly by species having planidium type larvae
(Clausen 1940/62). Many Tachinidae show great host specificity, but there are some
species that have a much wider host range than is known in any other group of
parasitic insects. An example of the
latter is Compsilura concinnata Meig, for which ca. 100
different hosts are known in the United States alone (Webber & Schaffner
1926), these representing 3 orders and 18 families. Generally, it may be that the species of this family are less
restricted in their host range than the Hymenoptera. Many native Tachinidae show a degree of parasitization and
effectiveness comparable to that attained by introduced species. In northern Japan, Centeter cinerea Ald.
destroys ca. 90% o of adult Popillia
japonica beetles in alternate
years, within 6-10 days after their emergence. These are in years of beetle abundance, while when populations
are low the parasitization is much lower.
The species is undoubtedly responsible for suppressing the pest to a
nonproblem level in Japan (Clausen 1940/62).
In North America, the red-tailed tachinid fly (reported as Winthemia quadripustulata F.) frequently destroys 50% of the armyworm
population, and sometimes parasitization nears 100%. Trichopoda
pennipes F. similarly builds up to
a high level at destroys up to 80% of the adult squash bugs of several
Pentatomidae. Ernestia rudis Fall. in
Europe parasitizes a high percentage of the pine moth, Panolis flammea
Schiff. Paradexodes epilachnae
Ald. shows the remarkable capacity for increase of tachinids in the
field. Colonies of 100-200 were able
to build up to a parasitization of 50% or more in relatively heavy
infestations of Epilachna over a
radius of several miles from the point of release within two
generations. Overall the Tachinidae may be considered beneficial, for most
species are primary parasitoids of plant pests. Unlike the major families of parasitic Hymenoptera, which have
a varying portion of their species hyperparasitic, there are none in this
category among Tachinidae. However,
certain species are harmful because of their direct attack on hosts that are
themselves beneficial. Several
tachinid species that attack silkworms in Asia are very harmful. Before adequate control was developed, the
loss of silkworms from tachinid parasitism in Japan was sometimes 80%. The tachinids also serve as vectors of the
pebrine disease. Other cases of
parasitization on beneficial insects are found in the species that attack
adult Carabidae and honeybees.
However, these are not widely distributed and usually are not abundant
enough to seriously affect the host population. Many species of Tachinidae have been used successfully in
biological control. An landmark
example is the biological control of sugarcane beetle borer, Rhabdocnemis obscura Boisd. in Hawaii through the importation of Ceromasia sphenophori Vill. from New Guinea in 1910. Also, in 1925 the biological control of
coconut moth, Levuana iridescens B. B. in Fiji by Ptychomyia remota Ald. was achieved even though the parasitoid attacked a
related host, Artona in its native
Malayan range. In many other cases
less spectacular results utilizing tachinids have, nevertheless, led to some
reduction in the targeted pests. In
the United States a high parasitization of the gypsy moth and other
Lepidoptera has been obtained by Compsilura
concinnata Meig., imported from
Europe during 1906-1911. This
parasitoid is credited with having significantly reduced infestations. Chaetexorista
javana B. & B. of Japan was
established in the northeastern United States where it exerts partial control
of the oriental moth, Monema flavescens Wlk. although complete
control was prevented by the parasitoid's inability to tolerate occasional
extreme cold in winter (Clausen 1940/1962). Tachinidae have been deployed for control of the sugarcane moth
borer, Diatraea saccharalis F. in the neotropics. Metagonistylum
minense Tns, or "Amazon
fly" was originally discovered in the Amazon basic of Brazil. It was colonized in British Guiana and
islands of the West Indies since 1933, where it has evoked a significant
reduction of the infestation in some areas, Guyana in particular. Tachinidae
is a very large family with over 12,000 species known by 1993. They are distributed worldwide. The greatest number of species have been
described from the tropics. Important
morphological characters include a bristly body, especially on thoracic
dorsum and 4th to 6th abdominal segments; postscutellum of mesothorax well
developed, protruding posteriorly; pteropleural and hypopleural bristles
present. The body is small to medium
in size, gray or dull colored. Cell
R-5 is narrowed or closed at the wing tip. Most tachinids are primary, solitary, endoparasitoids, but some
are gregarious parasitoids. They have
a wide host range, with all major groups of insects serving as hosts. They are important to biological control,
as >16 species have been introduced into the United States alone. Many more have been employed worldwide,
mainly against Lepidoptera. A discussion on host preferences of Tachinidae is probably best
done on the basis of principal subfamilies.
The Exoristinae are the dominant group, both in number of genera and
in species known to be of importance as parasitoids of crop pests. Host preferences cover an extensive range,
with the majority of species parasitic in lepidopterous larvae and in adult
beetles of the families Chrysomelidae, Scarabaeidae and Carabidae. Genera parasitizing caterpillars include Anetia, Compsilura, Winthemia, Sturmia, Zenillia and Exorista. On adult beetles, Eubiomyia, Centeter and
Chaetophleps are
representative. Relatively few genera
and species parasitize larvae of sawflies.
A number of species attack larvae of Curculionidae, Chrysomelidae and
Tenebrionidae, and sometimes there are species parasitic in larvae of
Tipulidae and Vespidae and in adult phasmids, earwigs and locusts. Species of genera Doryphorophaga, Meigenia
and Paradexodes attack chrysomelid
larvae, and several species of Siphona
are included among the few parasitic natural enemies of tipulid larvae. Important subfamilies are the
Gymnosomatinae, Tachinae, Rutiliinae, Dexiinae and Oestrinae. Biology
& Behavior
Tachinidae have sustained extensive investigation since the
beginning of the 20th century. Early
detailed studies were by Nielsen (1909, 1912, 1918), Pantel (1910, 1912) and
Baer (1920, 1921). Townsend
(1934-1939) contributed much information on the reproductive behavior and egg
of 1st instar larvae. Tachinidae show
exceptional uniformity in host relationships as compared with parasitic
Hymenoptera and Diptera. The larval
stages are entirely internal during the feeding period, with one exception
being Myiobia bezziana Baran., which Beeson & Chatterjee (1935) found to be
an external parasitoid of caterpillars of a wood boring cossid in India, which
was nevertheless questioned by Clausen (1940). Most Tachinidae are solitary.
Even in gregarious species it is only in rare instances that more than
3-4 develop in a single host, although sufficient food seems available for a
larger number. A maximum of 16
individual Eubiomyia calosomae Coq. was recorded from a
single Calosoma beetle, while up to
28 Palpostoma subsessilis Malloch reach maturity in scarab beetles in
Australia. The largest number
recorded was 110 Achaetoneura samiae Webber from a single Samia cocoon, and 550 A. frenchii
Will, were reared from 44 S. cecropia L. cocoons. Surplus 2nd or 3rd instar larvae are
usually killed in cases of superparasitization, resulting from an apparent
overcrowding or starvation, but also at times by direct combat. If the number is very high, all the larvae
may die and the host continue to maturity or also die. Clausen (1940) remarked that in one case a
total of 147 dead larvae of Achaetoneura
was found in a single S. cecropia larva. Female Reproduction.-- Females have several
modifications of the internal reproductive system which relates to the type
of eggs or larvae deposited (please refer to Clausen, 1940, for
diagrams). The basic type is one that
produces the heavy shelled macrotype egg and in which not much uterine
incubation occurs. C. cinerea
is typical of this group. Each ovary
has 9-10 ovarioles and a short oviduct leads to the uterus. The latter is also short and membranous
for its entire length, with the stalks of the three spermathecae attached
near the middle. In gravid females,
each ovariole may have one mature egg and a series of immature ones. One mature egg is usually present in each
oviduct and a single fertilized one in the posterior part of the uterus. Daily egg production is low, but extends
over a protracted period. Species that oviposit on leaves produce microtype eggs, which
have several adaptations. Zenillia libatrix Panz. is representative. Dowden (1934) found each ovary to comprise 80-100
ovarioles. In young females each of
these contains ca. 14 eggs in various developmental stages. The paired oviducts are long and slender,
and the long posterior uterus is thick-walled for most of its length. The spermathecae are attached near the
bases of the paired oviducts. After
fertilization when eggs descend to the uterus, the uterus becomes greatly
enlarged and may have several thousand eggs packed irregularly within. The coiled and expanded uterus may fill a
large part of the abdominal cavity.
All species of this group have many ovarioles, ranging to 460 in Leschenaultia exul Tns. (Bess 1936).
Many genera of the Exoristinae
have this kind of reproductive system and lay microtype eggs. Epidexia
(placed in Dexiinae by Townsend) is the only member of the group known to
produce microtype eggs. Anetia nigripes Fall represents a third type
that injects its larvae into the host's body. Each ovary consists of 12-14 ovarioles, and in unmated females
the anterior and posterior uteri are about equal in length, with the
spermathecae attached near the juncture.
After mating, the eggs descend past the spermathecal openings into the
posterior uterus, which becomes very much elongated, and lying in four
coils. Eggs which most recently
passed into the uterus lie transversely in an even row, but as they progress
downward, they lie longitudinally and usually paired, with the head end
directed cephalad with respect to the body of the parent female. Thus, the larva is ejected with its caudal
end first. In gravid females of some
larger species this "strap-like" uterus is very long, reaching its
greatest development in Latreillimyia
bifasciata F. where it measures 110
mm (Townsend 1936). Eggs range from
fully incubated at the posterior end of the uterus to undeveloped at the
anterior end (Clausen 1940/1962).
This kind of reproductive system allows for the deposition of a
relatively small number of larvae daily, although larviposition may extend
over a long period. However, in Ernestia ampelus Wlk. and Compsilura
concinnata all the eggs in the uterus
are in the same stage of development.
This allows the deposition of the full quota of larvae in 2-3 days
(Tothill 1922). Maggots of this
species lie in the reverse position in the uterus and are thus ejected with
their heads first (Culver 1919). Many of the tachinids which deposit their maggots on foliage or
on the surface of the soil represent a fourth type. The uterus is somewhat coiled and strap-like before
fertilization. After fertilization it
becomes much distended and filled with enormous number of eggs in all stages
of development, these often lying in precise transverse rows in the anterior
portion and longitudinally in the posterior section (Clausen 1940/1962). The eggs of Echinomyodes are arranged with great precision in as many as 24 rows. This kind of reproductive provides for the
deposition of a large number of larvae in a short period of time (Townsend
1936). Among species which deposit
larvae or fully incubated eggs, the posterior uterus serves as an incubation
chamber, with its walls abundantly supplied with trachea to satisfy the
oxygen requirements of incubating eggs. The rhythmic development of the eggs of P. epilachnae was
observed by Landis (1940). The total
number of ovarioles ranges from 16-54, each containing 6 eggs in various
developmental stages, which represents the full reproductive capacity. Each egg has a brood relative in the
entire series of ovarioles, and together they descend into the oviduct at
about the same time. The successive
broods descent in turn, and they may be distinguished in the uterus by the
stage of incubation attained, the first batch being fully incubated and each
following one being less developed than that preceding it. Females of specie that inject eggs or larvae into the host body
require an extensive modification of the external reproductive
structures. The form taken depends on
the kind of host and the amount of force necessary to penetrate the
integument. A simple adaptation is
that of species attacking caterpillars, which have relatively thin and
flexible integuments. Adaptation for
penetration of such hosts is found in such common genera as Compsilura, Anetia and Lydella. The 6th abdominal segment is developed
into a curved, very pointed, thornlike process. This structure, or piercing organ, is deflected downward and in
some species lies along the mid ventral line when not being used. It is grooved along the outer convex side,
and the ovipositor glides along this groove in the deposition of eggs or
larvae. Therefore, there are two
distinct acts involved in larviposition, (1) the puncturing of the host
integument and (2) the insertion of the ovipositor in the wound for laying an
egg or larva. In species that
larviposit they are usually placed directly between the peritrophic membrane
and the cellular wall of the mid intestine rather than free in the body
cavity of the host, and are then left to seek their ultimate destination
(Clausen 1940/1962). Chaetophleps setosa Coq. shows a striking adaptation
for deposition of the egg or larva internally. This is a parasitoid of chrysomelid beetles of genus Diabrotica, which was described by
Walton (1914) as Neocelatoria ferox Walt. and studied by Bussart
(1937). The 6th abdominal segment is
modified into a very long piercing organ, more than 1/2 the length of the
abdomen. The 2nd abdominal segment is
greatly extended ventrally into a laterally compressed structure which bears
at its tip a large number of heavy, flattened, spine-like processes directed
a bit caudad. During larviposition,
the fly pounces on the beetle, and the latter turns on its back, after which
its abdominal region is grasped in pincer-like fashion between the piercing
organ and the extension of the 2nd segment.
The ovipositor is then driven through the elytra and into the
abdominal dorsum. However, many
beetles are attacked while in flight or after alighting but while the wings
are still spread and the ovipositor is inserted through the thin dorsal
integument of the abdomen (House & Balduf 1925). Bussart (1937) found that oviposition
occurs only while the beetles are in flight.
This modification of the abdominal structure is clearly an adaptation
for holding a hard bodied host during insertion (Clausen 1940/1962). Celatoria
diabroticae Shim. also attacks Diabrotica beetles and is similarly
modified. Clausen (1940) noted that a
similar adaptation is found in the Conopidae, which parasitize adult
bumblebees and some larger wasps. Adult Behavior.-- In most Tachinidae adult
emergence occurs during the early morning, although in some species of
crepuscular or nocturnal habit emergence is in late afternoon and early
evening. Adults feed on honeydew
secreted by scale insects, leafhoppers and aphids and also on various plant
secretions, particularly nectar glands.
Some Dexiinae, such as Prosena
sibirita F., possess a very long
probosis which is used to feed at blossoms, mainly Umbelliferae. In the laboratory Landis (1940) found that
females of Paradexodes confined in
cages and fed sugar and raisins lived longer if the cage was dirty. From this it was inferred that the yeasts
and other materials obtained from waste material have a nutrient value for
the flies. Tachinidae only infrequently feed on host body fluids, which is
possible only in species having the piercing type of ovipositor. Host feeding was observed in Doryphorophaga doryphorae Riley, a parasitoid of Colorado potato beetle larvae
(Bruneteau 1937); and it is found also in Anetia
nigripes (Clausen 1940/1962). Although crepuscular and nocturnal species frequently correlate
their attack with similar habits of the host, there are exceptions as shown
by Hamaxia incongrua Wlk. This
parasitoid is active in late afternoon and early evening, even though one of
its principal hosts, Popillia japonica Newm., is wholly diurnal and
feeds during the morning and early afternoon. Other scarab hosts, particularly Sericinae, are wholly night
feeders, but there is not much difference in the extent of parasitization
because of this seemingly favorable behavior. The Australian Palpostoma
subsessilis is also crepuscular,
and Cryptomeigenia theutis Walk. is nocturnal as are
adult Phyllophaga, its host. Tachinids usually mate soon after adult emergence and in most
species this is during the morning hours during bright sunshine. However, Carcelia gnava Meig.
and other species mate at dusk. In
the laboratory mating is often best secured by caging 1-2-day old males with
newly emerged females. Temperature,
light and humidity are important for influencing mating, the optimum range
often being quite narrow. Various
artificial stimuli have been tried to secure mating, such as exposure in
vials to bright sunlight, vigorous shaking of cages, lining cage walls with
green cloth, etc. Cleare (1939) in
studies on Metagonistylum,
determined that light intensity is the most important factor and that mating
occurs only within a very narrow range. Males are able to inseminate a large number of females, as shown
by Dowden (1933) for A. nigripes, where one male successfully
inseminated at least 13 females over ca. 4 weeks. Late mating revealed a progressive reduction in the proportion
of the eggs that were fertilized, however.
Females have been observed to mate repeatedly in several species
(Clausen 1940/1962). Gestation varies considerably in Tachinidae. A minimum of 2 days was recorded for Ptychomyia remota, and Winthemia
required a little over 2 days. These
tachinids deposit unincubated eggs, and it should be expected that the
gestation period is short. The
shortest period recorded for those depositing fully incubated eggs or maggots
is that of Palpostoma subsessilis. This species was found to contain fully developed larvae within
4-6 days after adult emergence (Burrell 1935). Usually larviparous species have a gestation of 8-14 days,
although this extends to as much as 4 weeks in Ernestia ampelus. In the latter all maggots in the uterus
are of the same developmental stage, and deposition may be completed in 2
days or extend over one month (Tothill 1922). Townsend (1908) distinguished five modifications of tachinid
reproductive behavior, based on the position of placement of the eggs or
larva with respect to the host, as (1) host oviposition, (2) leaf oviposition,
(3) suprocutaneous host larviposition, (4) subcutaneous host larviposition
and (5) leaf larviposition. This work
was based largely on dissection of gravid females, and it was found that no
only can the type of egg be determined by such dissections, but valuable
clues as to the oviposition habit may be secured. Pantel (1910) presented a different classification,
distinguishing 10 groups and using as a basis the female reproductive system,
the type of egg, the stage of incubation of the egg and placement of the egg
or larva at deposition (please see Clausen, 1940, p. 345 for details). Townsend (1934) listed 39 groups, the
majority of which represent Tachinidae, on the same basis employed by Pantel,
but utilizing also the general characters of 1st instar larvae (Clausen
1940/1962). This researcher
distinguished oviposition from larviposition. Oviposition was considered to refer to the deposition of all
eggs adapted for attachment to surfaces, whether flattened or provided with a
pedicel, and of which the embryo may be in any developmental stage. Therefore, the deposition of all microtype
macrotype and membranous eggs are defined included in oviposition. On the other hand, larviposition refers to
the deposition of fully formed maggots, followed by their immediate activity,
irrespective of whether they are naked or enveloped in the membranous chorion
(Clausen 1940/1962). In Tachinidae, reproduction may be oviparous, ovoviparous or
larviparous, and various adaptive modifications occur as a consequence of
these differences. There are four
general types of eggs, the microtype, macrotype, pedicellate and
membranous. These are associated with
certain morphological modifications of parent females and serve to aid the
parasitoid to reach the body cavity or some internal host organ (Clausen
1940/1962). Females of species
depositing incubated eggs or larvae directly on their hosts and require the
stimulus of the host presence for normal oviposition or larviposition can,
under stress, deposit them at random in order to reduce pressure in the
uterus. Those which inject them into
the host body will retain them indefinitely if hosts are not available, and
they may finally die as a result of penetration of the body cavity by
imprisoned larvae, as was observed in Ernestia
ampelus (Tothill 1922). Reproductive Capacity.-- This
varies greatly among different groups and species, being directly related to
the position in which the eggs or larvae are deposited with respect to the
host, and to hazards encountered before the larvae reach the host body
cavity. The minimum deposition of
eggs or larvae occurs in species where females inject their eggs or larvae
into the host body or deposit them directly on the body. In this group are many of the most common
and important species, such as Compsilura
concinnata, Trichopoda pennipes,
and Winthemia quadripustulata. In these
species reproductive capacity is c. 100-200, with some depositing
<100. Phorocera agilis R.D.
has a capacity of >200 eggs, laid at the rate of 4-5 daily (Prell 1915,
Burgess & Crossman 1929). A second group includes
species that deposit larvae or fully incubated eggs in the vicinity of the
host. Bigonicheta setipennis
Fall. places the eggs very near the host, or at times on it, and its total is
relatively low (ca. 25), while others in which the association is not so
close produce a higher number. Those
placing eggs or larvae on foliage and of which the hosts are fee living
caterpillars deposit 400-1,000 eggs or larvae. When hosts are enclosed in a tunnel in a plant stem, as in cane
borers, and the larva is deposited near the entrance hole, the number is
usually around 1,000. Gravid females
of Theresia claripalpis v.d.W. contain >500 eggs and larvae (Jaynes 1933),
and Metagonistylum minense parasitic on Diatraea and having the same general
habits as Theresia, produces
500-700 maggots (Myers 1934b). A third group are
parasitoids of white grubs, which deposit their eggs or larvae on the soil
surface. Prosena sibirita often
contains over 800 larvae and eggs in various stages of development. Townsend (1934) found that ca. 2,000 eggs
were in the uterus of a female Microphthalma
disjuncta. A fourth group includes
species which deposit microtype eggs and those in which the larva is attached
to the substratum by a membranous cup, which consists of the old eggshell,
enveloping the caudal segments. The
microtype eggs must be ingested by the hosts, while the fixed larvae are
dependent on passage of the host larva within reach. The chances of reaching the host are more
or less equal, as shown by the reproductive capacity of the two forms. Reamur (1738) estimated that a female Echinomyia contained 20,000 maggots,
but this figures was considered too high by later researchers of this species. Clausen (1940) remarked that Von Siebold
estimated 7,000 for Echinomyia fera L. However, Townsend mentioned 13,000 for Echinomyodes, which is the highest number determined by actual
count. He also found ca. 3,2000 eggs
and maggots in the uterus of Eupeleteria
maginicornis Zett., which was
thought not to represent the full reproductive capacity. Records for species with microtype eggs
range from 2-6,000. This method of reproduction in which the minute eggs are
deposited apart from the host larvae and must be ingested by the latter
before hatching, is known to occur in a large number of species, principally
in the Exoristinae. Townsend (1908)
recorded 14 species and Pantel (1910) listed European species of 8 genera
with this habit. Since then a large
number in many genera have been found to reproduce in this manner. Well known genera are Exorista,
Gonia, Frontina, Parachaeta, Masicera, Sturmia, Gaedia, Chaetogaedia, Leschenaultia, Prosocilipes
and Pales. This kind of oviposition was first observed in S. cilipes
Macq. (= sericariae Rond.) by U.
Sasaki in 1873, and the first record is that by H. Pryer in his catalogue of
the Lepidoptera of Japan, published in 1884, in which he stated, "I have
noticed that the Uji, a diptera, which is parasitical upon it and causes an
immense amount of damage, deposits its eggs about the larva on the leaves and
not on the insect." (Clausen 1940/1962). Dr. C. Sasaki, son of the discoverer, published an extended
account of the habits of S. cilipes in 1886. Such behavior was so bizarre at the time
that little credence was given to it for many years. It was not until 1908 when Swezey
described the behavior of Chaetogaedia
monticola Big. and Townsend that of
several other species in the same year, that full credit was accorded to
Sasaki's contribution. Clausen (1940)
remarked that the parallelism between the course of events here outlined for
Tachinidae of this type, and that which takes place in the Trigonalidae in
Hymenoptera, is especially interesting, because in both cases the eggs are
minute, hard of shell and deposited on the food plant of the host. They are capable of remaining viable for
long periods, and are consumed by the host and finally hatch in the digestive
tract from which position the larvae migrate into the general body cavity. Host Stage Attacked.-- Most Tachinidae attack
the larval stages of the host. This
behavior is consistent among those which parasitize Lepidoptera, and none is
known to attack the pupa directly, although some complete feeding in that
stage. Several species, such as Zenillia libatrix, delay development beyond the first instar until host
pupation. Those which deposit
macrotype eggs on the host body usually limit themselves to the late larval
instars, while others may gain entry at almost any time during the larval
period. Dexia and Prosena are
able to parasitize scarab grubs in any stage, but they do not complete
feeding until the latter are mature.
Tachinids parasitizing chrysomelid larvae, such as Paradexodes, sometimes successfully
attack the prepupa and pupa (Clausen 1940/1962). Edelsten (1933) reported an exceptional case of emergence of a
tachinid maggot from an adult moth. A
female Zygaena lonicerae Esp. emerged normally but lived only two days, during
which she laid 30 eggs. Ten days
later a mature maggot of Phryxe vulgaris Fall. emerged from the body,
after having completely consumed the contents. Previously the emergence of a larva of this species form a living
female of Nyssia lapponaria Boisd. was reported. Emergence of Manduca atropos L.
moths from pupae that had yielded tachinid maggots, the latter having emerged
from the wing pads, has been also observed (Clausen 1940/1962). A large number of tachinids attack only the adult stage of their
hosts, which is true of almost all those which parasitize Orthoptera and
Hemiptera. It is interesting that
many species attacking adults tend to limit their oviposition to the female
sex., which may be important from the point of view of natural control. Centeter
cinerea lays 80-95% of its eggs on
female beetles of Popillia japonica Newm., and other examples of
this behavior are Hyalomya aldrichi Tns., attacking mostly adult
females of the false chinch bug, Nysius
ericae Schill., and Thrixion halidayanum Rond., which is restricted to female Phasmidae. By contrast, most chrysomelid hosts of Chaetophleps setosa are male (Clausen 1940/1962). Silvestri (1910b) studying Erynnia
nitida R.D. found a very unusual
seasonal differentiation in host selection, where the larvae of the elm leaf
beetle were attacked by the summer broods of the parasitoid and only the
adult beetles by the last brood.
Clausen (1940) noted a comparable peculiarity in behavior with a few
species of Hymenoptera. Among tachinids that deposit eggs directly on the host body, most
are often found to be largely confined to a particular part of the body. Species of a single genus may differ in
this respect, even though they attack hosts of the same group which are
similar in size, form and behavior.
Therefore, Centeter cinerea places most of its eggs on the
dorsum of the thorax of female P. japonica (Clausen et al. 1927). While ovipositing, the female parasitoid
usually attacks pairs of beetles who are mating and dashes diagonally across
the female's thorax, lowering the tip of the abdomen momentarily to deposit
the egg. By contrast, C. unicolor
Ald. attacking Anomala and Phyllopertha beetles, places its eggs
ventrally on the posterior portion of the abdomen (Parker 1934). Among Tachinidae attacking caterpillars, the variation in
position is also great. On free
living caterpillars, either the last two thoracic segments or the last
abdominal segments are usually chosen.
In W. quadripustulata the thoracic position on Cirphis unipuncta Haw.
is a provision for the eggs' protection, as a high percentage of those placed
farther back are crushed by the host's mandibles (Allen 1925). The presence of these eggs apparently
causes some irritation, which causes the host to try to brush them off or
destroy the. An unusual position for
egg placement was recorded by Ainslie (1910) for Exorista larvarum L. on
larvae of Hemileuca oliviae Ckll. The latter is attacked only while in
motion and at the moment when the posterior portion of the abdomen is
raised. The egg is usually placed on
the sole of the psuedopod, within the crescent of hooklets. On Pentatomidae and other Hemiptera, the
tachinid egg may be placed on the side of the abdomen or thorax, which is
common, on the venter of the prothoracic margin, as in Gymnosoma fuliginosa
R.D., or on the dorsum or sides of the abdomen while the wings of the host
are spread, as in Phasia crassipennis L., Siphona geniculata DeG.
and S. cristala F. (Roubaud 1906), which parasitize tipulid larvae,
oviposit on the stigmatic crown, presumably because this is the only portion
of the host body exposed (Clausen 1940/1962). In Carcelia gnava the pedicellate egg is attached
by the tip of the pedicel to a hair of the caterpillar host (Clausen 1940),
while in C. evolans Wied. the egg is placed on one of the thoracic segments
of the bagworm host, a position which is obligatory because that is the only
part of the body that is ever extruded from the bag (Skaife 1921b). In hosts that inhabit soil, the eggs or larvae of the tachinids
are laid on the soil surface, although Davis (1919) found that the female of Microphthalma disjuncta Wied. places them in crevices. They are nevertheless probably placed in
proximity to host grubs in considerable numbers rather than singly. Species attacking hosts in plant stems,
such as Theresia and Metagonistylum, which attack sugarcane
moth borer, place larvae near the entrance of the host tunnel. They must burrow through the frass that
fills the entrance before reaching host larvae. This behavior is found in a great many species attacking hosts
that are concealed but that have an open entry or later make holes for other
purposes in fruits, stems or seeds (Clausen 1940/1962). Female Rondanioestrus apivorus Vill. pounce on worker bees
while they are in flight, but touch the body only lightly to lay the maggot
on it (Skaife 1921a). Tachinids that
lay macrotype eggs seem to show no discrimination in their choice of hosts. This often results in individual hosts
receiving an excessive number of eggs.
A certain portion of these eggs is lost through molting of the host,
this varying with the length of the incubation period and the interval between
host molts. If the egg incubation
period is the same as the length of the host larval stage, there would be
virtually a complete loss of the eggs before hatching, while if the egg stage
is, for example, 3 days and the host larval stage 6 days, the loss from this
cause would be ca. 50% (Clausen 1940/1962).
It seems that early portions of the host stage, immediately after
molting, are preferred for oviposition, which would reduce the loss
considerably. At time there is a failure in successful parasitization even when
oviposition is extensive. Toward the
end of outbreak periods of the nun moth, Lymantria
monacha L. in Europe, almost every
caterpillar bears tachinid eggs and yet the attack is rarely successful, with
the hosts developing and emerging normally.
In a collection of 235 gypsy moth caterpillars, each bearing 1-33 eggs
of Exorista larvarum, only four parasitoids were produced. Another collection of 252 did not yield a
single parasitoid. Although only part
of this loss can be explained by the molting factor, it probably accounts for
the loss of a considerable portion of the reproductive capacity of species
laying unincubated eggs on caterpillars (Clausen 1940/1962). One field collected larva of Datana
minestra Drury yielded a maximum of
228 macrotype tachinid eggs (Clausen 1940). A larva of Samia cecropia with 40 tachinid eggs was
still able to reach adulthood.
Tothill et al. (1930) noted where 72 eggs of Ptychomyia remota were
deposited on a single Levuana
larva, in which only one parasitoid can develop to maturity. Observations on Winthemia indicate that the number of eggs laid on different
hosts and on various instars of the same host vary directly with the host's
size (Allen 1925). Clausen et al.
91933) showed in Centeter cinerea that there was no selective
oviposition, but that the egg distribution appeared random. Microtype eggs are laid on plant foliage that serves as host food
and adhere to the leaf surface by a mucilaginous material that is partially
water soluble. Sometimes a particular
plant or plant group serves as the oviposition stimulus, which is thus
independent of the host itself. Other
times the attraction seems to be to foliage bearing, or visited by, host
larvae. Dowden (1933) observed that
cut leaves, as well as the presence of host larvae, stimulate oviposition by Zenillia libatrix, which simulates the condition existing while the host
larvae feed. Eggs of Racodineura are laid on any plant
material upon which earwigs have fed the previous night. In most species, the eggs are laid on the
undersides of leaves, scattered about, but in other species the are placed at
the leaf margins. Females of Gonia capitata Deg., parasitic on Porosagrotis
in North America and Europe, lay most of their eggs on the upper sides of
leaves of Graminae, in particular the bluejoint grass, Agropyron smithii
(Strickland 1923). However, the host
feeds mainly on the cultivated grains and attacks the bluejoint grass only
when other preferred vegetation is not available. It also feeds extensively on alfalfa, but the parasitoid does not
oviposit on that plant. The
parasitoid's value is thus restricted, for host larvae on their preferred
food plants are able to avoid attack.
The extent of oviposition of Gaedia
puellae Nishik. on mulberry foliage
is correlated with the infestation of aleyrodid, Bemisia myricae Kuw.,
on the secretions of which the female flies feed (Nishikawa 1930). The silkworm, principal host of this
tachinid, does not occur on mulberry in the field, although species of Acronycta, Bombyx and Porthesia,
some of which are usually present, are more normal hosts (Clausen 1940/1962). Immature Development (Egg Incubation, Hatching and Host Entry).--
There is a wide range of behavior regarding incubation, hatching and host
entry. Activities of 1st instar
larvae in penetration of the host vary as much as any other group of
parasitic insects (Clausen 1940/1962).
The frequent occurrence of partial or complete uterine incubation of
the several types of eg produced, which is relatively rare in other dominant
parasitic groups, particularly Hymenoptera, serves as one means to overcome
or avoid certain hazards that might otherwise be disadvantageous during
external incubation. Macrotype eggs generally undergo the entire embryonic development
outside the parent female's body.
Occasional rare exceptions this occur, such as in Ptychomyia remota
(Tothill et al. 1930), where there is a partial and variable degree of
uterine incubation. The normal period
of external incubation in this species is 36-50 hrs; but some eggs have been
found to hatch in 30 min., and other required 4 days. Among other species in which the
incubation period has been determined, it most frequently requires 2-3 days,
with a minimum of one day. The eggs
of Eubiomyia calosomae are almost fully incubated at the time of deposition,
usually hatching in <24 hrs., though some hatch in <3 hrs (Collins
& Hood 1920). Two ways exist in which hatching and entry into the host body are
accomplished by larvae from macrotype eggs.
Centeter, Trichopoda and Meigenia and others have the indehiscent form of macrotype egg
where the larva bores directly downward through the thin chorion on the
ventral side of the egg and through the heavily chitinized integument. Of course there is no external evidence to
indicate that hatching has occurred.
This method is particularly common among those species attacking adult
Hemiptera and Coleoptera. Larvae from
eggs of this type are usually provided with teeth on the distal margin of the
mouth hook, which serve to rasp (see Clausen, 1940 for diagram). An exception is shown by E. calosomae,
in which young larvae escape from the egg through a hole in the thin ventral
chorion. However, instead of
continuing into the host body immediately, they emerge from beneath the egg
and enter at some other point on the host.
In the second form, hatching occurs by the lifting of a definite lid,
or operculum, at the anterior end of the egg. The fracture takes place along a horizontal line around the
front of the egg, a bit below the median transverse line and often extending
slightly dorsad at each end. The
larva emerges partly from this opening and with the caudal segments still
enclosed within it, braces itself and penetrates the integument just in front
of the egg. Ernestia, Phorocera and
Winthemia have this behavior,a nd
it is probably common among the species attacking caterpillars and other
relatively thin-skinned hosts (Clausen 1940/1962). In some species the larva abandons the eggshell altogether and
enters through the intersegmental membrane or at some other vulnerable spot. In the majority of tachinids that lay microtype eggs, uterine
incubation is also partial or complete, and the entire quota of the female
may be present in the uterus and partly or completely incubated before any of
them are laid (Clausen 1940). Such
species, as Zenillia libatrix, are thus able to lay a large
number in a short time. Such eggs,
whether or not incubated at the time of laying, are protected from
desiccation and injury by the heavy and variously sculptured chorion. Hatching does not occur until they are
ingested by the host larva. They
remain viable for 3-5 weeks in most species, with a record from Gonia ornata Meig. being alive 2/5 months after laying, at which time
the caterpillars of the 2nd generation of Euxoa
were present in the field (Sakharov cited by Clausen, 1940). This ability to persist in an inactive
condition for a long time is advantageous to the species and counteracts,
partly at least, the disadvantage of being laid apart from the host. Several genera and species with apparently
mature eggs in the uterus show very little evidence of embryonic development,
being laid before this is complete (Townsend 1908, Clausen 1940). In these species, ingestion of the eggs by
host larvae cannot lead to successful parasitization unless the eggs are at
least 1-2 days old. Clausen (1940) pointed out that opinions vary concerning the
manner in which hatching of microtype eggs takes place. The chorion of the ventral side of the egg
is thin and membranous, contrasted with a thick walled dorsum. Townsend implied that the action of the
digestive juices provides the stimulus for hatching and considered that the
heavy chorion served to protect the embryo from injury by the host mandibles
while the egg was being swallowed.
Swezey (Clausen 1940) questioned this, and believed that the chorion
was cracked by the mandibles of the host, thus allowing the larva to
escape. He supported his conclusion
by the snapping open of the shell, and the escape of the larva, when pressure
is applied to it. Also, hatched
larvae were found in large numbers in the crop of caterpillars almost
immediately after the eggs were eaten.
Severin et al. (1915), Nishikawa (1930) and Dowden (1933) immersed
eggs of Chaetogaedia in the fluids
ejected from the mouths of Heliophila
larvae; hatching of some of the individuals resulting in less than one minute
and 97% hatching within 3 hrs. The
same results was obtained with the juices of other caterpillars. The presence of such fluids stimulates the
larva, and the thin ventral chorion of the egg is broken as a result of its
movements. Many eggs hatched when
immersed in distilled water for 36 hrs, and this was brought about by an
increased turgidity of the larva (Clausen 1940/1962). The egg absorbs a lot of fluid, often
resulting in a doubling in size. In
some species the thin ventral chorion bulges out, blister-like, so that the
greater part of the larval body is enveloped by only a thin membrane. As a result, the rupture of the heavy
dorsal chorion is not required. Nishikawa confirmed the conclusions of his colleagues with
respect to Gaedia, stating that
hatching occurs only after immersion in the digestive juices; but he found
that a higher percentage hatch resulted with a higher concentration. The mouth hook apparently breaks the
ventral chorion of the egg. Dowden
(1933) reported that the mere immersion of the eggs of Z. libatrix in the
digestive fluids of the host fails to induce hatching but does result in a
pronounced swelling which renders them susceptible to rupture due to
variation in pressure within the host's digestive tract. Hatching takes place in this species at
any point in the digestive, while in others it is mostly in the fore intestine. Most Tachinidae in which the eggs undergo partial or complete
incubation while still within the uterus of the parent produce the membranous
type of egg, and in some cases actual hatching of all eggs in the uterus
occurs. Female Prosena sibirita never
deposit eggs and the eggshells are retained in a "brood pouch,"
whereas in Dexia ventralis they are voided at the time
of larviposition. In the latter,
uterine incubation is usually complete, with hatching taking place before
larviposition. During periods of
extensive reproductive activity, the brood pouch may be emptied before all
the eggs are fully incubated, and some of them may require as much as two
days of further development before they can hatch (Clausen 1940/1962). Larvae that are laid externally as such or that arise from the
membranous type of egg usually enter the host through the intersegmental
membranes or at some other point where the integument is thin. This is especially true of the species
attacking heavily armored hosts, such as beetles and locusts. In several species, such as Lixophaga diatraeae Tns., Siphona
cristata, Eubiomyia calosomae and
S. geniculata, it was found that entry is through a spiracle. Clausen (1940) expressed some doubt about
this, however. Strickland (1923)
noted that many of the planidia of Bonnetia
comta Fall. are bitten off and
killed by the cutworm larva while they are trying to penetrate the
integument, and Muesebeck (1918) noted that brown-tail caterpillars make
frantic efforts to dislodge or destroy the larvae of Sturmia nidicola Tns.
as they bore into the body. This
mortality factor does not operate in the case of species with microtype eggs,
and is comparable to the loss of macrotype eggs through the same agency. Most hosts, however, show no discomfort
during the time maggots are penetrating. Stimuli attracting planidium type of larvae to their hosts are
not completely understood. Many
attach themselves to almost any moving object within reach. Those of Archytas analis F. are
attracted to many species of caterpillar in which they cannot develop, but
they show no interest in certain other species. Generally, the larvae that find the host and enter its body
through their own efforts do so immediately after coming into contact with
it. Complete penetration is often
achieved in 15 min., though some species require more time. The variation is probably due mainly to a
difference in the thickness and toughness of the host's integument. However, in the case of the planidia of A. analis,
which, when they reach the host exude a liquid that fastens them horizontally
to the skin of the host. They may
remain in this position for 24 hrs. or longer before attempting to penetrate
(Clausen 1940/1962). First instar larvae of Dexia
ventralis Ald. and other species of
Dexinae, must search through soil for their hosts and have the planidium type
larva. The penetrate the integument
at almost any point as soon as the host is located. Experiments have shown that the larvae do not discriminate
among grubs, but that differences in parasitization of different host species
and instars is related to the thickness and hardness of the integument. In this way, under comparable conditions
there was a parasitization of only 18% of mature Anomala grubs and 85% in 2nd and early 3rd instar grubs of Phyllophaga, the integument of the
latter being very thin and bare.
Although initial parasitization was high, the parasitoid could not
mature in the latter, however. Many species, such as Bonnetia
comta, Ernestia ampelus, Archytas analis and Eupeleteria magincornis, use the egg shell as a
cup-like device that serves to anchor the larvae to the substrate while
awaiting a host. The shell is
fastened to the leaf or other surface by a mucilaginous substance from the
colleterial glands of the parent female.
it closely envelopes the caudal end of the larva (see Clausen, 1940
for diagram). B. comta larvae stand
erect in the shell, even when resting, with the anterior segments retracted,
whereas larvae of A. analis lie horizontally on the substratum
when resting (Allen 1926). The
presence of the collapsed eggshell is not essential to the well-being of the
planidia. Both of these species
frequently leave the shell entirely when excited, such as occurs when a host
approaches. They are still able to
assume the erect position at will. It
does not seem that this adaptation serves any essential purpose, however
(Clausen 1940). The majority of species that inject their membranous eggs
directly into the host body partially incubate their eggs in the uterus. However, a few regularly lay them before
much embryonic development has occurred.
The pedicellate egg, which is attached to a hair or to the
caterpillar's integument, hatches quickly after deposition. The young larva seeks out a vulnerable
spot on the host where it enters. Its
activities thus do not differ much from those of other forms of larvae from
membranous eggs, except that it is spared effecting the initial contact with
the host. Larval Activity in the Host.-- Clausen
(1940) mentioned that there is much less diversity in behavior among
Tachinidae once the host is penetrated.
This was because for all larvae the same general medium is inhabited,
even though the final destination after entry differs a lot among different
types of larvae and species. Most
species do not associate with any particular organ, yet others have a regular
habit in this regard. The organs with
which they may be associated are nerve ganglia, gonads, salivary glands,
intestines, muscles and fat body. The
greatest adaptations are found in microtype larvae, which find their way into
the body cavity from the intestine.
Young Sturmia cilipes larvae enter one of the nerve
ganglia of the silkworm, usually the 2nd to 5th, and during its stay of about
1 week in this position causes a proliferation or enlargement of the
ganglion, which also changes in color to white. Clausen (1940) remarked that the occurrence of these larvae in
the ganglia resembled the same localization of attack by some species of the
hymenopterous family Platygastridae.
In Gonia capitata, the maggot first remains for
a period of 4-28 days in the mesenteron of the intestine without
feeding. Then it progresses to the
salivary gland, and finally reaches the supraesophageal ganglion. A few species are known to inhabit the
salivary gland throughout this period.
Gaedia puellae, parasitic on silkworms, has the habit and may
occasionally be found in the reproductive organs. The period passed within the salivary gland by some species
varies directly with the age of the host larva, which may be from 4-22
days. The maggot of Leschenaultia exul may reach the gland within 2 hrs. after ingestion of the
eggs by the caterpillar, and it remains there for 8-10 days. It lies in the gland itself, rather than
in the duct, and a pronounced malformation is produced. The first-instar larva of Zenillia libatrix may be found in a muscle, the salivary gland or
occasionally in a histoblast. S. scutellata
R.D. consistently passes the first stage in a muscle, and its presence results
in hypertropy of the tissue. In Ghaetogaedia monticola this period is passed in a sac, apparently formed by an
enlarged tracheal tube, near one of the host spiracles. However, this may be only a respiratory
funnel plus a membranous sheath.
First instar larvae of Racodineura
antiqua Meig. lie free in the body
cavity of the host, thereby departing from the usual habit of this group. The planidium and tachiniform larvae which gain entry to the host
through the body wall, either by direct penetration or by injection by the
parent female, are not known to enter nerve ganglia or the salivary gland at
any time. Maggots of Plagia trepida Meig. and P. ruricola Meig. enter a muscle and pass
the entire first stage therein, a habit they have in common with S scutellata. However, in such instances the tissue is
killed and thus no enlarged pouch or sac is formed (Thompson 1915b). The larvae of Rondanioestrus apivorus,
parasitic in adult honeybees, feed in the abdomen throughout the
developmental period, while those of the sarcophagid, Myiapis angellozi
Seguy, are often found lodged in the thoracic muscles (Seguy 1930). The majority of these species lie free in
the body cavity of the host, for the greater part, if not all, of the first
stage, but a number are intimately associated with the intestine (Clausen
1940/62). Young maggots of Compsilura concinnata are always found between the peritrophic membrane and
the cellular wall of the mid-intestine, and the same is found in Anetia hyphantria Tot. A. piniariae
Hart and A. nigripes are found in the mid-intestine, frequently attached to
the walls by spiracular hooks. The
young maggot of Zygobothria nidicola (Muesebeck 1922) lies free in
the body cavity for 10-14 days after penetration and then enters the esophagus,
where it lies dormant for ca. 9 months, while that of Archytas analis remains
for a period up to 15 days between the skin and the hypodermal layer before
entering the body cavity. The larvae
of Chaetophleps setosa are often located in the fat
body (Clausen 1940/62). There may be an obligatory association of certain species with a
salivary gland, muscle, nerve ganglion or intestine, but not when in a gonad
or the fat body (Clausen 1940).
Species associated with a definite host organ as 1st instar larvae
usually leave it right before or after the first molt, and most then assume a
fixed position in the hot body for the rest of the feeding period, which is
related to respiration. A large number of species have 1st instar larvae that pass a long
period of time without feeding or apparent growth, especially those which are
association with host organs.
However, one this starts it is extensive before the molt occurs. Zenillia
libatrix, e.g., increases from 0.23
mm. to 2.0 mm. in length before the end of the 1st instar. Some free living species show migratory behavior during
development, as shown by Centeter cinerea. The egg is laid on the thoracic dorsum of the female beetle,
and young larvae bore directly downward into the host. Feeding and the first molt occurs in the
thorax, and the 2nd instar then enters the abdomen right after the molt. It slowly works its way to the tip of the
abdomen, turns and reenters the thorax.
The host dies and the second molt occurs. The thorax contents are consumed after which the larva reverses
its rout to complete feeding in the abdomen.
However, in male hosts the second molt occurs in the abdomen rather
than the thorax. First instar larvae
of A. analis persist in the host caterpillar until the latter pupates,
after which it molts and then positions itself in a wing pad, in which it
causes an easily recognized characteristic bulge. The respiratory funnel is formed at this position (Clausen
1940/1962). In some tachinids the larvae are partially or completely
enveloped by a membranous sheath, which like the respiratory funnel, is of
host origin. It is soft and flexible,
of varying thickness, and almost opaque in species that induce its greatest
development. Clausen (1940) believed
it could be the result of a defensive reaction on the part of the host
similar to that where phagocytes attack a foreign body, and thus it differed
from funnel formation, which results from healing. The sheath of Sturmia
is made of hypodermal cells, leucocytes, and compressed fat cells and
envelops only the funnel and the posterior portion of the body (Muesebeck
cited by Clausen, 1940). In Siphona cristata and other species, young overwintering larvae are
completely enclosed in the sheath, but older individuals have the sheath open
at the anterior end. A closed sheath
also envelops young larvae of Acita
diffidens Curr. and Winthemia quadripustulata, which occur in caterpillar bodies only in
midsummer, and thus the sheath must be permeable to host blood from which
larvae derive nourishment. Host death does not necessarily occur in the same stage as that
which is originally attacked. Among
Lepidoptera, initial attack is frequently on larvae, usually when they are
half grown or larger. In most host
species, death does not occur in that same stage, but a large number
consistently pupate. A few tachinid
species are indiscriminate in this regard, and the stage at which the host is
killed depends on the age of the larvae when parasitized. Many tachinids kill the host early with
respect to their own stage of development, often when they are in the 2nd
stage, and death is followed by a rather complete liquefaction of the host
body, this conditions being distinct from putrefaction. Cirphis unpuncta caterpillars parasitized by Winthemia die two days after the
parasitoid larvae penetrate, but the latter are still able to complete
development. Death of worker bees
parasitized by Rondanioestrus apivorus occurs suddenly, often they
are stricken while in full flight and die within minutes after falling to the
ground. The mature larva emerges from
the body within 10 min. thereafter.
Because of the continued activity of the affected bees until the
parasitoid larvae are mature, it is possible that the latter feed only on the
body fluids and that, when ready to emerge, they cut the nerve cord and thus
cause almost instantaneous death (Clausen 1940/1962). There are several variations in the manner of emergence of the
mature larva from the host, which depends on the host stage and whether or
not it is alive. In larval hosts, the
mature tachinid larvae usually make an incision in the ventral area of the
abdomen, at which point the integument is very thin. Some researchers believed that this was
done by use of mouth hooks, but other thought that it occurs by pressure of
the caudal end, aided by the solvent action of body secretions. The emergence of Bessa selecta Meig. is
with its rear end foremost (Nielsen 1909).
Microphthalma michiganensis Tns, attacking scarab
grubs, dissolves a large opening in the body wall of the grub permitting exit
(Petch & Hammond 1926). The
aperture may be made some time before actual emergence and may be used in the
meantime for respiration. Emergence
from pupae of Lepidoptera usually occurs at some point on the body venter and
sometimes from the wing pads. In hemipterous hosts, many species are still alive at the time of
emergence of mature tachinid larvae.
In Nezara and Anasa, parasitized by Trichopoda pennipes, Eurygaster by
Clytiomyia and Dysdercus by Alophora
and Catharosia, the larvae leave
the body through the anal opening or through the intersegmental membrane
close by. The host does not die for
several days. Coleopterous hosts also
show a similar condition at times.
The larvae of Minella chalybeata Meig. emerge from the
chrysomelid beetle, Cassida deflorata Suffr. through a dorsal
aperture between the 1st and 2nd abdominal segments. Because the vital organs are not affected,
the host does not die until later. The
death of parasitized earwigs also follows, rather than precedes, parasitoid
emergence from the body, and maggots exit through the intersegmental
membranes near the posterior abdomen.
Larvae of Thrixion emerge
from the body of the phasmid host through the wound at the side of the thorax
which had been previously used for respiration, and thus the mechanical
injury that occurs at this time is slight. Several species, such as Zenillia
pexops B.B., that attack larval
hosts in exposed situations, pass winter as mature larvae within the dry host
skins. Thus they are exposed to
sudden changes in both temperature and humidity and must adapt to such
conditions. These larvae are golden
yellow in color owing to large quantities of fatty substances stored within
the body, and the integument is much heavier than in species not so exposed
(Clausen 1940/1962). Tachinidae generally have three larval instars, but four and
possible five may occur in some as in Microphthalma
michiganensis. In Actia
diffidens, Paradexodes epilachnae,
and others, it was noted that the inner wall of the puparium is lined with a
distinct transparent membrane, the true cast skin discarded after a short but
definite prepupal stage (Clausen 1940). Pupation.-- Tachinidae pupate at variable
sites, especially those attacking Lepidoptera. Most form the puparium outside the host, those from free-living
caterpillars usually enter soil, while others developing in stalk borers,
leaf rollers, etc. usually pupate within the burrow or leaf roll. Some such as Voria ruralis Fall, Echinomyia fera and Sturmia nidicola pupate in the host larval
skin. In solitary parasitoids the
puparium is oriented the same way as the host, but if gregarious the puparia
lie transversely in an even row. Species
reaching maturity in the host pupa often form the puparium within the pupal
case. Pupation on sawfly larvae
usually occurs either within the larval skin or in the soil. When attacking Coleoptera, tachinids
usually pupate within the larval skin.
When adult beetles are hosts, most species pupate within the abdomen
of dead beetles, with the head at the posterior end of the abdomen, thereby
facilitating emergence. This is
especially obvious in Centerer, Hamaxia, Trophops and Erynnia. Some gregarious species also pupate in the
host body, especially Palpostoma subsessilis, where as many as 28
develop in one beetle, and Cryptomeigenia
theutis. In Eubiomyia calosomae, that may produce up to 16
individuals in a single Calosoma
beetle, ca. 40% pupate within the host abdomen, and the rest do so externally
in the space between the abdomen and the elytra. A few solitary species emerge from the host beetles for
pupation, among which are Degeeria funebris Meig. and Minella chalybeata. Mature larvae
of Chaetophleps emerge from the
dead or dying chrysomelid host through an incision at the juncture of the
head and thorax and pupate in the nearby soil (Clausen 1940/1962). Tachinids that attack Hemiptera usually emerge from the host body
to pupate in the soil. However, Trichopoda pennipes differs in that the summer generation pupates in the
soil while puparia of overintering broods are found in the dead hosts. Tachinids attacking earwigs pupate outside
the host body as do the few species with dipterous hosts. Mature larvae of Prosena, Dexia and
other white grub parasitoids leave the body and pupate in the soil, ca. 2-4
cm. below the host remains (Clausen 1940/1962). The normal habit of dipterous larvae, which enter the soil for
pupation is to reverse their position so that when the puparium is, formed
the head end points upward (Thompson 1910).
Flies emerge by using the ptilinum and the backwardly directed spines
of the body in order to force their way through the soil. Landis (1940) found that in Paradexodes, 46% of the pupae had
their head end directed upward, but 29% had it directed downward, and 25%
were laying horizontally. When pupating within the skin of a larval host, gregarious
tachinids often adopt a regular transverse position. Those pupating outside the host are usually
scattered, or they may be closely packed but without any order. An exception is found in Sturmia cubaecola Jaenn. and S.
protoparcis Tns. where several
dozen puparia from one host are cemented together in an upright position in a
disk-like mass (Greene 1921). Temperature and humidity changes affect tachinid pupae
variably. Those pupating in soil are
protected from sudden changes and are not able to tolerate prolonged exposure
to temperatures or humidities that differ a lot from those experienced under
natural conditions. Resistant forms
are found in those that pupate above ground and pass winter as pupae. Such pupae withstand large fluctuations in
both temperature and humidity. A
maximum emergence is secured in culture from species pupating above the
ground when they are held under comparatively low humidities, while with most
soil-inhabiting forms an almost saturated atmosphere is most favorable. Maximum emergence from puparia of Parasetigena segragata Rond, was secured by Grösswald (1934) from material
which had been stored at 7°C. and 100% RH. Experiments with W. quadripustulata
resulted in 33.4% emergence at 7.1% RH and 100% at RH of 73-100% (Hefley
1928). The viability of puparia of
both summer and winter broods is proportional to the atmospheric humidity up
to 73%, above which it remains at 100% (Clausen 1940/1962). By contrast the emergence of host adults
from healthy pupae that are found in the soil is inversely proportional to
RH. Therefore, optimum conditions for
the host are disadvantageous to the parasitoid and vice versa. Parasitoid pupal stage duration is also
much influenced by changes in RH; at 17°C. the range was from 26.3
days at 7.1% to 15.7 days at 73%, and then lengthened to 20 days at 100%. Larval & Pupal Respiration.--
Cutaneous respiration satisfies the oxygen requirements of tachinid larvae
within the host body. They
temporarily tap an air sac or tracheal branch of the host, or they establish
a fixed source of exchange with the outside air through the host tracheal
system or directly through the integument.
The latter involves the formation of a respiratory funnel within which
the caudal end of the parasitoid larva, having posterior spiracles, is
enclosed. Cutaneous respiration is
the only way in which young larvae embedded in muscular tissue, a ganglion or
a salivary gland are able to secure their oxygen requirements. As they mature this source probably proves
inadequate and there is a greater need and another source is secured. Many tachinids that are free living in the host body have
posterior spiracles equipped with sharp, heavily chitinized hooks, which are
used to puncture an air sac or tracheal branch. This is especially common among the parasitoids of adult
beetles, and permits free movement, for the connection is only
temporary. Many of these larvae
persist in a free state throughout the larval period, although the spiracular
hooks may be lacking in the later instars.
Spiracular hooks in Anetia
spp. are present on the 1st instar larvae, while in Centeter spp. and Hamaxia
incongrua they are found only in
the 2nd instar. They are employed to
puncture an air sac to provide for respiration in Centeter, whereas in Anetia
piniariae and Compsilura concinnata
they also serve to hold the larva in a definite feeding position. The larval respiratory funnel, in which the posterior end of the
body with the functional spiracles is fixed, is an adaptation of general
occurrence in Tachinidae. However, it
is found elsewhere in only a very few highly specialized parasitic species of
the closely related Sarcophagidae, but in no other families of parasitic
Diptera. The only case of the
development of an apparently similar relationship elsewhere is found in the
chalcidoid family Eucharidae, of which two species of Orasema are internal parasitoids of larvae (see Clausen, 1940 for
diagrams). Funnels in Tachinidae may
be integumentary in origin, giving direct access to the outside air, and
formed at the point of entry of the young larvae or at some other point by a
larva that has already passed a period of free live in the host body. Or it may be of tracheal origin, usually
arising on one of the main lateral trunks or on a spiracular stalk, but sometimes
on a tracheal branch or air sac. There are two classes of respiratory funnels, based on the manner
of origin . Those which develop at
the point of entry of the parasitoid larva into the host body are called
primary and are always integumentary.
Others arising as a result of the activities of the larva from within
the body are secondary. The latter
may be either intergumentary or tracheal in origin. However, there is not much difference in form or function of
the two (Pantel 1910-1912). When the respiratory connection is made with the formation of a
funnel is variable. In a large number
of species it takes place at the time of initial entry into the host, and
funnel formation can frequently be distinguished within a few hours after
penetration. Species that inhabit
Lepidoptera and have this habit are Ernestia
ampelus, Bonnetia comta, Ptychomyia remota, Phorocera agilis, Winthemia quadripustulata
and Sturmia inconspicua Meig. Bigonicheta setipennis parasitic in earwigs and Dexia ventralis in scarab
grubs similarly fix themselves at the point of entry. However, in hosts that are in the adult
stage when attacked, the immediate formation of the funnel at the point of
entry is rare. Funnels formed at the point of entry are often relatively consistent
in position for a given species. The
planidium of Bonnetia comta usually penetrates the
integument of the noctuid caterpillar host on the dorsum of the 1st thoracic
segment. Bigonicheta does this in the intersegmental areas of the
thorax. In these cases the location
of the funnel is a matter of choice by the planidium, but in species that lay
macrotype eggs it is determined by the location of the egg on the host
body. In Winthemia and others that have a dehiscent egg, the larva remains
with its posterior end in the eggshell while penetration is being attempted,
and the funnel is thus found just in front of the egg. Those species making the respiratory attachment after a period of
free life in the host or of confinement in a definite organ, it may be either
with the integument or with the tracheal system. There is an approximately equal division between the two points
of attachment. Among species having
caterpillar hosts, Actia diffidens Curr. is invariable found in
an integumentary funnel on the mesothorax, while Sturmia nidicola and Leschenaultia exul are located in the posterior region of the abdomen (Prebble
1935). Gonia capitata and Archytas
analis, which form the funnel only
after the host has pupated, are located in a wing pad, while Zenillia libatrix chooses a point between any of the ventral plats of the
head sclerites. Of species making their attachment to the tracheal system, S. cilipes
and Compsilura concinnata do so with the short stalk leading from a spiracle to
the main longitudinal trunk, often close enough to the spiracle as to be
considered connected with it. Most
species make their attachment to the longitudinal trunk itself, but near the
base of a spiracular stalk, and in caterpillars and coleopterous larvae a
spiracular stalk in the 1st or 2nd abdominal segment is the preferred
location (see Clausen, 1940 for diagram).
The connection is with one of the smaller tracheal branches permeating
the fat body of the Pyrausta larva
in Zenillia roseanae B.B., whereas Eubiomyia
calosomae forms the funnel on a
tracheal branch in the beetle's metathorax.
An unusual adaption was recorded by Matthey (1924) in Exotista larvarum L., where occasional larvae are found to have broken the
longitudinal tracheal trunk of the host and to have used the broken end of
the trunk itself as a funnel enveloping the posterior body. Gymnosoma
rotundatum L., parasitic in
Pentatomidae, makes its connection during the intermediate larval period,
with one of the air sacs in the thorax (Clausen 1940/1962). Species making respiratory connections with the tracheal system
rarely reveal any external evidence of their presence until near the end of
feeding. If the funnel is formed
close to a spiracle, it may sometimes be visible. However, integumentary funnels are usually visible almost
immediately after the connection is made, not only because of the actual
perforation but by the dark funnel showing through the host integument if the
latter is thin and not heavily sclerotized (Clausen 1940/1962). The tracheal funnel represents a defensive reaction of the host
to the irritation caused by making the perforation and to the persistence of
the posterior end of the body of the parasitoid in the wound. Prell (1915) called it a wound-scab
formation, which if true should cause it to have a constant makeup whether it
arises from the integument or a trachea.
Tothill et al. (1930) believed that the tracheal funnel of Compsilura consisted of an inner
chitinous layer, a median hypodermal layer and an outer basement membrane. There is much variation in size and form of the funnel, both
among species and among different parasitoid instars. It is relatively short, flat and almost
button-like in Thrixion (Pantel
1898), parasitic in Phasmidae.
However, in most cases it is cup-like and closely envelops several
posterior segments of the parasitoid body.
It increases gradually in size with larval growth, and the basal
portion may appear as a slender stalk.
The funnel is usually much darkened in color, this being more
pronounced near the point of attachment where the wall is thickest. In a few species the funnel is almost
colorless. At times it has a
distinctly segmented appearance due to a different in size and form to
accommodate later instars (Clausen 1940/1962). One or both molts may occur during the period of connection with
the funnel. When this takes place,
the exuviae remain in the form of a wrinkled lining on the inner wall of the
funnel instead of being matted into its base, where they would interfere with
respiration. The first exuviae are
more often ejected from the mouth of the funnel (Baer 1921). This is possible in species forming the
funnel at the point of entry, for the opening is often large. Anetia
nigripes, shows an unusual kind of
molting. The 1st instar larva, lying
in the mid gut of the caterpillar host, forms a transverse split just above
the caudal spiracles, after which the skin is cast over the head (Clausen
1940/62). When the host molts, the union between the funnel and integument
is severed, and thus no interruption of function or injury to the parasitoid
larva occurs. However, sometimes it
was found that the funnels of 1st instar larvae have been pulled out of the
wound as the skin was cast; but the larvae remained in situ, with posterior
portions of the body extruded from the wound. They had no problem with forming new funnels at the same spot. In Compsilura, the
initial perforation in the tracheal tube is made by the larva's mouth hooks
(Tothill et al. 1930). In Thrixion and other species this is
accomplished with the posterior end of the body (Pantel 1898). The 3rd instar larva of Paradexodes epilachnae in the larva of Epilachna
varivestris Muls. frequently
abandons the funnel that it has used and makes a final respiratory opening on
the host's dorsum. To do this the
kidney shaped spiracular plates are flexed against the inner side of the
integument until it is cleared of muscular and other tissue. They are then pressed closely to the
surface, and a partial vacuum is created, which causes the integument to come
into close contact with the serrate edges of the plates, and the rasping
action of the plates results in perforation (Landis 1940). Usually a single respiratory attachment is made during the life
of the larva, and its position remains fixed in the host body from the time
of formation of the funnel until it is abandoned for feeding in the 3rd
stage. However, Archytas analis,
occupying a funnel in the wing pad of the host pupa, leaves this one in the
early 3rd stage and makes a second breathing pore in the head or anterior
thorax. In Paradexodes, the first funnel, arising from a spiracular stalk,
is formed at the end of the first stage, and this is abandoned at the second
molt and attachment made to another spiracular stalk. This second funnel is for a portion of the
stage only, following which a third opening is made, this time in the
integument, or the larva enters the intestine and lies at either end, using
the natural openings of the host to respirate (Clausen 1940/62). Initial funnel formation may occur right after entry into the
host or during a later larval stage.
When this is done is relatively constant for any given species. In most species it is during the first
stage, but it occurs only after the first molt in Leschenaultia exul, Zenillia roseanae and Gonia capitata. A small group of species do not make a definite attachment or
induce the formation of a respiratory funnel at any time. Among these are Anetia piniariae and A. nigripes,
which are parasitic in caterpillars Hamaxia
incongrua, Centerer cinerea and Palpostoma subsessilis in adult scarab grubs (Boas 1893), Trichopoda pennipes in adult Pentatomidae and Fortisia foeda Meig. in
Lithobius. Dissection is usually not adequate to
detect the larvae, but it is thought that most of them if not visible from
the exterior, are somehow attached to the internal tracheal system. Cutaneous respiration is normal in the first larval stage of many
species, especially those inhabiting one of the host organs, and spiracles do
not occur in Racodineura and a few
other species. Tachinids that are
free living as larvae secure their air during later stages largely by means
of frequent temporary connections with an air sac or tracheal branch. The time when the connection with the
respiratory funnel is broken varies with the species and the physical
condition of the host. Clausen (1940)
thought that the determining factor was probably whether or not the larva
could reach its food source during this period. In Degeeria luctuosa Meig. on Haltica beetles, the connection is maintained until the end of
the 3rd stage. This is possible
because the attachment is to a tracheal trunk near an abdominal spiracle and
all organs of the abdomen on which the larva feeds are within reach. On the other hand, the larva of Eubiomyia calosomae is attached to a tracheal branch in the metathoracic
region. Thus, to complete feeding,
the connection is broken very early in the 3rd stage, and thereafter it
wanders free in the body of the live host.
The persistence of the respiratory connection reaches an extreme in Siphona geniculata, where larvae sometimes maintain their connection with
the funnel even after emergence from the host (Rennic & Sutherland 1920). Other adaptations that relate to respiration are known. The young larva of A piniariae which
inhabits the mid gut of the host, resumes feeding in springtime which is
marked by the cutting of two openings in the intestinal wall. One of these is near the head for feeding
purposes, and the caudal spiracles are thrust through the second
opening. In this position many
tracheal branches are held by the spiracular hooks, and the air supply is
derived from them. The mature larvae
of Ginglymyia acrirostris Tns. that are found in aquatic larvae of Elophila fulicalis Clem. (Lloyd 1919), extrude the large stalked spiracles
completely through the dorsal integument of the host (see Clausen, 1940 for
diagrams). Just before pupation, the
host replaces its thin, web-like covering with a heavy, oval, roof-like
structure, making up the cocoon, which has several openings at each end. Water passes freely beneath this covering,
and a large bubble of air formes at its center. The forked spiracular structure of the parasitoid extends from
the middle region of the host body into the bubble, thus securing for the
mature larva and pupa an adequate air supply. Clausen (1940) speculated on the manner in which this
adaptation may have arisen.
Transitional forms are not known, and these presumably would not
fulfill the requirements. It is
probable that the parasitoid adapted concurrently with the host, for the
latter is one of the few species that has adapted an aquatic mode of life. Another respiratory adaptation is in the 1st instar larva of Plagia trepida (Thompson 1915b), which bears a well-developed anal
vesicle in the form of a large plate occupying the greater portion of the
ventral surface of the last segment and in the center of which the anal
opening is located. This vesicle is
formed of large cells appear like the striate wall of the middle
intestine. This larva normally lies
in a dead host muscle, and the great development of the vesicle is in
response to the need for oxygen.
Other larvae confined to host organs are bathed in abundant secretions,
resulting partly from the hypertrophy of the parts involved, and thus the
need for respiratory adaptations is not great. Pantel (1898) recognized a respiratory purpose for this
structure in Tachinidae. It has not
been found in any genus of the family except Plagia but is present in bilobed form in Conopidae. The structure appears to be homologous
with the anal vesicle of some parasitic Hymenoptera, where it is highly
developed (Clausen 1940/62). Information on tachinid pupal respiration reveals that the
spiracles of the puparium, representing those of the 3rd larval exuvia, are
not utilized directly. There is no
connection with them. Species having
extruded prothoracic cornicles may obtain their air supply from outside, but
the extensive development of the internal spiracles in all species indicates
that these are the principal organs serving the purpose. Where the cornicles do not penetrate the
puparium, the larval spiracles may remain sufficiently open to permit the
passage of air into the general cavity of the puparium. Snodgrass (1924) concluded that the
anterior larval spiracles of Rhagoletis
pomonella Walsh provide the
channels through which air enters the puparial chamber. At molting, the tracheal branch or stalk
that leads to the spiracle is withdrawn from the body, and it remains
distended, even though broken, on the inside of the puparial shell. Clausen (1940) thought that the same
adaptation may occur in Tachinidae. Adult Emergence.-- As true for many Diptera,
emergence from the puparium is by an expansion of the ptilinum which forces
off the two parts of the operculum.
The fly then works its way out.
Soil penetration or other material is accomplished by alternate
expansion and retraction of the ptilinum, aided by the backwardly directed
spiens of the head and thorax, and by use of the legs. The way adult flies emerge from puparia
that are still enclosed within host cocoons or burrows has caused
speculation. Where the puparium is
contained within the dead host skin, this is usually thoroughly dried and
adheres closely to the outer surface of the puparium. It is broken by the outward pressure of
the expanding ptilinum. In dead
beetles on or in the soil, the intersegmental membranes of the abdomen are so
thin and weakened by decay that the fly has no difficulty in forcing the
segments apart (Clausen 1940/62). Cocoons of Tenthredinidae and Lepidoptera ore different, however,
for the walls are composed of varying quantities of silk and other
material. This makes them too tough
and heavy to be broken by pressure alone.
The manner in which emergence is accomplished by Diplostichus janitrix
Htg. from the cocoons of Diprion pini L. differs. After feeding in the body of the host, the
parasitoid larva emerges and prepares for a later exit by cutting a circular
groove with its mouth hooks around the inner wall of one end of the cocoon
(Robbins 1927). When the fly emerges
from the puparium, it forces this cap away, which is only lightly attached by
the outer silken layer. Cocoons
containing unparasitized sawfly larvae do not show this inner groove, and
adult sawfly emergence is effected by cutting away a cap, which is a bit
larger than that removed by Diplostichus,
with the mandibles (Prell 1924) . De
Fluiter (1932, 1933) found that the parasitoid may emerge from either end of
the host cocoon, thereby establishing definitely that the cap structure is
related only to the parasitoid.
However, most do emerge from the anterior end. Even Lepidoptera with very heavy walled cocoons sustain tachinid
parasitization. Chaetexorista javana
seems to have no difficulty in emerging from the egg-like cocoons of Monema flavescens, that are very hard and tough and can be cut only with
a sharp knife. The cap, which is
usually forced off by the host itself, is similarly removed by the
parasitoid. The circular line of
union of the cap with the rest of the cocoon is relatively weak, and an oral
secretion may serve to soften the lining so that the pressure the fly is able
to exert causes the braking away. Some
Lepidoptera that spin silk cocoons leave a definite opening or loosely woven
area at the anterior end of the cocoon, through which the parasitoid larva or
adult is able to emerge without trouble.
Winthemia datanae Tns. parasitic on Samia cecropia shows that sometimes there is difficulty in emergence
with some hosts. The larvae seem
unable to penetrate the cocoon wall, and they die from desiccation even
before pupation. Life
Cycle
Tachinids have life cycles ranging from 10 days in Metagonistylum minense in the tropics to a full year for others. Most multibrooded species complete the
cycle in 3-4 weeks in summer. The
incubation period is vary variable, for many species are larviparous or ovoviviparous,
and the microtype eggs, though fully incubated at oviposition, must be eaten
by the host before they can hatch. In
some species they remain viable for as long as 2 1/2 months. The unincubated macrotype eggs hatch in
2-4 days. Free living larvae, which
await a host or must search for it, are able to survive for up to 10 days in
the case of Ernestia ampelus. Summer broods have short larval feeding periods, in some cases
being only 4-6 days as in W. quadripustulata, 7 days in Lixophaga, and 6-8 days in Centeter spp. However, 12-16 days is more frequent for
feeding of summer broods. In Bigonicheta the period is nevertheless
variable, ranging from 21-90 days, the duration depending on the amount of
food available in the individual hosts (Mote et al. 1931). In contrast, the developmental period of Paradexodes epilachnae larvae is not affected in this way, and the duration
of feeding is the same, whether in young or mature larvae, prepupae or pupae
(Clausen 1940/62). Summer developing broods have a pupal period ranging from 5-7
days in Hyalomya aldrichi to 25-30 days in Sturmia nidicola, with 8-12 days as a general average for the
family. Dowden (1933) observed that
the pupal stage of female Zenillia libatrix is 1-2 days longer than for males,
which Clausen (1940) thought was probably a consistent difference for the
sexes generally. Tachinids hibernate mainly in the pupal stage, although there are
some departures from this. The next
most common behavior is in an early larval stage within the living host larva
or pupa, which occurs commonly in lepidopterous hosts. Of the Dexiinae, the single brooded Prosena sibirita is in the first stage within the host grub, while Dexia ventralis and Microphthalma
are in the second stage during that period.
Species which attack adult beetles that persist through the winter are
usually in the early larval stage within the body. Species of Chaetophleps,
Degeeria, Erynnia, Eubiomyia, Erynnia, and Stomatomyia show this behavior.
Several species of Winthemia
hibernate in the mature larval stage in the soil. Zenillia pexops (Wardle 1914) that develops in
sawfly larvae, overwinters in its mature larval stage within the dried host
integument, and a few other species have this behavior. Hibernation of a large number of North
American species were recorded by Schaffner & Griswold (1934). North American tachinids do not overwinter
as adults in temperate regions, although in Japan and Korea Hamaxia incongrua apparently does so (Clausen 1940/62). Hibernation in the pupal stage is most often the result of low
temperatures, but this is not the only factor responsible. A large number of species pass into a
state of diapause irrespective of prevailing temperatures. Sometimes this can be broken prematurely
by subjecting pupae to a period of pronounced cold. Thompson (1928) found that Bigonicheta
setipennis appears to have two
types of puparia. In one the fly
develops and emerges rapidly, while the second persists until the following
spring, when the adult flies appear.
Emergence of the first form cannot be indefinitely retarded by low
temperatures, nor can that of the second type be forced by high
temperatures. According to Pantel,
the two types of pupae ar found most often in species restricted to a single
host or to a very small number of hosts and is an adaptation that increases
the opportunities of the brood for finding hosts, inasmuch as a portion
emerge during autumn and the rest in springtime. The generations per year ranges from one in a large number of species
of temperate regions to 8-10 in Lixophaga
(Scaramuzza 1930) and Prosopaea indica Curr. in the tropics. The number in Metagonistylum is probably much higher, for host stages suitable
for parasitization are available throughout the year, and there is no
hibernation or estivation (Clausen 1940/62).
Species parasitic in adult chrysomelid beetles and Pentatomidae often
have a much greater number of generations per year than do their hosts. This is especially true in those which
overwinter in an early larval stage in their hibernating hosts. Degeeria
luctuosa has this habit and
completes its development early enough in springtime to produce an additional
generation on the hibernating brood of beetles. Chaetophleps setosa usually passes through 5
generations annually under temperatures prevailing in Illinois. Clytiomyia
helluo F. completes 4 generations,
and sometimes 5-6 in Eurygaster,
which has an annual cycle (Jourdan 1935). For most species the seasonal cycle of the parasitoid is
correlated with that of the preferred host, but in quite a few cases thee is
an obligatory alternation of hosts.
This is true especially among multibrooded species that pass winter in
the early larval stages in the bodies of the live hosts. Therefore, a species having this habit
would be unable to exist solely on a host species that hibernates in the egg
stage. Compsilura concinnata
is one of these parasitoids. The
gypsy moth is a favored host, but the parasitoid would be unable to persist
on it alone. An exceptional adaptation
to bridge the winter has been developed in Erynnia nitida, a
parasitoid of the elm leaf beetle.
The two summer generations develop in the larvae whereas the
overwintering brood develops in the hibernating beetles (Silvestri 1910,
Clausen 1940). In Dexia ventralis the seasonal is of interest
because of an unusual alternation of hosts (Clausen et al. 1927). This species is a solitary internal
parasitoid of some scarabs in Asia.
In Korea there are normally three generations annually. The larvae produced by the spring brood of
females attack the grubs of Popillia
spp., the 2nd generation occurs in Serica
spp. and the overwintering generation in miridiba
koreana N. & K. Thus, the successive generations during
the season develop each upon a different scarab subfamily. However, part of the population has only 2
generations annually, and in this case the overwintering host is P. castanoptera
Hope rather than Miridiba. Popillia
grubs pupate ca. one month later than those of Miridiba, and Dexia
development is delayed also, so that the following generation is directly on Serica, the intervening one on Popillia being omitted. The parasitoid thus shows considerable
adaptability, but the complexity of the annual cycle and the host preferences
suggest that the species requires at least one alternate host to become
numerically abundant. Adults appear
in the field around the same time as adult beetles of the host brood on which
they had developed. Therefore, with a
single host having a strictly annual cycle, only 1st instar grubs would be
available for attack for a prolonged period.
This makes it difficult for the planidia to locate them in soil, and
they are not in a suitable physical condition for extensive parasitization. Clausen (1940) therefore questioned
whether the species would be able to maintain itself on a single host species
unless the latter had at least a partial 2-year cycle, as a result of which
grubs in a stage of development suitable for parasitization would be
available at all times. Sex Ratio & Parthenogenesis.-- Sexes
are not superficially easily distinguished in Tachinidae. Reports from New Zealand on Hystricina lupina Swed. by E. S. Gourlay (Clausen 1940) shows a sex ratio of
4:1 in favor of females. In Paradexodes the ratio is ca. 50:50,
with a tendency toward proportionally more females at higher temperatures. Parthenogenetic reproduction was reported in G. puellae by Nishikawa
(1930). He found that uninseminated
females lay few eggs, usually <10, in contrast to several thousand laid by
mated females, and that these unfertilized produced normal larva when
ingested by silkworms. Unmated Ptychomyia remota females sometimes lay eggs, though these do not
hatch. Compsilura concinnata
may puncture the host caterpillar, as in normal larviposition, but no eggs or
larvae are laid. it seems to be the
general habit to retain the eggs in the ovarioles until after mating. If unmated, the eggs are broken down and
resorbed. However, Webber (1932)
found that the eggs of unmated females of Carcelia
laxifrons Vill, Phorocera agilis, etc. descent into the uterus and may be laid, whereas
this does not occur in Sturmia inconspicua. There are wide fluctuations in relative abundance of the two
sexes during different times. In some
multibrooded species, females predominate during autumn. Allen (1926) concluded that a definite
sexual segregation took place in Archytas
analis in the field at times. It is believed that most females migrate
to new areas soon after mating, in case suitable hosts are not abundant
locally, leaving males to mate with whatever females may emerge later. The dispersal tendencies of females seems
lacking in the males. Allen (1925)
noted the occurrence of large gyrating swarms of Winthemia quadripustulata,
consisting entirely of males, in localities lacking in host infestations and
food sources. Parasitism Effects on Hosts.--
Tachinid parasitism effects adult hosts variably, depending on their age at
the time of attack and the rate of the parasitoid's larval development. During early larval stages, feeding is
principally on body fluids and fat bodies, which inhibits development or
causes atrophy of the reproductive organs of the host, a condition of
parasitic castration similar to that in Hymenoptera. In C.
cinerea attacking Popillia beetles, parasitization
usually occurs very soon after the beetles emerge, and this combined with the
lapse of only 6 days from egg laying to host death, ensures that little or no
oviposition by the latter will occur.
Overwintering beetles of Galerucella
that harbor the young larvae of Erynnia
nitida are killed in springtime
soon after feeding starts, and no eggs are deposited. In Eubiomyia
calosomae, with several generations
per year and which attacks Calosoma
beetles having an adult life of 2 or more years, the summer broods kill the
host in 9-12 days. Overwintering
parasitized beetles die within a few days after the beginning of activity in
springtime. The effect on the host
population is much less than in the cases previously noted, because a large
portion of the reproductive potential may already have been realized (Clausen
1940/62). When parasitizing Orthoptera, parasitoid larval feeding seems to
be restricted mostly to the blood, which results primarily in a reduction of
the fat bodies. Mature larvae of Ceracia ajarifrons Ald. have been taken from several locusts that had
oviposited during the preceding 12 hrs (Clausen 1940). Pantel (1898) had shown that the
degeneration of the reproductive system of female Phasmidae as a consequence
of parasitism by Thrixion was only
temporary and that in some cases these females were again able to produce and
deposit eggs after the parasitoid larvae had left their bodies. In the European earwig, which is
frequently attacked in the late nymphal instars, there is a partial atrophy
of reproductive organs, and death usually occurs before eggs can be laid. Clausen (1940) noted that in the case of parasitism of Hemiptera,
the false chinch bug, Nysius ericeae Schill. is rarely able to
deposit eggs when parasitized by Hyalomya
aldrichi, but adult females of Anasa tristis DeG., that contain larvae of Trichopoda pennipes
oviposit, seemingly without serious interruption, until the final larval
stage of the parasitoid is attained.
The parasitized individuals of the autumn brood do not reach sexual
maturity, however (Worthley 1924). For detailed descriptions of immature stages of Tachinidae,
please see Clausen (1940/62). References: Please refer to <biology.ref.htm>, [Additional references
may be found at: MELVYL
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W. 1925. Biology of the red-tailed tachina-fly, Winthemia quadripustulata
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Bull. 12. 32 p. Blanchard,
E. E. 1956.
Parapolios grioti, nuevo Actiino útil argentino
(Dipt.). Rev. Soc. Ent. Argent. 19:
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des Phasiinae (Dipt. Larvaevoridae): leur signification taxonomique
différente selon les lignées. Proc. Intern. Cong. Zool. 14: 474-76. Dupuis, C. 1957a. Contributions à l'étude des Phasiinae
cimicophages (Diptera Larvaevoridae). XIX.
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71-9. Hertig, B.
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Die Raupenfliegen (Tachiniden) Westfalens und des Emslandes. Abh. Landesmus. Naturkde. Münster Westf.,
Jhg. 19. 40 p. Landis, B. J. 1940. Paradexodes
epilachnae, a tachinid parasite of
the Mexican bean beetle. U. S. Dept.
Agr. Tech. Bull. 721. 31 p. McLeod,
J. H., B. M. McGugan & H. C. Coppel.
1962. Commonwealth Inst. Biol.
Contr., Tech. Comm. 2: 1-216. Mesnil,
L. P. 1955.
Contributions à l'étude de la faune entomologique du Ruanda-Urundi
(Mission P. Basilewsky, 1953).
Diptera Tachinidae. Ann. Mus. R.
Congo Belge 40: 359-67. Mesnil, L. P. 1956. Trois nouveaux
Tachinaires d'Afrique (Dipt. Tachinidae).
Entomophaga 1: 76-80. Mesnil, L. P. 1957. Nouveaux Tachinaires d'Orient. Mém. Soc. R. Ent. Belgique 28: 1-80. Mesnil, L. P. 1962. Die Fliegen der
Palaearktischen Region 8: 753-848. Paramonov, S.
J. 1957. Notes on Australian Diptera. XXIII. Notes on some Australian Ameniini (Dipt., Tachinidae). Ann. & Mag. Nat. Hist. 10: 52-62. Peris, S.
V. 1956.
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H. J. 1956. A synopsis of the tachinid genus Leucostoma (Diptera). J. Kan. Ent. Soc. 29: 155-68. Reinhard, H. J. 1957. New American muscoid Diptera
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J. D., T. H. C. Taylor & R. W. Paine.
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