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Entomology: HYMENOPTERA
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Kingdom: Animalia, Phylum: Arthropoda
Subphylum: Hexapoda: Class: Insecta: Order: Hymenoptera
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General Summary of
the Hymenoptera
The order has two suborders,
the Symphyta and the Apocrita: The
Symphyta includes species with a
very generalized form, both as adults and as larvae. None of them show the
specialized habits that typify most of the other Apocrita, and they are primarily phytophagous. The first
abdominal segment is not completely fused to the metathorax nor does the
constricted waist that is characteristic of the remaining Hymenoptera
accompany the fusion. The ovipositor
is used as an apparatus for piercing plant tissues. The trochanter has two
joints. Larvae are eruciform and in addition to thoracic legs some the
abdominal segments may have prolegs that are without distal crotchets or
spines (Please see Glossary for terminology).
This group includes the
wood-wasps, the ovipositors of which are used as drills for perforating
timber in which the eggs are laid. The 6-legged, strong larva bores through
the wood (in the case of Sirex gigas, this stage lasts for two
years). Pupation occurs near the
surface of the affected timber, from which the adult bites its way out. The
sawflies with saw-like ovipositors, are most important as agricultural pests,
and are different from the wood-wasps by having softer bodies, their smaller
size, and by the presence of two apical spurs on the anterior tibiae.
The Apocrita includes all the other
Hymenoptera. The second abdominal segment is constricted to form a narrow waist
or petiole, the first segment being firmly joined with the thorax. Larvae are apodous when full-grown. Ichneumon flies have slender curved
antennae, and there is a stigma on the wing. The ovipositor is generally long
and projects forward from the tip of the abdomen. The larvae of Lepidoptera
and of sawflies are their usual hosts.
Cyanamid
"flies" also have slender antennae, but there is no stigma on the
wing and there is a reduced venation.
Many of these form galls on plants, while others are parasitic
on fly larvae.
Chalcid wasps also have the
wing venation reduced with no closed cells. The antennae are geniculate or
elbowed. Though most of these small wasps are parasites, e.g. of
lepidopterous and dipterous larvae, and of homopterous nymphs, a few feed on
plant tissues.
Ichneumonids, chalcids and
cynipids have the ovipositor coming from beneath the abdomen well in front of
its tip, and these insects differ in this feature from the Proctotrypidae where
the ovipositor is terminal. Dipterous
larvae are often parasitized by these insects, as are also the eggs of
Orthoptera and Hemiptera. Many hyperparasites occur in this family.
The ants (Formicoidea)
are social, polymorphic insects in which two segments form the abdominal
petiole. Moreover, this petiole always has one or two nodes. The females have a well-developed stinging
apparatus which is a modified ovipositor. Polymorphism reaches its highest
degree of complexity in this group, as many as 30 different castes having
been found. Some of these are
pathological phases due to infection by parasites, e.g. nematode worms, or
other Hymenoptera. In colonies that have winged forms of both sexes, mating
takes place during a nuptial flight in which several colonies are involved at
the same time. This permits interbreeding between individuals from different
colonies. The females then cast off their wings and begin colonies in the
ground.. The workers are sterile females, whose ability to lay eggs in
certain circumstances may be restored. For example, when a colony loses a
queen several workers may, under the stimulus of diet, take her place.
In addition to the environmental
complexity, which a social existence involves, association with other
organisms complicates the lives of ants. Some have adopted an agricultural
habit, living on fungi that they cultivate. Others gather seeds from which
they destroy the radicle to prevent germination. Special chambers or
granaries in the nest are constructed for their storage. A pastoral habit is
found in others, a symbiotic relation being set up with such insects (e.g.
aphids) because they exude fluids that are coveted to the ants. There are numerous other associations of a
different nature that range from the symbiotic to the parasitic. Finally
there are the slave-makers: Formica sanguinea, e.g.,, captures from
the colonies of F. fusca pupae which on emergence serve as slaves in
the colony which has adopted them (Borradaile &
Potts, 1958).
The wasps of the superfamily
Vespoidea are both social and solitary in habit. In these, the
abdominal petiole is smooth and, species with a worker caste are always
winged. The prothoracic tergum extends back towards the wing base. Wasps are
primarily carnivorous. Rarely are they phytophagous as in some
solitary masarine wasps that feed their larvae on pollen and honey. Among
solitary species may be mentioned Odynerus which deposits caterpillars
in its nest when its larvae are developing. Pompilid wasps are entirely predatory
on spiders. Other groups have adopted
the 'cuckoo' habit, laying their eggs in the nests prepared and provisioned
by other species. Social wasps, e.g. Vespa,
live in nests usually constructed of paper obtained in the form of wood
pulp. The larvae, living in cells on horizontal combs, are fed on insect food
gathered by the workers. In early summer the social wasps feed on such
insects as plant lice, etc., but later in the season they search for sweet
fruits, which make them annoying, both in the garden and in structures. In
autumn the colony dies, fertilized females being the only survivors. The
inability to store animal food on which the larvae rely explains the
disappearance of colonies in autumn. Only in tropical regions where food is
plentiful throughout the year do wasp colonies persist.
Closely resembling the Vespoidea are the wasps belonging to
the superfamily Sphecoidea, the distinctive character of which is the
possession of a prothoracic tergurn that does not extend back as far as the
wing bases. These are all solitary predaceous forms, which sting their prey
and paralyze them before placing them in the larval cells, which have been
previously prepared.
The superfamily Apoidea includes
social and solitary bees. Bees are recognizable by their dilated hind tarsi
and the plumose hairs of the head and body that acquire pollen. There are
also inner metatarsal spines of the posterior legs that comb the hairs free
of pollen. The pollen is then
transferred to the outer upturned spines (pollen basket) of the hind tibia of
the opposite side. These legs are further adapted with spines for the
manipulation of wax plates when being removed from the abdomen. The median
glossa is also typical and in certain solitary forms, e.g. Anthophora and
all the social bees, e.g. Apis and Bombus, is greatly elongated
along with the parts other than the mandibles for gathering nectar from
deep-seated flower nectaries. Larvae are fed exclusively on pollen, nectar
and salivary fluids. Megachile, the leaf-cutter, is a solitary bee
that makes cells of neatly cut leaf fragments. Each cell containing an egg is
stored with honey and pollen. Such cells are commonly made in the walls of
houses, the mortar being removed for this purpose. Andrena constructs burrows
in the ground and, though solitary, is usually found in groups of individuals
occupying a common terrain that may include a 'village' of several hundred
nests. Nomada has adopted the 'cuckoo' habit (Borradaile
& Potts, 1958).
Bombus
spp. are similar to the Vespa
spp. in that only impregnated females survive the winter. The colony of
the honeybee Apis mellifera is more permanent, only the males dying
off in the autumn to leave the rest of the colony to hibernate. The nest is
constructed of wax, an exudation from abdominal glands of the worker (sterile
female), and a material of vegetable origin fastens
parts of the nest together thereby making the whole weatherproof.
The workers of Apis are
graded according to age into nurses, who see to the welfare of the
larvae by incorporating salivary juices with their food, ventilators who,
by wing-fanning, set up currents in the nest or hive to reduce the
temperature and to evaporate the honey, scavengers or cleaners, and
foragers who collect pollen and nectar. The changes from nursery work
to housework and to fieldwork are necessitated by changes in glandular
capacity as age increases. Though the density of the population of the colony
determines to some extent when a queen with a number of workers will depart
from the hive as a swarm, it appears that this event is also dependent on
other factors not as yet clear, one of which is the relative proportions of
the above age-groups among the worker caste. The sexes are determined by a
cytological mechanism. Thus, in bees, wasps and ants, haploid parthenogenesis
results in the production of males. A fertilized (diploid) female has control
over the fertilization of eggs that she lays. If an egg is fertilized by
sperm from the spermatheca a female (diploid) offspring develops; if not, a
male offspring (haploid) develops. Whether a young female becomes a worker
(sterile) or a queen (capable of fertilization) depends on nutrition (Borradaile
& Potts, 1958).
The mouthparts of the Hymenoptera are
adapted primarily for biting and often for sucking. There are two pairs of membranous wings joined by
hooks on the anterior border of
the hind wing joined with a groove on the posterior border of the
forewing. The hind wings are
smaller. The first segment of the
abdomen is fused to the thorax, and a constriction behind this segment
usually is present. There is always an ovipositor that is modified for
piercing, sawing, or stinging.
Metamorphosis is holometabolous.
The larvae are usually without legs and rarely erusiform with thoracic
and abdominal legs. The pupae are
exarate
And
regularly protected in a cocoon.
The Hymenoptera are remarkable for
their great specialization of structure, for their varying degrees of social
organization and for the highly developed condition that parasitism has
reached.
The mouthparts are complex in some cases but they seem hardly ever to have
wholly lost the various parts recognizable in the generalized Orthoptera body
plan. The high point of their development is in Apis, the honeybee,
and their least modified condition is in sawflies.
There also have
developed certain parts of the head capsule that are common to all but the
more generalized Hymenoptera. Thus the head articulates with the thorax by a
narrow neck and the occipital foramen that is small and bounded below
by a strong hypostomal foramen. The union of the post genae forms
this. Great mobility is thereby
possible of the head; the hypostomal bridge forms a strong base for the
attachment of the maxillo-labial complex. In all Hymenoptera this complex is
formed by the union in a common membrane of the maxillae and labium that are
thus placed in a close working relationship with each other. The working of
maxillae and labium as a functional unit is further ensured by their basal
segments, cardo and stipes, submentum and prementum, being so arranged as to
bend in a common plane. Folding of
the mouthparts under the head when at rest, as well as their forward
extension when in use, is facilitated. In these several features, most of
which are present in generalized forms, there are the foundations on which
the structural evolution of the higher forms is based and without which these
might never have developed (Borradaile & Potts,
1958).
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Among the sawflies are to be found
the most generalized mouthparts. Wasps, too, are easily referable in these
respects to the primitive omnivorous types with the additional feature of
adaptation to licking of fluids by an extension of the bifid glossa and the
setose maxillary galea. The mandibles here are well suited by their toothed
form to feeding on solid food.
At the other end of the scale of
specialization there is the elaborate elongated and extensible mouthparts of Apis,
the honeybee. The mandibles are
large, smooth, spatulate structures articulated to the gena of the cranium.
They are used for manipulation of wax and pollen within the hive and not for
the gathering of food.
The labium has a short
triangular postmentum, to the front border of which is articulated a long
prementum. From this there projects forwards a long tongue, formed from fused
glossae, and which is setose externally and grooved ventrally. At the base of
the tongue are the short curved paraglossae, holding it in such a way as to
conduct fluid from the ventral glossal groove to the upper surface of the
tongue base and so to the mouth that lies above. Arising also from the distal end of the prementum are the labial
palps consisting of several long segments whose inner surfaces, being
concave, can partly encircle the bee's tongue ventrally for the whole of its
length.
In line with the postmentum
lies the maxillary cardo at each side. Basally each cardo is articulated to a
cephalic apodeme that projects inwards to the head cavity. At its distal end
it articulates both with the stipes and with a V-shaped sclerite, the lorum.
This lies in the membrane that unites the labium with the maxillae and
probably develops as a specialization of it. The locum thus connects the two
maxillae with each other, and into its apex fits the proximal angular border
of the postmentum. The stipes of each maxilla lies at the side of the
prementum and is of about the same length. Distally, on its outer side, lies
the much-reduced maxillary palp, and on its inner side a similarly reduced
lacinea. From between these two there projects the curved, blade-like, long
galea. The two galeae have concave inner surfaces, like the labial palps, and
with these latter complete the encirclement of the tongue dorsally.
Food can be drawn up the
ventral groove of the tongue by capillary action, but it can also pass in
larger quantities into the space surrounding the tongue enclosed by the galeae
and the labial palps, passing within the folds of the paraglossae and being
thereby directed to the mouth, which opens above this point. Such a
feeding mechanism is the climax in an evolutionary process which has involved
in succession the fusion of the glossa lobes, as in the sawflies, the
lengthening of the basal joints of the labium and maxilla as in Colletes, and
the elongation of the glossa, e.g. Apis and Bombus.
The
highly complex social organization in the bees, ants and wasps, in which
caste development is of prime importance, is foreshadowed in the interesting
behavior of solitary wasps and bees. The supply of food to the larva by progressive
feeding, instead of mass provisioning, appears to help the parent to
become acquainted with its offspring, and this establishment of family life
may be regarded as the forerunner of the complex social state of the higher
forms. For instance, in the wasp Odynerus the egg is laid in a cell
and sufficient caterpillars stored to serve as food for the whole of the
larval life Some African species of this genus supply their growing larvae
daily with fresh caterpillars (Borradaile & Potts,
1958).
.
Another important aspect in
the development of social life has been that of trophallaxis. Among wasps the worker
taking food to a grub receives in turn a drop of saliva from the grub. The
workers eagerly look for this, and it is thought that it is the mutual
exchange of food between young and adult that creates in the adult an
interest in the welfare of the colony. That the exploitation of a particular
form of abundant food has contributed to the development of the social system
is obvious. As examples there is pollen and honey for bees and dung as a
basis for the simpler social life of some beetles. No feature determining
cohesion of the bee colony seems to be of such paramount importance as the
ability of the queen to satisfy the craving for a secretion produced by her
(queen substance) which all members of the colony experience. The absence of
a queen is rapidly sensed by the colony and its communal behavior
consequently greatly disturbed. Ants and termites appear to be similarly
dependent on the queen.
The complex environment in
which a social insect lives has produced a form of behavior simulating
intelligence. Bees, for example, can with great effect inform one another of
the presence of a food source. They can further inform each other by scent
and, dance of the position of the food source with considerable precision
provided the sky is not wholly overcast.
The direction of the dance movement refers to the position of the sun
in the sky relative to the hive. Because this position can be determined by
the bees in a sky when the sun is not visible, though in which some blue sky
is present, the ommatidia of the compound eye enable the bee to analyze the
degree of polarization of light emerging from a blue patch. It may be concluded that associated with
the social state's development, there has come about a complexity of behavior
that ultimately depends in turn on the enhanced sensitivity of the members of
the colony.
A kind of parasitism known
as "parasitoidism" is highly developed in the Hymenoptera, with the
ichneumons, chalcids and proctotrypids being almost entirely parasitic.
Almost all orders of insects are affected by the activities of these groups,
the egg, larval, pupal and adult stages all being parasitized. Insects with parasitic habits are divided
into (1) Koinabionts
and (2) Idiobionts
(Please see <koiidio.htm> for comparison ).
The Hymenoptera contains
some of the most economically important insects. The sawflies are important
as agricultural pests. Flower-visiting bees are of great value in the
pollination of flowers. Carnivorous wasps devour other insect pests such as
aphids, while to a large extent the parasitic Hymenoptera are useful in
regulating the populations of phytophagous insects as has been proven by
numerous biological control campaigns.
Two main types of larvae are found in this order, the
legged larva of the sawflies and the legless form of bees, wasps and
ants. The sawfly larva has an outward
resemblance to the lepidopterous caterpillar, but is easily distinguished by
its single pair of ocelli and the absence of crotchets or spines on the
abdominal legs. The prolegs of the abdomen occur on different segments (Borradaile
& Potts, 1958).
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Detailed Morphology
& Habits
The Hymenoptera ranks second in
the number of insect species next to the Coleoptera. They also contain the greatest number of
beneficial insect species. Two-paired
and clear wings characterize them the front wings being largest. A stigma often occurs on the coastal margin. Venation is very variable, however. Some species are wingless, which is
especially true in the parasitic groups.
Hamuli
or hooks are present on the costal margin of the hind wing, which engage with
a ridge on the posterior portion of the forewing to hold the wings
together. This enables the wings to
operate together as a single unit. Three ocelli are usually present.
The mouthparts are usually of the
chewing type, but there many species with a lapping-sucking type. The mandible is used for chewing, cutting,
molding wax, forming cells, etc. The
labium and maxillae are considerably modified in the bees.
The meso- and metathorax are well
developed, but he prothorax is reduced.
The first segment of the abdomen is fused with the thorax and is
called the propodeum. The second abdominal segment is often a petiole and
the remaining portion the gaster. Most narrow-waisted species are beneficial
while broad waisted species are usually harmful (e.g., sawflies).
The ovipositor is used to bore,
pierce or to reach into crevices in order to deposit eggs. It is often associated with poison glands
and ducts. For example, some wasps
sting lepidopterous caterpillars with just enough poison to immobilize
them. This then serves as fresh food
for the developing wasp larvae.
The pupae are exarate and may be
either naked or enclosed in a cocoon, but much variation may occur within one
family.
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Subdivisions
and Classification
There are two suborders: Symphyta (Chalastogastra) are the sawflies
and Apocrita (Clistogastra) are all other groups.
The Symphyta have
eruciform larvae and their prolegs are without crochets may occur on all abdominal
segments. The adults have the abdomen
broadly joined with the thorax. The
ovipositor is adapted for piercing so that their eggs may be laid in hard
wood. There are many pestiferous
species in this group.
The Apocrita contains
the largest number of species of Hymenoptera. Their larvae are grub like without legs. Some develop as grubs on other animals and
their mother nourishes some. The
adults have a distinct petiole, and in some ant species both the second and
third abdominal segments may form the petiole. A node is usually present.
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Selected Families
& Subfamilies of Hymenoptera
[Please see <familyhy.htm>]
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Details of Insect Taxonomic Groups
Examples of beneficial species occur in almost every
insect order, and considerable information on morphology and habits has
been assembled. Therefore, the
principal groups of insect parasitoids and predators provide details that
refer to the entire class Insecta.
These details are available at <taxnames.htm>.
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References
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