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Invertebrate Zoology Kingdom: Animalia, Phylum: Arthropoda Subphylum: Hexapoda, Class:
Insecta (Contact)
CLICK on underlined file names and for
greater detail:
Hexapoda: Class Insecta Some authorities have suggested that the Insecta
contain more species than all the rest of the Animalia. Insects are primarily terrestrial and they
are spread out to almost every conceivable ecological niche, except the
marine environment, which is practically devoid of insect life. Because of their extreme importance as
pests of food crops and as pests and vectors of disease in public health the
special disciplines of Entomology
and Biological Control have been
established. For greater detail on
the various groups of insects please See <Entomology> General Characteristics of Insects Typical characters are a head with one pair of antennae, one pair
of mandibles, two pairs of maxillae (the 2nd pair fused into a labium), a labrum, which does not represent a pair of appendages, and usually
one pair of compound eyes accompanied by several simple eyes or ocelli. The thorax consists of three segments: prothorax, mesothorax and metathorax, which bear the walking legs. The abdomen has a variable number of segments,
with a maximum of 11. There are few
signs of abdominal appendages, with the possible exceptions of (1)
ovipositors and copulatory structures, (2) abdominal structures found on
primitive insects and (3) prolegs that occur in the larvae of some
groups. Details of the various morphological
parts of representative groups of insects may be found at Respiration. -- Blood does not play a major role in oxygen transfer. The tracheae perform this function
primarily. Life Cycles. -- An indirect metamorphosis is involved in the majority of
insects. The sexes are separate and
copulation occurs and the eggs are fertilized internally. Taxonomic Characters. -- The mouthparts, wings and metamorphosis are used
more than any other characters in classifying insects. Mouthparts:
include chewing, which the most common and considered to be the most
primitive. Included are the beetles,
caterpillars and grasshoppers.
Sucking mouthparts are characteristic of butterflies and moths. Chewing-sucking mouthparts are found in
honeybees. Piercing-sucking
mouthparts are found in mosquitoes, horseflies, deerflies, fleas, sucking
lice and the true bugs (Hemiptera/Homoptera). Wings:
Insects are either wingless (apterous), winged with two pairs or
winged with one pair either fore or aft.
The apterous condition is found in very primitive forms, such as the
silverfish or in highly evolved forms such as fleas, chewing and biting lice
and bedbugs. The wings are used to designate two subclasses: (1) Apteragota (wingless) and (2)
Pteragota (winged or those that have lost wings). In the primitive condition of the Pteragota the wing
pairs are alike, membranous and possess many cross and longitudinal veins
(e.g., dragonflies). In the more
advanced forms there may be two equal and transparent wings, or the first
pair have developed into a cover, or there is the presence of scales, or one
or the other pair of wings is lost or there is a complete loss of wings,
which is found mostly among parasitic insects. Metamorphosis. -- Several kinds of metamorphoses
may be found among the Insecta as follows: Ametabolous (= no
metamorphosis). This is the most
primitive type. It includes all
insects that lack wings and or never had wings. Members of this group resemble miniature adults. Paurometabolous (= gradual
metamorphosis). As in
grasshoppers there are no wings in the immature stages, which are called nymphs. They
gradually assume adult proportions. Hemimetabolous (= incomplete
metamorphosis). As in the
dragonfly, immatgure stages are aquatic and called naiads. At this stage there
is little resemblance to the adults.
Then the change to the adult form is abrupt ( = gradual development up
to a certain point after which the adult stage is rapidly formed.) Holometabolous (= complete metamorphosis). As in the caterpillar and cocoon there is
a progression from the egg to the larva, pupa and finally the adult. The larvae in this group are often referred
to as grubs, caterpillars, worms or maggots.
The pupa is referred to as the resting or dormant stage, which is
non-feeding and enclosed in a heavily sclerotized cuticle. The adult is the sexually mature stage. ------------------------------------
Type Animal. -- The Grasshopper because of its
size and representative body parts is a good subject for studying the various
parts of an insect. Body Form. -- The head, thorax and abdomen are clearly defined. Mouthparts. -- The chewing mouthparts consist of a labrum, or upper lip,
mandibles that are heavily chitinized, the first pair of maxillae that assist
in chewing, a tongue-like hypopharynx and a labium, which is the fused 2nd
maxillae. Digestive Tract. -- There are three sections: (1) the fore gut, (2) midgut
and (3) hindgut. The fore gut consists of a mouth, pharynx, esophagus,
crop, gizzard and salivary glands that open into the pharynx. The mid gut includes the stomach and gastric caecae. The latter are outpocketings of the
stomach, which secrete digestive juices. The hindgut includes the intestine, rectum, anus,
rectal glands that serve to conserve water and Malpighian tubules. The latter end blindly into the haemocoel,
and empty into the digestive tract at the junction of the mid and hindguts. Circulatory System. -- The heart is usually confined to the abdomen and is
provided with paired ostia. The blood
is usually pumped forward through the thorax in the dorsal aorta, which opens
near the brain. Blood then slowly
percolates through the haemocoel but does not play a significant role in
distributing oxygen. Respiratory System. -- Tracheae lead in from openings in the body wall, which
are called spiracles.
Smaller Tracheoles then lead off the tracheae to supply
all internal parts of the body with oxygen.
The system also removes carbon dioxide. Ventilation is accomplished in part by muscular pulsations of
the abdomen. The blood plays little
or no role in carrying oxygen. Excretion. -- The Malpighian tubules serve for excretion. Locomotion. -- Legs and wings accomplish locomotion. Immature stages of some groups move about in
a worm-like fashion through contractions of the body. Sense Organs. -- The compound eyes are composed of ommatidia, each of which is an independent visual unit. Therefore, the vision obtained is a
mosaic. Ocelli respond to rapid changes in the intensity of illumination, but
their function is not well understood. The antennae and palps serve a tactile and olfactory
function. Sensory hairs are tactile
and auditory organs are located on the sides of the thorax. Sound Production. -- Some members have a stridulating apparatus whereby the tibia of the hind leg is rubbed against a vein on
the hind wing. This serves for sexual
recognition and is pronounced in crickets. Nervous System. -- The dorsal brain consists of the protocerebrum, deutocerebrum and tritocerebrum, and they may represent the paired
ganglia of the original segment of the head.
There is also a ventral, double and solid nerve cord. Reproductive System. -- Female insects have one pair of ovaries that
consist of ovarioles.
They have a paired oviduct, a medium oviduct, a copulatory bursa, a
seminal receptacle and an ovipositor. Male insects have one pair of testes that consist of
follicles. There are two vas
deferens, a median vas efferens, a seminal vesicle and a copulatory organ. Development. -- Deveopment in the grasshopper is paurometabolous or
gradual. ------------------------------------ The Insect Orders (Increasing Complexity Arrangement →) For greater detail and illustrations of the following please CLICK on the Order Name: Thysanura. -- bristle tails. They have a long caudal filament plus two
cerci. They are primitively wingless (apterous)
and ametabolous. Food consists of
starchy products and thus they are household pests, e.g., silverfish and
firebrats. [Illustrations: <THYSU2>] Ephemeroptera. -- The name meaning
"short-lived" is derived from the fact that adults live for a very
short time. The immature stages are
called naiads, which are predaceous and long-lived. A sub imago stage occurs between the naiad and the adult. [Illustrations:
<EPHEM2>] Odonata. -- Included are the dragonflies,
damsal flies, snake doctors and darning needles. The immature stage naiads are predaceous and long-lived. Adults are longer-lived than Ephemeroptera
and possess a primitive wing venation.
They are also predaceous and able to cause severe bites if annoyed. The Odonata is the only order of insects
in which there are no wingless forms (all species are winged in the adult
stage). [Illustrations: <ODONATA2>] Orthoptera. -- The name means,
"straight-winged" and the order includes grasshoppers, katydids,
locusts, preying mantids, walking sticks and crickets. They have a gradual or paurometabolous
metamorphosis. They have a
generalized body plan for insects and thus are widely adapted for laboratory
study. Many species are pests of
agricultural crops. [Illustrations:
<ORTHOP2>] Dermaptera. -- The earwigs are common household pests, but some species
may cause extensive damage to garden vegetables. [Illustrations:
<DERMAPT2>] Isoptera. -- The name means "similar
wings" and includes termites.
Their food is wood and wood products, and a close relationship has
developed with symbiotic flagellates termed obligate symbiosis.
The symbionts digest the cellulose in the wood for the termites. They are notorious destructive pests of
wooden structures, especially in the tropics. Termites are social insects with casts. The reproductive cast includes a king and
a queen. There are also secondary of
accessory kings and queens, workers and soldiers. Winged termites appear only during the breeding season and
only among the mature males and females.
[Illustrations: <ISOPTERA>] Embioptera. -- Embioptera are small
cylindrical insects with elongated and flattened bodies; two pairs of similar
wings with reduced venation. The
females are apterous, while their cerci are 2-jointed; in males the cerci are
asymmetrical. Metamorphosis
is not present in females but occurs only slightly in males.
Plecoptera. -- The Plecoptera (stone flies)
are mandibulate insects with a hetero-metabolous metamorphosis. Although they
possess two pairs of well-developed wings, they are frail fliers, and do not
move far from their aquatic breeding grounds. They have prominent elongated antennae and. There are 3-jointed tarsi. The wing
venation may represent a primitive type.
There is considerable variation in venation in the order. [Illustrations:
<PLECOP2>] Zoraptera. -- Zorapterans are minute insects,
winged or wingless, with 9-jointed moniliform antennae. They have biting mouthparts. The wings, of which there are two pairs,
have a reduced venation, and are capable of being shed by basal fractures as
in the Isoptera. They have a wide
distribution in the warmer parts of world. They live in colonies and some
caste differentiation exists. [Illustrations: <ZORAPT>] Psocoptera. -- Psocids are small insects, that
have both winged and wingless members.
They have biting mouthparts and their thoracic segments are
distinct. The wings have a reduced
venation and cross-veins are largely absent.
Metamorphosis is slight.[Illustrations:<PSOCOP2>] These insects occur on bark and leaves of trees. They
feed on lichens and dry vegetable matter. The eggs are laid on plant bark or
leaves and covered by a protective sheath of silk by the female. Phthiraptera (= Anoplura) -- The sucking lice are parasites primarily of
mammals. They are vectors of European
Typhus. They are secondarily wingless
and metamorphosis is almost absent. [Illustrations:
<Anoplura>] Mallophaga. -- The bird lice hve chewing mouthparts. They are primarily parasites of birds, but
some species also occur on mammals. They
have a flattened shape and appendages that are adapted for crawling abount on
bires. They are secondarily wingless
and metamorphosis is almost completely absent. [Illustrations:
<MALLOPHA>] Thysanoptera. -- The thrips are tiny insects with asymmetrical
piercing mouth parts with a short labial proboscis; prothorax large and free;
tarsus 2- or 3-jointed with terminal protrusible vesicle; two pairs of similar
wings, provided with a fringe of prominent long hairs, veins few or absent;
metamorphosis slight, including an incipient pupal instar. Hemiptera. (Usually assigned to Heteroptera) = The "True Bugs" include squash bugs, chinch bugs, boxelder bugs,
bedbugs, water striders, backswimmers and lightening bugs. They have
piercing-sucking mouthparts. Their first wings, or hemelytra, are partially
membranous and particularly leathery.
Some species are predaceous and others are parasitic on plants and
animals. The "kissing bug" is a vector of Chagas Disease. [Illustrations: <HEMIPT>] Homoptera. (Usually assigned to Heteroptera). All species are plant parasites and include the aphids,
leafhoppers, mealybugs and spittlebugs.
They are of considerable economic importance because of the attacks on
crop plants. Some members such as the
leafhoppers are vectors of plant pathogens. The 17-year locust has a long life cycle. Populations emerge in tremendous numbers
all at one time in localized areas.
Eggs are laid in twigs and nymphs drop to the ground, burrow into the
soil where they feed for 17 years on plant roots. Damage occurs to plants at the site where the eggs are
laid. [Illustrations: <HOMOPT>] Neuroptera. -- The alderflies, lacewings and ant lions have two pairs of
large, broad wings, the posterior ones having a large posterior or anal
field. The longitudinal veins branch freely and cross-veins are common, particularly
behind the anterior border. A pterostigma
or pigmented area placed laterally on the anterior border is either
absent or poorly defined. At rest the wings are held over the back in a
roof-like manner.
Coleoptera. -- The name means, "shield
wing" and includes the beetles and weevils. The first pair of wings has developed into a very hardy
sclerotized structure or elytra, which covers the 2nd membranous wing pair. They are holometabolous. The larva, or grub, frequently lives in the ground. The pupa is often undetected. The adult is recognized as the beetle. Some species are serious pests of crop
plants. [Illustrations: <COLEOPT2 >] Strepsiptera. -- The twisted-wing parasites are
characterized by some remarkable biological phenomena. Their parasitic
activity is limited to the growth stages and the adult female. The adult males are free living. There is
marked sexual dimorphism where the females are prothetelous, i.e. they have a
larval body form while they are sexually mature. [Illustrations:
<STREPS2>] Mecoptera. -- This is small order of insects
recognized by their vertically directed and elongated head capsule that
carries the biting mouthparts at its end.
They have two pairs of similar wings with simple venation in which a
number of cross veins divide the whole area into a number of almost equal
rhomboidal cells. Metamorphosis is
complete. Trichoptera. -- Caddis flies are medium-sized
insects with bodies and wings well adorned with hairs. Their mandibles are vestigial or absent;
the maxillary and labial palps well developed, and there are two pairs of
nearly similar membranous wings with few cross- veins. These are held in a roof-like manner when
at rest. Metamorphosis
incomplete. These obscurely colored insects have considerable powers of
flight and at sexual maturity may producing mating swarms. Oviposition may
occur directly into the water or the eggs may be laid on plants above the water
where they will await immersion with the winter floods. [Illustrations:
<TRICHO2>] Lepidoptera. -- The butterflies and moths are
holometabolous. The larva is a
caterpillar with chewing mouthparts.
The pupa is a chrysalis that is sometimes enclosed in a cocoon. The adults have wings with scales and
sucking mouthparts. The group is of
considerable economic importance from the larvae, which have chewing
mouthparts. Silkworms are one of the
valuable species as well as butterflies that are cherished for their beauty
and relative tameness. [Illustrations: <LEPID2>] Diptera. -- The flies, mosquitoes and midges
include many pests of public health importance. Some species are vectors of malaria, elephantiasis and yellow
fever. The genus Drosophila has been an important species in the
study of genetics. The first pair of wings is present and the 2nd pair is
represented only by halteres, which act as gyroscopes. [Illustrations:
<DIPTERA2>]
Siphonaptera. -- The group has piercing-sucking
mouthparts adapted to sucking blood, such as the fleas. They are vectors of typhus and bubonic
plague. [Illustrations: <SIPHONAP>] Hymenoptera. -- The name suggests, "clear
wing," and the group includes bees, wasps, ants and sawflies. They are of great economic importance both
as pests, as in the sawflies, and beneficial as plant pollinators and in
biological control. Some bees live in social communities and have casts
such as female queens, male drones and degenerate female workers. [Illustrations:
<HYMEN2>] ------------------------------------ Subclass: Monocondylia, Order: Archaeognatha (Microcoryphia) These are wingless insects, called jumping bristletails. They are
among the least evolutionarily changed insects that arose in the Devonian
period along with the arachnids. The name Archaeognatha comes from the Greek Archaeos
or"ancient" and gnatha meaning "jaw". This refers
to the articulation of the mandibles, which has a single condyle, where all
higher insects have two. An alternate name, Microcoryphia comes from the
Greek micro "small" and coryphia "head". The Order Archaeognatha has previously been combined with the Order
Thysanura, or bristletails, both of which groups possess three-pronged tails
comprising two cerci . [Illustrations -- MICROCOR] ------------------------------------ Please see
following plates for Example Structures of the Insecta: Plate 80 =
Phylum: Arthropoda, Class: Insecta -- Section of cuticle and hypodermis Plate 81 =
Phylum: Arthropoda, Class: Insecta -- Petrobius
maritimus mouthparts Plate 82 =
Phylum: Arthropoda, Class: Insecta -- Histology of the insect gut Plate 83 =
Phylum: Arthropoda, Class: Insecta -- Section through gut of Rhodnius prolixus Plate 84 =
Phylum: Arthropoda, Class: Insecta -- Trachea & tracheole structure Plate 85 =
Phylum: Arthropoda, Class: Insecta -- Closing of spiracles Plate 86 =
Phylum: Arthropoda, Class: Insecta -- Regulation of tracheal respiration Plate 87 =
Phylum: Arthropoda, Class: Insecta -- Sensillae of insects Plate 88 =
Phylum: Arthropoda, Class: Entognatha -- Basic morphology of Collembola Plate 89 =
Phylum: Arthropoda, Class: Insecta, Order: Thysanura -- Lepisma saccharina & Petrobius
maritimus Plate 90 =
Phylum: Arthropoda, Class: Insecta, Order: Ephemeroptera -- Morphology Plate 91 =
Phylum: Arthropoda, Class: Insecta, Order: Dermaptera -- Forficula auricularia male Plate 92 =
Phylum: Arthropoda, Class: Insecta, Orders: Mecoptera & Neuroptera-Panorpa, Sialis, Mymeleon spp. Plate 93 =
Phylum: Arthropoda, Class: Insecta, Order: Neuroptera -- Sialis lutaria Plate 94 = Phylum: Arthropoda, Class:
Insecta, Order: Trichoptera -- Hydroptila, Odontocerum, Hydropsyche,
guttatipennis & Phryganea spp. Plate 95 =
Phylum: Arthropoda, Class: Insecta, Order: Diptera -- Mouthparts of a
mosquito Plate 96 =
Phylum: Arthropoda, Class: Insecta, Order: Diptera -- Calliphora sp. mouthparts Plate 97 =
Phylum: Arthropoda, Class: Insecta -- Pseudotracheal membrane structure Plate 98 =
Phylum: Arthropoda, Class: Insecta, Order: Diptera -- Calliphora sp. labellar lobes Plate 99 =
Phylum: Arthropoda, Class: Insecta, Order: Siphonaptera -- Ctenocephalus mouthparts Plate 100 =
Phylum: Arthropoda, Class: Insecta, Order: Hymenoptera -- Apis sp. mouthparts Plate 101 =
Phylum: Arthropoda, Class: Insecta, Order: Strepsiptera -- Elenchinus sp. male & Polistes sp. host Plate 102 =
Phylum: Arthropoda, Class: Insecta, Order: Anoplura -- Pediculus humanus Plate 103 = Phylum: Arthropoda, Class:
Insecta, Orders: Hemiptera & Homoptera -- Aradus sp. & Aphanus sp. mouthparts Plate 104 =
Phylum: Arthropoda, Class: Insecta, Order: Homoptera --Psylla sp. stylet penetration into plant tissue ============== |