What is an Insect?

Insects are members of the animal kingdom and are contained in the Phylum: Arthropoda. Arthropods are animals which have:

  • A hard outer skin
  • bilateral symetry (one side of the body is a mirror image of the other) and
  • The body is segmented with paired limbs

The Arthropoda Phylum includes many Classes one of which is Insecta – the insects. Some other classes under the Phylum Arthropoda are: Arachnida (spiders, ticks etc), Crustacea (crayfish, prawns, crabs etc), Chilopoda (centipedes), Diplopoda (millipedes) etc.

The Orders under the Class Insecta, in Australia, include the following:

Bristletails are similar in appearance to Silverfish but they have a more cylindrical body, a strongly arched thorax, large eyes which touch, and the ability to jump. The most common species are to be found in cracks in coastal rocks, sometimes as close to the water as the splash zone. Food includes algae, lichens and vegetable debris. Reproduction is usually sexual but parthenogenesis is known for some species. Females take up a spermatophore previously deposited by the male on the ground.
Cockroaches are usually ground-dwelling, being found under bark, logs or stones during the day. Their most distinguishing feature is an enlarged shield-like pronotum which overlaps the head in many species. Eggs are produced in an ootheca, or “egg-case”, which can sometimes be seen protruding from the female’s abdomen. Nymphs resemble adults but lack wings. Diet appears to be omnivorous.
Beetles are the largest order of insects and the most important factor contributing to their success has been the development of elytra which are the hardened forewings that protect membranous hindwings that can be folded away for protection. This allows the beetles to inhabit enclosed spaces and the elytra also form a space which enclose the metathoracic and abdominal spiracles thus reducing water loss. Beetles are to be found in almost all terrestrial and aquatic habitats and their diet includes a wide range of food resources including plant tissue, animal dung, carrion, fungi and invertebrate prey. Their life history consists of egg, larva, pupa, adult but some species are ovoviporous.
Springtails rarely exceed 3 mm but are most noticeable when they form masses on water surfaces to feed on microscopic algae. They are also a significant part of the leaf litter fauna and are to be found in rotting logs where they feed on micro-organisms. Many species leap considerable distances when disturbed by releasing the flexed fork-shaped furcula held beneath their abdomen which then hits the ground and catapults them into the air.
Earwigs are readily recognised by their forceps-like cerci which are used for the capture of prey, to fold the wings, and in defence. Nymphs generally resemble adults. Earwigs are usually nocturnal but are attracted to lights at night. Being thigmotactic (responding to touch), they live in cracks and crevices of bark and debris on the ground, preferring damp conditions. Food consists of a wide range of living and dead, plant and animal matter. Eggs are laid in a burrow which the female guards until the nymphs have moulted once or twice.
Diplurans are mainly found in damp soil under logs and stones in the wetter parts of Australia. Most are small but some species grow to 50 mm. They have well developed abdominal cerci, either long and filiform or reduced to forceps-like pincers. Those with pincers lie buried in the soil and use them to seize small, passing arthropods. Other species are vegetarian. Development is slow and moulting continues throughout their life.
Flies, as the name of their order implies, have only two wings, in most species the hindwings are reduced to knob-like halteres which act as stabilisers to assist them when flying. Mouthparts are adapted for sucking liquids, either by piercing with elongated stylets or through an enlarged sponge-like labellum. Two suborders are recognised: the Nematocera which have long filiform antennae and are generally slender and the Brachycera with short antennae and are typical stout flies. Nematocera includes mosquitoes, sandflies, midges, craneflies and gall flies. Brachycera is a large group which contains flies such as March- flies, House-flies, Blow- flies, Robber-flies; Fruit-flies and Hover-flies. There are aquatic, terrestrial and also parasitic species. Life history is egg, larva, pupa, adult although some flies shorten the life cycle by laying living larvae. Egg development requires a blood or protein meal. Flies can be vectors of disease, carried in the saliva of biting insects or on the body of those which feed on carrion.
Web-spinners live in silken galleries and are rarely seen in the open. Alate males, which may be seen when they are searching for mates, are small and dark and can be recognised by their ability to run backwards, very quickly, in a straight line – apparently an adaptation for their life in a silken tube. Under magnification, the expanded tarsae on their forelegs, which contain the silk glands, confirm their identification. Galleries are extended to reach new food sources which are entirely vegetable. The diameter of the gallery is maintained by the web-spinner rotating its body as it spins, keeping its sensitive cerci against the wall as a gauge.
Mayflies spend their nymphal stages in running water before emerging as a drab, winged subimago. Soon after it moults again to produce the more brightly coloured reproductive adult. These two adult stages are the “dun” and “spinner” of the trout fisherman. Adult mayflies live for only a few hours, or days at most, and so most of this time is spent with swarms of males flying over specific markers, such as rocks or bushes or the water surface, with females entering the swarm to mate. In many species, emergence is synchronised. Both adult and larval mayflies are recognised by their three long abdominal cerci.
Bugs are a large group of insects covering such diverse groups as cicadas, aphids, scale insects, backswimmers and those more recognisable as bugs. They are all recognised by their tube-like suctorial mouthparts. Traditionally they have been divided into the Heteroptera in which the forewings are thickened in the basal half and membranous towards the tips, and the Homoptera which have fully membranous forewings. This latter suborder is now considered to comprise two groups: the aphids, psyllids and scale insects; and the cicadas, leafhoppers and spittle-bugs. Most bugs feed on plant fluids, some being serious economic pests while predacious or parasitic species attack both vertebrates and other invertebrates. Some plant-sucking bugs introduce viruses into their hosts while the saliva of others induces gall formation. Bugs which suck sap need a high throughput to ingest sufficient amino acids and proteins. The excess sugars are exuded as honeydew which is a valuable source of nutrition for other insects, and also the cause of sooty moulds on bug infested plants. Nymphs resemble adults except that they achieve wings and reproductive organs only in the final instar. Parthenogenesis occurs in some groups.
Wasps, Bees and Ants are the most diverse order of insects which is divided into two suborders: the Symphyta which do not have a distinct waist between the thorax and abdomen and the Apocrita which do. The Symphyta includes the saw-flies with their well known caterpillar-like larvae that form clumps in trees during the day and defend themselves by exuding a sticky eucalyptus fluid. All other wasps, bees and ants comprise the Apocrita which can be categorised generally as either parasitic, social, or non-social. There are also some in which the larvae form plant galls. Parasitic wasps lay their eggs in or on another invertebrate and the larvae feed on the host without killing it until pupation occurs. Hyperparasitism, in which a wasp parasitises another parasite, is also found in this group. The ovipositor of parasitic species has developed purely for egg-laying but in many of the other species it forms a sting. Social wasps, bees and ants form colonies in which there is a caste system of reproductive females and males and a worker caste of immature females. Non-social hymenopterans provision a cell with a paralysed spider or other insect, or with pollen, and lay their eggs in it for the developing larvae to feed. The nest can be excavated in the ground or in the pith of plant stems, or an existing cavity can be used. These species are able to memorise landmarks that allow them to recognise their nest as they transport the prey to it.
Termites are small, soft-bodied insects which live in colonies that contain a number of castes: reproductives, soldiers, workers. They digest cellulose with the aid of protozoa within their gut and are, therefore, very destructive to trees and wooden structures. Nests can be constructed in galleries below ground, in logs or the trunk of a living tree, or in distinctive mounds of cemented material. Erroneously called “white ants” they can be distinguished because they do not have elbowed antennae or the petiole “waist” of true ants, and their fore and hind wings are alike and held flat over their body.
LEPIDOPTERAButterflies and Moths are probably the most well recognised group of insects with their coloured scale-covered wings , their caterpillar larvae, and cocoon or chrysalis pupal stage. Most larvae are plant grazers but there are also sap-feeding leaf-miners and occasional carnivores. Generally adults suck nectar through a coiled haustellum formed from elongated mouthparts (galeae). Almost every plant habitat has been exploited by larvae of the lepidoptera: roots, trunk, leaves, flowers, galls; seeds, and some members feed on animal products such as wool or keratin. Some species have developed relationships with ants which tend them in return for secretions from dorsal glands. There are many pests of agricultural crops which are members of this order.
MANTODEAMantids are well known from their “praying” posture: an upright stance with their spiny, raptorial forelegs held in front of the body. Cryptic coloration, coupled with immobility, enables most species to wait unseen until prey moves within seizing distance. Cannibalism of the male by the female during mating is part of folk lore but may be an artefact of observing captive specimens. Eggs are laid in a distinctive ootheca (egg-case) for each species, from which hatch pronymphs that then moult to resemble adults except for the absence of wings and genitalia.
MECOPTERAScorpion-flies or Hanging-flies are predominantly found in two families, the Panorpidae in which the males have an enlarged genital tip to the abdomen somewhat like a scorpion’s sting; and the Bittacidae whose members hang from plants by their elongated forelegs. They resemble crane-flies but have two pairs of wings and a rostrum or beak. Hanging-flies capture insects with their long raptorial hindlegs as they fly over vegetation, particularly flowerheads, and some offer the food to females prior to mating. Larvae are aquatic or live in moist organic matter so adults usually are found in damp environments.
MEGALOPTERAAlderflies are fairly rare insects resembling lacewings but distinguished, in the adult, from the wing venation which lacks forking at the terminal ends. They are crepuscular or nocturnal insects found near water, and often sluggish. Larvae are aquatic with long, lateral abdominal gills and chewing mouthparts. They leave the water to pupate in a burrow that they construct under stones or in the ground. The 26 species found in Australia comprise about 10% of the world fauna.
NEUROPTERALacewings are recognised from their net-veined wings which are held in a tent-like manner over the abdomen. Most species are predators with one group, the mantispids, being quite distinctive with raptorial forelegs like a mantis. Some larvae are aquatic, while ant-lions dig conical pits in sandy places and actively flick sand onto passing ants to make them tumble into the pit. Others stalk aphids and psyllids. Larvae have conspicuous jaws which are hollow and used for sucking. Most species are nocturnal or crepuscular and many are attracted to light.
ODONATADragonflies and Damselflies are an ancient order of insects with Australian representatives of the Anisoptera (“unequal wings” = dragonflies) and Zygoptera (“equal wings” = damselflies). Their larvae are aquatic and are easily recognised from their “mask”, formed from the labium and labial palpi, which can be shot out from under the head to catch other invertebrates. Mating takes place in the “wheel” position where the male grasps the female behind the head with its anal claspers and the female curls its body so that its genitalia contact a secondary sperm reservoir on the male’s modified segments 2 and 3. Eggs are deposited in the tissues of water plants or washed off the abdomen tip as the female dips into the water. Adults catch insects on the wing with their legs held forward like a spiny basket.
ORTHOPTERAGrasshoppers, Locusts and Crickets generally have hindlegs modified for jumping, an enlarged pronotum with lobes that extend downwards, and biting mouth parts. Females have a well developed ovipositor. Song is another characteristic of this group and it is produced, normally by the males, by rubbing modified parts of the forewings together or by rasping a hindleg along the abdomen. The order is divided into those species with long antennae and those with short ones. Most orthopterans feed on plant material some becoming serious pests of crops, but there are also some carnivorous species. Nymphs resemble adults and moult through a number of stages until they reach sexual maturity.
PHASMATODEAStick Insects or Phasmids are usually long and slender, modified with spines or protuberances and coloured to look like sticks or leaves for camouflage. They are slow moving, tending to remain immobile except for swaying movements when disturbed. Phasmids are leaf-eaters. Usually populations exhibit a low density even though food is abundant, but outbreaks of plague proportions can defoliate foodplants in some species. Parthenogenesis is widespread which may be some counter to the difficulties in finding mates in species with a patchy distribution. Eggs are dropped or flicked from the ovipositor to lie in the leaf litter. Nymphs ascend the first vertical surface they encounter in search of suitable foliage. Nymphs show only minor differences from the adults.
PHTHIRAPTERALice are divided into the chewing or biting lice (Mallophaga) and the sucking lice (Anoplura), and they have their mouthparts modified accordingly. All are ectoparasites of birds or mammals and are highly modified with flattened bodies and hooked tarsi to cling to feathers or fur of the host. Most species are host specific. Lice usually cement their eggs to feathers or hair and nymphs generally resemble the adults except for colour.
PLECOPTERAStoneflies have an aquatic larval stage and a flying adult. Adults are poor fliers and rarely found far from water, and they are a particular feature of alpine habitats. Most adults are winged and they hold their wings flat against the body almost looking as if they are wrapped in them. Adults and most larvae are herbivorous but there are some carnivorous larvae.
PSOCOPTERAPsocids or Book Lice are usually small but do range up to 10 mm in length and can be either winged or wingless. If winged, they are usually held roof-wise over the abdomen. A bulging post- clypeus and long antennae make them relatively easy to identify, once known. They have chewing mouthparts to feed on algae, lichens and fungal hyphae, though pests of stored products also occur. Some species tend to congregate under communal webs, whilst others are solitary. Courtship often involves a nuptial dance before mating, and development takes many forms including diapause and multiple generations in a season.
PROTURAProturans are pale, slow-moving arthropods, less than 2 mm long, found in leaf litter, mosses, soil or rotting wood. They are recognised from their habit of holding their forelegs forward to act as antennae. Little is known of their development biology except that immatures resemble adults but increase the number of abdominal segments with each of the four moults. They probably feed on mycorrhizal fungi.
SIPHONAPTERAFleas exhibit a number of characteristics which are due to their specialised existence as ectoparasites on birds and mammals. They are wingless, laterally flattened, are covered with backwardly pointing spines to facilitate movement through feathers or hair, and have well developed hindlegs for jumping. Fleas feed on blood and quickly leave a dead host to seek a new one so are effective vectors of disease. Some species are quite host specific whilst others have been recorded from a wide range of mammals, marsupials and rodents. Even aquatic animals such as the platypus or water rat are hosts to fleas. A blood meal is usually required before mating and egg-laying and the insect has both a grub-like larva and a pupal stage.
STREPSIPTERAStrepsipterans exhibit extreme sexual dimorphism with a free-living winged adult male but a female and later larval stages which are parasitic principally on plant-hopper bugs and social wasps or bees. Both sexes are small and the parasitic stages are recognisable only as a cephalothorax protruding from the host. Males have conspicuous eyes and enlarged fan-like hindwings; forewings are reduced to a form like the haltere of flies. Some females emerge from the host to pupate but the majority of species complete their whole life cycle within the host. The first instar larvae are free-living and capable of jumping on to a host by using elongated caudal setae.
THYSANOPTERAThrips are small insects that range in size from 1 to 15 mm. Winged and wingless species occur and wings, when present, are fringed with hairs. Mouthparts are formed into a cone which punches a hole into plant matter so that liquids can be pumped out. Food includes leaf and flower tissue and fungi hyphae. Both winged and wingless forms can be distributed by wind currents, even achieving sea crossings in the aerial plankton. Some species are commercial pests, damaging flowers, causing the formation of galls, or acting as a vector for the transmission of viral diseases.
THYSANURASilverfish are well known household pests, but there are many species which live in other terrestrial habitats such as bark crevices, leaf litter and under stones. Some live as inquilines in the nests of ants or termites and others are cave dwellers. Larvae closely resemble the adults with the silvery scales that give them their common name appearing after the third instar. Their food is debris, particularly of a starchy nature, hence the prevalence of some species in domestic situation.
TRICHOPTERACaddisflies are moth-like insects which have aquatic larvae many of which live in cases made from vegetable matter or sandgrains. Other, free-living larvae make capture nets of silk or live in silken retreats. Adults are covered with hairs, rather than scales as in the moths, and have long, filiform antennae and large compound eyes. Forewings are usually elongate and hindwings broad and fan-like; they are weak fliers. They have reduced mouthparts and either do not feed at all, or imbibe only water or nectar. Larvae feed on plant matter as suspension feeders, chewers, sucking plant fluids or as predators on other invertebrates.


Click here to see a list of butterflies (not including skippers – Hesperioidea) recorded from Victoria

Reference Books – Where to go for Further Information

The following are a selected list of standard references which contain further information on Insects in general and in particular Australian insects.

The Complete Field Guide to Butterflies of Australia by Michael F. Braby (Nov 1, 2004)

Butterflies of Australia: Their Identification, Biology and Distribution by Michael F. Braby (Aug 15, 2000)

A field guide to insects in Australia. Paul Zborowski and Ross Storey. Reed Books, 1995.

An Introduction to Australian Insects. J. Johnston & P. Hadlindton. New South Wales University Press. 1982 Reprinted 1987.

Introductory Entomology for Australian Students. T.R. New. UNSW Press, 1992.

The Insects: an Outline of Entomology. P.J. Gullan and P.S. Cranston. Chapman & Hall, 1994.

The Insects of Australia. CSIRO Division of Entomology. (A comprehensive two volume publication available in the reference section of most Australian Libraries).

Common, I.F.B & Waterhouse, D.F. 1981. Butterflies of Australia. (Revised Edition). Angus & Robertson, Sydney

D’Abrera, B. 1971. Butterflies of the Australian Region. Lansdowne Press, Melbourne. 415pp.

Dunn, K.L. & Dunn, L.E. 1991. Review of Australian butterflies: distribution, life history and taxonomy. Parts 1-4. Published by the authors, Melbourne.

McCubbin, C. 1971. Australian Butterflies. Thomas Nelson Melb.

Flying Colours: common catepillars, butterflies and moths of south-eastern Australia. Pat & Mike Coupar. UNSW Press, 1992.

Australian Cicadas. Max Moulds. UNSW Press. 1990

Worms to Wasps: an illustrated guide to Australia’s terrestrial invertebrates. Mark S. Harvey & Alan L. Yen. Oxford University Press 1989.


The Australian Entomologist. Subscriptions payable in advance to the Business Manager, The Australian Entomologist, P.O. Box. 537, Indoorpilly, QLD. 4068.