Insects are an incredibly diverse Class of the Phylum Arthropods, arguably the most successful Phylum on earth. For the most part insects live on land or fresh water, leaving the oceans to their cousins, the Crustaceans. All insects share a common body plan: six jointed legs; three body parts (head, thorax and abdomen); an exoskeleton that must be shed in order for them to grow; a ventral nerve chord plus a dorsal one-way gut with a mouth in the head and an anus at the end of the abdomen. Insects are segmented and, with the rest of the Arthropods, evolved from Annelids back in the Cambrian Period. The Onychophora or velvet worms are a surviving Phylum which show this step, being segmented worms with simple jointed legs.
The most primitive insects are the wingless forms of the subclass Apterygota. Five orders of insects were once included in this group, which never have wings at any stage of their development. The first three Orders are now considered by most taxonomists to be separate Subclasses: the Collembola or springtails, the Protura and the Diplura. All three groups have six legs but are different enough from other insects to deserve subclass status. The most common are the springtails, which are found in soils throughout the world and have a unique method of escaping from predators. Their tails are V shaped and can be hooked under their abdomen and then released to toss them up and away from danger.
The true Apterygota are the Archaeognatha and the Thysanura or firebrats. Archaeognatha are so small and seldom seen that they have no common name. The firebrats are also called silverfish and because they have invaded our homes, clothes and books, they are fairly well known. There are some 330 species of silverfish and most are free-living. They are omnivorous, eyeless and have long antennae and three-pointed tails.
The rest of the insect orders belong to the Subclass Pterygota or winged insects. The most primitive are the Mayflies, order Ephemeroptera. These frequent streams, with the eggs and nymphs living aquatic lives. The winged adults are short-lived, have no mouths and do not feed. Their sole purpose is reproduction. Mayfly nymphs are an important food source for fish and other aquatic animals. Adults have four wings with the front pair being larger than the hind pair.
The next Order is the Odonata, the dragonflies and damselflies. These predatory insects are excellent fliers and have good eyesight. Their front and back wings are hooked together and cannot be folded back like more advanced insects so we often see them sitting with their wings outspread. Their legs are short, weak and adapted for seizing prey on the wing rather than walking. Dragonfly nymphs are aquatic and predatory. In turn they and the adults are preyed upon by fish, frogs, birds and reptiles plus other arthropods. The adults can be large, brightly coloured and are usually active in the daytime.
Cockroaches are next: Order Blattodea. These insects have wings but the front pair are modified into protective covers and they are usually seen scurrying along floors in homes or the ground in nature. Because some are pests, cockroaches have a bad reputation but there are about 4000 species world-wide and only a few are pests. Most are nocturnal and hide under logs or leaf litter during the day. Most are omnivorous. They are parasitised by wasps and are eaten by other terrestrial arthropods as well as insectivorous vertebrates.
Termites belong to the Order Isoptera and are closely related to the cockroaches. They mainly occur in the tropics and subtropics with an estimated 2000+ species world-wide. They are soft-bodied insects with cryptic, underground habits. They live in colonies with highly evolved social structures and behaviours. Like the cockroaches, they have a bad reputation due to the wood-eating habits of a few economically important species. However most are important parts of their ecosystems, being responsible for the recycling of much plant material and helping to create and aerate soils. The colonies are founded by winged alates' that emerge from the old colony, disperse, mate and then drop their wings before descending underground to form new colonies. Pairs of males and females form the basis of these new colonies, so, unlike ants, termites have both king and queen. The queen's first brood are workers who tend the royal couple. Later broods become either workers or soldiers, the soldiers having much larger mandibles. Termites feed on wood, grass, fungi and other vegetable matter, both living and dead. The wood-eaters are able to digest cellulose with the help of their gut bacteria.
Order Mantodea are the praying mantids, named after the way they hold their front legs in readiness to grab their prey. These large-eyed insects are found in all warm parts of the world. Mantids are solitary, diurnal insects with most living in trees or shrubs. They can fly but often run or take an aggressive stance if threatened. They are preyed upon by birds and parasitised by wasps. They are an important predator of other insects and have been considered as a possible biological control method for economically important insect pests. They seem more intelligent than most insects and can make interesting pets although they are best as single pets because they are cannibals.
There are two small, cryptic orders next: the Zoraptera and the Grylloblattodea, neither of which have common names. Little is known about them so I will move on to the Dermaptera or earwigs. These terrestrial insects are immediately recognisable by their modified cerci that look like forceps. There are about 1200 species worldwide and they are found everywhere except polar regions. They are usually nocturnal and they eat a wide range of living and dead plant and animal material. The front wings are small and hard and only the back wings are used occasionally for flight. Their primary predators are birds.
The nymphs of stoneflies, Order Plecoptera, are common in freshwater habitats and an important food source for fish. The adults have four wings and frequent streams as well. There are about 1000 species worldwide. They have long antennae, well developed eyes, weak mandibles and strong legs. Trout fishermen probably know more about them than anyone else.
Now we come to the first great order of insects, the Orthoptera, which includes the grasshoppers, locusts and crickets. These insects are important economically because of the damage they do to agricultural crops. They are distinctive because their hind legs are adapted for jumping. They can fly but usually walk and use jumping to escape predators. They are mostly herbivorous. Many orthoptera 'stridulate' using their legs to make strange noises, even songs, during courtship.
Order Phasmatodea are the stick insects. These strange insects can get very large, are slow-moving, usually found in trees and depend on their camouflage to protect them from birds and other predators. There are about 2500 species worldwide, mostly in the tropics and they are never very abundant. All stick insects are herbivores.
One of the least known Orders of insects are the Embioptera or web-spinners. They are small animals, probably related to termites and earwigs and are essentially tropical. Less than 200 species are known world-wide. As their common name suggests, they can spin silk webs in which they live together, but without any division of labour or castes. They eat dead plant matter.
The next Order is slightly larger but not well known: the Psocoptera or booklice. There are about 2000 species world-wide of these small, free-living insects which are probably related to the true lice. They feed on plants and look a little bit like true bugs but have mandibles, not sucking mouthparts. They are preyed upon by other insects and spiders. The true Lice, Order Phthiraptera, are much better known but not well liked because of their parasitic habits and probably evolved from the psocopterans. There are three distinct groups of lice: the biting lice (suborder Mallophaga), the elephant lice (Rhynchophthirina) and the Anoplura (sucking lice). Biting lice plague mammals and birds by chewing their feathers, fur and skin while sucking lice drink blood with modified sucking mouthparts. Lice are highly adapted to their lifestyle, having secondarily lost their wings and having small, flattened bodies. Lice transmit diseases, including typhus, and lice infestations can cause heavy production losses in many agricultural industries. About the nicest thing that can be said about them is that they are so host specific that they can sometimes show evolutionary relationships in higher animals.
The order Hemiptera contains the only insects that can accurately be called Bugs. Bugs and leafhoppers feed by sucking mouthparts. Most suck plant juices, but some are predatory and can suck animal juices. They are the largest group of insects known as exopterygotes, which are all the groups discussed so far except the more primitive Apterygotes. Exopterygotes have wings that develop externally on the larvae, known as nymphs.
There are a great many sorts of true bugs, divided into two suborders: the Homoptera in which both pairs of wings are similar, and the Heteroptera where the two pairs of wings are different. Homoptera include the aphids, the psyllids, the cicadas, spittle bugs, leaf hoppers and numerous smaller groups. The aphids are the most important economically and the cicadas are the noisiest. The heteroptera include ectoparasites, predatory bugs, lace bugs, assassin bugs, bark bugs, and many of the flattened bugs we see on citrus trees and garden flowers. This group also contains the water striders, back swimmers and water boatmen.
There is one more exopterygote order: the Thysanoptera or thrips. These small insects have feathery wings and are usually seen around flowers. They are poorly studied and may be pests. They don't have sucking mouthparts but do feed on and possibly damage fruits and flowers of important crops.
All the rest of the insect orders are Endopterygotes in which the wing sheaths develop internally inside the larvae and there is a pupal stage before the larvae turn into adults. Megaloptera (alderflies), Neuroptera (lacewings), Strepsiptera (no common name) and Mecoptera (scorpion flies) are small endopterygote orders. The annoying, ectoparasitic fleas, Siphonaptera, are also endopterygotes. But the most important insect orders, the big four, are the Coleoptera (beetles), Lepidoptera (butterflies and moths), Diptera (flies) and Hymenoptera (ants, bees and wasps).
The members of these orders are the most highly evolved of the insects, with complex life cycles that involve a distinct larval form such as the caterpillars in the Lepidoptera, followed by a pupation phase, in which the larva wraps itself in a protective container and completely changes its anatomy to emerge in the adult form. Exopterygote young usually just look like a smaller, wingless version of the adult. Endopterygote larvae are so different from the adults that they don't appear to even be the same species.
The largest order is the Coleoptera. This group has been so successful that it has radiated out into a bewilderingly complex number of species, but they are instantly recognisable by the elytra, which are the front wings changed into hard cases that protect the body and hind wings. There are anywhere from 270,000 to 350,000 species worldwide.
Some beetles can use their hind wings to fly but most prefer to walk and in some the hind wings have atrophied. In the weevils, the elytra have fused so the hind wings cannot be extended anyway. Most beetles eat plants but a few are predators. Most are terrestrial but about 5000 have adapted to an aquatic lifestyle. Among the many types of beetles there are: ground beetles, scarabs, stag beetles, fireflies, longhorned and shorthorned beetles, carpet beetles and fruit beetles, water beetles and whirligigs, ladybirds and leaf beetles, darklings and of course weevils, the largest family of all.
All flies belong to the order Diptera, which means two winged and refers to the fact that flies have altered their forward pair of wings into club-like halteres which act like gyroscopes and allow flies to use their hind wings for amazing feats of acrobatics such as flying upside down. There are about 150,000 species worldwide, characterised by legless larvae, sucking mouthparts in the adults and large compound eyes that dominate the head. Most adult diptera are free-living and they are found everywhere, from the aerial plankton to all terrestrial habitats to fresh water and even one extraordinary chironomid which lives in the sea.
Flies are important medical and veterinary pests. Bloodsucking species transmit such diseases as malaria, sleeping sickness, dengue fever and encephalitis. Most of the vector species are mosquitoes but some are tabanids (horse flies) and muscids (house flies). They also transmit worms, viruses and protozoa in domestic animals and birds. There are also many beneficial species of flies. Diptera are important plant pollinators, especially of orchids. Some fly larvae are endoparasites of other insects including several pest species. Fruit flies have played an important, if involuntary, part in the study of genetics.
The Order Lepidoptera contains some of the most beautiful insects on earth, with their wings covered in multi-coloured, overlapping scales. Butterflies are usually diurnal while moths dominate the night. Both use long sucking mouthparts to feed on plant juices. The fore and hind wings are coupled and move together. The success of this group is linked with the evolution of flowering plants. Caterpillars have chewing mouthparts and eat foliage. They are preyed upon by numerous insectivorous species, especially birds. Caterpillars, being fat and juicy, also may fall prey to parasites. Some caterpillars also contribute to the loss of agricultural crops: grass grubs, borers, leaf miners, forest defoliators, cutworms and armyworms. Balanced against this is the joy we experience in the spring when butterflies flit across the landscape and enchant us with their beautiful wings.
The last order is the Hymenoptera: bees, ants and wasps. They generally have chewing mouthparts and reduced venation on their 4 clear, coupled wings. Bees are one of the few insects that have been domesticated to provide us with honey, as well as being important pollinators throughout the plant kingdom. All three groups are characterised by highly evolved social systems, which include a queen and various caste members such as workers and soldiers.
Wasps are important predators and parasites of numerous other species. This can be both beneficial for humans in the case of predation and parasitism of economic pests and harmful to us in terms of being stung by wasps, which can be notoriously ill-tempered. Hymenoptera larvae usually have six legs but some are legless. They have chewing mouthparts and may eat plant material but some are fed and protected by workers in colonies, and others, especially wasp species, are internal parasites of other insects. The adult female uses her stinger' or ovipositor to penetrate the host, such as a caterpillar, and lay the eggs inside the body. The larvae hatch and destroy the host from the inside out. A few wasp larvae are predatory.
The insect world is bewilderingly diverse but can be better understood if one divides it into orders. Life styles and habits are reasonably consistent within these groups and helps one to sort out the diversity into some sort of pattern.
CSIRO 1979 The Insects of Australia Melbourne University Press
Borrer and DeLong 1971 An Introduction to the Study of Insects
Plus numerous other unremembered sources in my garbage can of a brain and my own personal experiences. All errors are definitely my own!