They first evolved around 400 million years ago - and insects today make up around 85% of all animal species. They may appear to be small and insignificant but they represent about 76% of total animal mass and much of life on this planet depends on them. They may be burying dung, acting as miniature vultures by clearing carcasses, pollinating plants or simply being eaten and so forming part of many food chains. Characterised by their six jointed legs and an exoskeleton made of chitin1 they have been able to evolve into many diverse forms with over one million species described by science and estimates for the total number of species ranging from 3.5 to 30 million. Insects display many different colours with some showing outstanding beauty. However, these colours are not due to the presence of pigments but rather small dimples on their exoskeletons, which refract light (iridescence).
There are three main parts to an insect's body: the head, thorax and abdomen. On the head is a pair of antennae with a variety of purposes. The mouth parts vary largely depending on what the species feeds on and are generally very complex. The legs are attached to the thorax, as are the wings, if the insect has them.
The size of insects is limited by the way in which they breathe. They rely on diffusion2, which is only effective over short distances, so the giant insects that are popular in many works of science-fiction are simply impossible. It is very important to have a large surface area compared to volume so diffusion can be at its most efficient. Insects have a series of 'holes' on either side of their bodies called spiracles. The insect can open and close them, letting air in and out. Within the spiracles are systems of air tubes, called tracheae, where gas exchange takes place. By moving their abdomens up and down, some of the larger insects can draw air through their spiracles allowing for greater activity, such as with bees and locusts.
This useful entry on taxonomy gives a brief guide to species classification. Insects have been divided into orders to make them easier to categorise, with all members of each order sharing similar physiological characteristics. Insects make up the class Insecta under the phylum Arthropoda in the animal kingdom. There are two sub-classes of Insecta: Apterygota (wingless) and Pterygota (winged). Apterygota is the oldest and most primitive sub-class. Pterygota is the largest sub-class and in fact comprises two divisions: Exopterygota (where the wings develop externally) and Endopterygota (where they are internal).
Below is a brief description of each of the insect orders, focusing on their adult form, under their sub-class and division.
Thysanura are bristletails. They're small insects with biting mouth parts. Some have compound eyes and others have none at all. Thysanura have abdomens divided into eleven segments. They are generally brown, grey or white in colour.
Diplura are the two-pronged bristletails. Their main difference from Thysanura are the entognathous3 mouth parts.
Protura are largely unknown, due to their very small size. The largest known species is only 2mm long. Species of the order Protura have piercing mouth parts, eleven-segmented abdomens and are lacking in antennae and eyes. They live mainly in moist soils such as turf.
Collembola are the springtails, so-called due to a hinged tail, which can be released causing the insect to jump as a means of escape. They have biting mouth parts but otherwise their physiology varies between species. Generally though, the antennae are four-jointed and the abdomen is six-segmented.
Pterygota: Division Exopterygota
Ephemeroptera are the mayflies. They are soft-bodied with short antennae. They have three long 'tails' (cerci) and two pairs of membranous wings. The nymphs are aquatic.
Odonata are dragonflies and damselflies. They are moderate to large in size with a long, normally slender body. They are vicious predators with large heads and eyes. Odonata have two pairs of membranous wings. The nymphs are aquatic.
Plecoptera are stoneflies. They are soft-bodied and of moderate to large size. The abdomen usually has long, jointed cerci and the antennae are long. Plecoptera have biting mouth parts. The nymphs are aquatic.
Orthoptera are formed by the crickets, grasshoppers and locusts. They are medium to large in size with a very well-developed exoskeleton. They usually have two pairs of wings with the forewings being more textured. For the most part, the hind legs are adapted for jumping. The cerci are not jointed and are quite short, whereas the antennae vary in length (longest in the crickets and shorter with the locusts).
Phasmida comprises the stick insects and leaf insects. They are mostly large with a long and cylindrical form, in the case of the stick insects, or flattened, as with the leaf insects. Occasionally they have one small pair of wings, though generally have none at all. The legs are all similar to each other and the cerci short and jointed. Phasmida have biting mouth parts.
Dermaptera, the earwigs, are of a small to moderate size. Earwigs have unspecialised, biting mouth parts. The body is quite flattened. The forewings are little more than leathery flaps and the hind wings are membranous and rounded. Several species have no wings. The cerci are not jointed and have evolved into 'pincers'.
Dictyoptera is made up of the cockroaches and mantids. They are of medium to large size with well-developed exoskeletons, as with the Orthoptera. Most have two pairs of wings, the forewings more textured. The mantids have forelegs adapted for clasping. The antennae are long and jointed. They have biting mouth parts.
Isoptera are the termites: moderately-sized, soft-bodied insects that live in colonies. They have general biting mouth parts and short cerci but otherwise their form is adapted for their role in the colony.
Psocoptera, booklice, are very small insects. Some species have wings, though many don't. They have long antennae with nine or more joints. Booklice have biting mouth parts.
Mallophaga are biting lice: parasitic insects that live on mammals. They are flattened in shape and are very small in size.
Siphunculata, sometimes called Anoplura, are the sucking lice. These small insects are external parasites of mammals with mouth parts adapted for sucking. The antennae are short and the eyes either reduced or absent. The body is flattened and the thoracic segments fused.
Hemiptera are the bugs and include among others the aphids, cicadas, leafhoppers, scale insects and plant bugs. They are of small to large size, usually with two pairs of wings, the forewings of a heavier texture. They have mouth parts adapted for piercing and sucking.
Thysanoptera, the thrips, are small, thin insects with long wings fringed with long hairs. A few species have no wings. Instead of claws, thrips have round structures at the ends of their legs. They have short antennae with between six and ten joints. The mouth parts are adapted for piercing.
Pterygota: Division Endopterygota
Mecoptera are the scorpion flies or hanging flies. Scorpion flies are so-called as they carry their genitalia in an upturned position that resembles a scorpion's tail. Some species hang from vegetation on their front legs and catch prey with their hind legs, hence 'hanging flies.' Mecoptera have beak-like heads, prominent and clear spotted wings. They are small to medium in length, with slender bodies and long antennae. Most of the larvae resemble caterpillars. The adults have biting mouth parts.
Neuroptera are divided into three suborders; these vary in their wing shape and the length of the thorax. Planipennia are the lacewings and antlions, Megaloptera are the alderflies and dobson flies, and Raphidiodea are the snakeflies. Neuroptera have extensive branching veins in their wings. Mouth parts vary greatly between species.
Strepsiptera are parasites and other insects and are consequently very small. The females remain in a juvenile form, living inside their hosts. The males will develop into adult form and have peculiar, twisted wings. Adult males have prominent antennae, raspberry-like eyes, reduced forewings and large hind wings. They are short-lived: their only purpose being to emerge from their host, find and then fertilise a female.
Coleoptera, the beetles, are very tiny to large insects with a hard exoskeleton. They are an extremely diverse and numerous order, with nearly 400,000 identified species. The forewings are modified to form wing-cases (elytra) for the hind wings. There are flightless species, but these still have the elytra. Beetles have biting mouth parts.
Hymenoptera include sawflies, ants, bees and wasps. They are of very small to moderate size with two pairs of wings. The hind wings are smaller than the forewings and are fastened to them by hooks. Except with the swordflies, there is a large constriction between the thorax and abdomen. Mouth parts are normally biting but a few are adapted for lapping.
Siphonaptera, the fleas, are small insects. They are unusual in that they are wingless. It is thought that this is because early on they evolved to become parasitic, and so never developed wings. Only the adults are parasitic, and have mouth parts adapted for piercing and sucking, which they use to feed on the blood of mammals and birds.
Diptera are the true flies. They are small to medium in size with one pair of wings. The other pair have been modified to form halteres, organs that act as gyroscopes making flies expert in the air. The mouth parts are normally adapted for sponging or sucking, but occasionally they are piercing.
Trichoptera, the caddisflies, are a hugely diverse order and all its members are aquatic in the larval stages. The adults are terrestrial and for the most part herbivorous. They are closely related to Lepidoptera and have a similar adult form to moths. However, they have hairs rather than scales on their wings and do not have a proboscis. Instead, some caddisflies have mouth parts adapted to imbibing liquids.
Lepidoptera comprise the butterflies and moths. They are small to large insects with two pairs of wings. The wings are covered in tiny scales, as is the rest of the body, along with little hairs. Lepidoptera have compound eyes. The antennae are either feathered, tapered or club-like. The mouth parts have a proboscis4 formed by adapted maxillae (mouth parts).