These are the insects. There are over a million described species and many more waiting to be described and even more waiting to be discovered. In the UK there are over 21 500 species. They are around 30 Orders, depending on which book you read. There are a huge amount of body shapes, but the "basic insect" body plan is a head, thorax, abdomen, one pair of antennae, mouthparts in three parts (mandibles, maxillae and a labium).. The head has the mouthparts and most of the sensory organs including the eyes and antennae. The thorax is subdivided into prothorax, mesothorax and metathorax; each bears a pair of legs - never more than 3 pairs in adult insects, and 2, 4 or no wings. The thorax is simply a box of muscles linked to the legs and wings. There are eleven or fewer segments in the abdomen and they contain the organs for digestion, excretion and reproduction. The internal organs are bathed in haemolymph which transports waste and nutrients and is moved around the body by a tube shaped heart and the movement of the insect. Adult insects have six legs, but larvae, e.g., caterpillars, maggots may have fewer or more than six legs (caterpillars have six true legs and up to ten prolegs). Most have three simple eyes as well as compound eyes. They range in size from less than 1 mm up to 20 cm in length.

The earliest fossil insects were found in Devonian rocks, were wingless and resembled modern springtails. The insects are thought to have started their colonisation of the land around 320 million years ago during the Carboniferous.

The secret of the success of the Hexapoda can be linked to the following features:
Tagmosis allowing specialisation in each area.
Jointed appendages and striated muscles giving efficient locomotion.
Exoskeleton which provides protection and prevents dehydration while still being flexible and light enough to allow movement. The hardening of the cuticle is a result of sclerotization, i.e. the bonding of protein molecules and their cross-links within and between the lammellae of the procuticle. This forms sclerotin, a resistant and insoluble protein.
Complex musculature allowing rapid movement.
Branched tracheae and tracheoles allowing direct passage of oxygen to the cells, so enabling a relatively high metabolic rate. The trachea end in spiracles which run down the sides of the abdomen and thorax.
Complex sensory organs of vision, chemoreception, hearing and touch; all enhancing awareness of surroundings.
Complex behaviour patterns leading to social organisation in some.
Metamorphisim reducing competition between individuals of different stages in the life cycle, e.g. caterpillar and butterfly occupy different niches.

Flight. Insects were the first animals to fly. They had functional wings over 100 million years before flying reptiles or birds. Insects that do not have two pairs of wings include those that are secondarily wingless; the Diptera (flies) which have one pair of wings and a pair of balancing organs (halteres), and the Strepsiptera which have a reduced forewing.

Some have wings that are modified such as the Coleoptera (beetles) where the first pair of wings forms a hard case (elytra) covering and protecting the second pair; and others may have wings for only a short time in their adult life, e.g. the sexual stages of ants and termites. There is no doubt that the evolution of wings greatly helped in the dispersal and radiation of insects. The wings are a double cuticular membrane containing veins which provide structural stability.

Generally the wings of species that evolved earlier e.g., dragonflies have more complex venation than those that evolved later e.g., wasps and bees. The drawing above right shows the very simplified venation of a parasitic wasp on the left, and the more complex venation of a dragonfly on the right. Wings can be naked or covered in scales (butterflies and moths) or hairs (caddis flies). The antennae (right) can vary greatly in size and shape (see below) even between sexes of the same species. They are separated into six Classes; five are wingless and one contains the insects with wings or those that are secondarily wingless, e.g., fleas.

There are a range of modifications to the Pterygote body parts that partly account for their great taxonomic and ecological diversity; leg modifications (see below and left) include:

1. The raptorial foreleg of preying mantises
2. The hind legs of grasshoppers, etc. that are modified for jumping .
3. The hind legs of social bees that have modifications for storing and packing pollen.
4. The paddle-shaped hind legs fringed with hair that are modified for swimming in aquatic beetles.

And the leg of the cat flea, Ctenocephalides felis, (not illustrated) enables it to jump 30 cm. This is because they have a hugely elastic material called resilin which is usually held compressed by the muscles in the thorax. When these muscles relax the resilin springs back to its original shape releasing a burst of energy which is transmitted to the hind legs.

The legs often have hairs and bristles which are sensitive to touch and air movement. Left is the typical insect leg. There are usually between 1 and 5 tarsal segments, usually ending in 2 claws. The number of segments is often important in identifying species. Tarsal hairs and pores are often sensitive to smell and taste. Crickets have their ears on their front legs at the top of each tibia.

Mouth. Although insect mouthparts vary enormously, they all fall into two main types; biting for solid food, and sucking for liquid food. At the sides of the mouth insects often have a pair of palps which help in detecting and tasting food. The type of mouthparts are a good guide to the kind of food the insect consumes, and include:
1. The piercing and sucking stylets of insects that must penetrate the tough external covering of skin or plant cells, e.g. mosquitoes and aphids.
2. The long proboscis required to reach the nectar of flowers with long corollas, e.g. in butterflies and moths.

3. The sponging or lapping mouthparts found in houseflies that are efficient at mopping up easily reached liquids.
4. The crushing, chewing mouthparts of predatory beetles.

There are also modifications for mating and ovipositioning (egg laying) that include:
· The "claspers" seen on the final segments of may male insects, e.g. locusts, which are used to hold the female while mating.
· Ovipositors allowing females to lay eggs in concealed sites and in hosts e.g. parasitic wasps; in social female bees and wasps this has been adapted to form a sting.
· Modifications to attract a mate, e.g. stridulatory mechanisms, pheromone release and antennal modifications, and bioluminescence.

more and also Insect Records and Flight Data >