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.

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 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).

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.

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.

The antennae (above) can vary greatly in size and shape even between sexes of the same species. Insects detect smells mainly using their antennae. The antennae are also used to touch, taste and in some cases, to detect sound.

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.