Insects exhibit the characters of Hexapoda, particularly a single pair of antennae, a tracheal system for gaseous exchange, and body segments grouped into head, thorax and abdomen. They are distinguished from other hexapods in having the mouthparts exposed (ectognathous), and larval insects retain the same number of body segments throughout their development (epimorphic), unlike the Protura.
Size and appearance varies in the insects (they may be 0.2 - 300 mm long). Eyes may be compound or simple, and present or absent; the number of simple eyes present may vary between taxa. The thorax may bear 0-2 pairs of wings; it consists of three segments, each bearing one pair of legs. Each leg has more than five segments unless secondarily reduced. Gas exchange is via pleural spiracles (pores) on the thorax and abdomen, but these may be variably reduced or absent (especially in immature stages). Water loss is minimized by the presence of a waxy cuticle and excretion of solid nitrogenous waste (uric acid). Efficient energy storage and visual and nervous systems make flight and navigation possible. Insect morphology and biology are discussed in greater detail in the first three chapters of Naumann, 1991, and by Gullan and Cranston, 1999.
Larval morphology and life-style is often different from that of the adult, so metamorphosis during development is common. Behaviour may be very complex, e.g. in social insects such as termites, bees and ants.