Each bears a pair of legs, and, in flying insects, the second and third of these segments also each bear a pair of wings. The wings are composed of an upper and a lower membrane, and between these two layers they are strengthened by a framework of chitinous tubes known as veins. The abdomen is the metabolic and reproductive centre, where digestion, excretion, and the sexual functions take place. In the female, there is very commonly an egg-laying instrument, or ovipositor, and many insects have a pair of tail feelers, or cerci. Most insects breathe by means of fine airtubes called tracheae, which open to the exterior by a pair of breathing pores, or spiracles.
Reproduction is by diverse means.
In most insects, mating occurs once only, and death soon follows. Growth and metamorphosis When ready to hatch from the egg, the young insect forces its way through the chorion, or eggshell, and growth takes place in cycles that are interrupted by successive moults. After moulting, the new cuticle is soft and pliable, and is able to adapt itself to increase in size and change of form. Most of the lower orders of insects pass through a direct or incomplete metamorphosis. The young closely resemble the parents and are known as nymphs.