1) Exoskeleton

The exoskeleton or body wall or integument is made up of chitin. The advantages of exoskeleton are many, viz., it provides a large area for attachment of muscles, provides complete protection to the vital organs from external injury, controls evaporation and so affords an excellent mechanism to protect against desiccation. Further, the exoskeleton gives the insects much physical strength without their being too heavy and also allows enormous modifications.

2) Small Size

Small size of insects favours subsistence and shelter. The chances of survival of the species are increased, it is easier for the individuals to subsist on small quantities of food and to take refuge in small niches against the vagaries of nature. Both these advantages favour increase in the number of individuals in the insect populations. With larger numbers resulting from smaller size the chances of variability and mutation also increase, thus making it possible for nature to conduct a larger number of trials in evolution. Lastly, the smaller size leads to greater proportionate efficiency.

3) Functional Wings

The power of flight greatly increased the statistical chances of survival and dispersal. Functional wings increased the feeding and breeding range, and provided a new ineans of eluding enemies, attacking a fast moving host, and finding a mate.

4) Hexapod (hexa-six, podos-leg) Locomotion

In insects the number of legs has reached an ideal stage of evolution. The large number of legs in millipedes makes locomotion