The insects are an important group, both as parasites and as vectors or intermediate hosts of parasites.
Parasitic insects have life cycles that are in most ways similar to those of free-living insects. They are dioecious, and the female produces eggs. In some groups, the egg hatches into a nymph which grows and completes its differentiation in a series of nymphal molts. In other groups, a larva emerges from the egg, which grows and them transforms into a pupa in which metamorphosis to the adult stage occurs. Insects parasitize terrestrial vertebrates. Some complete their entire life cycles on one host, whereas others leave the host for certain phases of the life cycle.
Insects are covered with a chitinous exoskeleton and have a segmented body typically divided into head (H), thorax (T) and abdomen (A). The head contains the mouthparts (M), eyes (E) and antennae (A). The thorax is the point of attachment for the three pairs of segmented legs (L). The abdomen has multiple segments and houses the majority of the reproductive (G), excretory and digestive systems.