Hexapoda: A Drosophila’s view of development

By any criterion, Insecta (as a subtaxon of Hexapoda) is the most successful taxon on the planet (see Chaps. 2 and 3 for the inclusion of the various subtaxa in Hexapoda versus Insecta). The number of described insect species nears one million, and they are found in almost every ecosystem, forming the major component of animal biomass in most. Within the insects, the largest group comprises the holometabolous insects. In the holometabolous insects, the outcome of embryogenesis is a larva, which normally has a relatively simple morphology, with no wings or reproductive organs and either no legs or rudimentary legs. The larva goes through several molts, pupates, and undergoes metamorphosis within the pupa, finally hatching from the pupa as an adult with limbs, wings, and reproductive organs. Most of the remaining species belong to the hemimetabolous insects, a paraphyletic group wherein the outcome of embryogenesis is a nymph, superficially similar to an adult, but with no wings or reproductive organs. The nymph undergoes several molts, each being slightly larger and closer in shape and size to the adult, with the final adult molt developing wings and reproductive organs, without going through a pupal stage or undergoing a dramatic metamorphosis. Finally, a small group of insects undergoes ametabolous development, wherein there is no terminal adult stage, and the animal continues to molt even after developing reproductive organs. These insects are all primitively wingless.