Insect metamorphosis boasts spectacular cases of postembryonic development when juveniles
undergo massive morphogenesis before attaining the adult form and function; in moths
or flies the larvae do not even remotely resemble their adult parents. A selective
advantage of complete metamorphosis (holometaboly) is that within one species the
two forms with different lifestyles can exploit diverse habitats. It was the environmental
adaptation and specialization of larvae, primarily the delay and internalization of
wing development, that eventually required an intermediate stage that we call a pupa.
It is a long-held and parsimonious hypothesis that the holometabolous pupa evolved
through modification of a final juvenile stage of an ancestor developing through incomplete
metamorphosis (hemimetaboly). Alternative hypotheses see the pupa as an equivalent
of all hemimetabolous moulting cycles (instars) collapsed into one, and consider any
preceding holometabolous larval instars free-living embryos stalled in development.
Discoveries on juvenile hormone signalling that controls metamorphosis grant new support
to the former hypothesis deriving the pupa from a final pre-adult stage. The timing
of expression of genes that repress and promote adult development downstream of hormonal
signals supports homology between postembryonic stages of hemimetabolous and holometabolous
insects.This article is part of the theme issue 'The evolution of complete metamorphosis'.