Endocrine control of metamorphosis explains the origin of the insect pupa
An invited review in Phil Trans R Soc B considers endocrine evidence in support of homology between juvenile stages of insects with complete and incomplete metamorphosis.
Image description: Alternative hypotheses for evolutionary relationships between holometabolous stages and those of a hemimetabolan ancestor (middle). Left: Holometabolous larvae arise by 'de-embryonization' (yellow) while hemimetabolous juvenile development is compressed to the pupa (orange). Right: Hemi- and holometabolous larvae hatch at an equal stage and the pupa is a modified late-stage hemimetabolous larva (green). Drawings: Martina Hajduskova (www.biographix.cz).
The 'miracle' of transforming caterpillars to beautiful butterflies has taunted thinkers since Aristotle. Particularly enigmatic is the origin of the pupa, a transitory stage between the caterpillar and the adult butterfly, which characterizes insects developing through 'complete metamorphosis' (holometaboly). The debate is on to this day. A theme issue of Philosophical Transactions of the Royal Society B published on August 26 (2019) confronts two historical concepts on the origin of complete metamorphosis. One hypothesis sees holometabolous insect larvae as early-hatching embryos whose discontinued development resumes at the pupa stage, which is said to comprise all juvenile stages of an 'incompletely' metamorphosing (hemimetabolous) ancestor. A competing view builds on a fact that insect larvae of both types leave their eggs at the same embryonic stage, and considers the pupa homologous to a late hemimetabolous larva from which it evolved by gradual modification. Our article advocates the latter hypothesis primarily based on recent discoveries of the hormonal control of insect development (https://royalsocietypublishing.org/doi/10.1098/rstb.2019.0064), which were in part achieved in our laboratory.
Jindra, M. (2019) Where did the pupa come from? The timing of juvenile hormone signalling supports homology between stages of hemimetabolous and holometabolous insects. Philosophical Transactions of the Royal Society B 374: 20190064.