Phylogeny of the Holometabolous Insects: The Most Successful Group of Terrestrial Organisms

Abstract

The radiation and diversification of the holometabolous insects stand as two of the grandest events in all of evolutionary history, representing an unprecedented explosion in species coupled with extensive anatomical and physiological specialization. The defining characteristic for Holometabola is complete metamorphosis: every insect in this group, with rare exception, passes through an egg, larval, pupal, and adult stage. This is in contrast to the non-holometabolous insect groups in which juveniles have more or less the same form as the adult, live in the same environment, and exploit similar resources. Although it has never been thoroughly tested, it is thought that the evolution of complete metamorphosis was the key innovation allowing these insects to partition habitats between adults and juveniles, resulting in a wider range of niches that could be occupied by the nascent species. And occupied they have. Holometabola includes well more than one million species representing roughly 80% of all described insect species and just more than half of the total number of described species on Earth today (Kristensen 1999, Wilson 1988). The immense size of this group and their unique morphological specializations present a serious challenge to phylogenetic systematics.