Larval character evolution in Holometabola (Insecta)

Larvae, a novel type of immature stage, are a crucial innovation of the Holometabola. Larval features therefore provide key evidence for reconstructing evolutionary transformations in this megadiverse group. Here we present the first formal reconstruction of larval character evolution, using maximum parsimony and Bayesian ancestral-state reconstruction. Our analyses are based on an updated matrix of 88 larval characters and a phylogenetic topology assembled from published transcriptomic and phylogenomic studies. A series of key larval transformations is associated with the origin of Holometabola. That includes partial reductions of antennal and labial structures, simplification of the tarsus and developmental shifts in the appearance of compound eyes and wing buds. The reconstructed holometabolan groundplan larva was terrestrial and non-predaceous, with an orthognathous head, well-developed biting mouthparts, well-developed thoracic legs with an undivided tarsus, no abdominal appendages and a relatively slender postcephalic body. Within Holometabola, larval character support for major clades was highly uneven. Some lineages, especially Neuroptera, Strepsiptera, Vespina, Glossata and Brachycera, showed dense clusters of inferred transformations and distinctive larval apomorphies. In contrast, several major clades, including Coleoptera, Lepidoptera, Mecopterida and Antliophora, were weakly supported or unsupported by larval synapomorphies, despite strong support from adult morphology in some cases. Major shifts in larval morphology were associated with ecological and functional transitions, including predation, parasitism, parasitoidism, phytophagy, aquatic habits, wood boring and development in moist substrates. Our results show a distinctive mosaic pattern in larval evolution in Holometabola, with major transformations largely restricted to some of the phylogenetic lineages and closely associated with key ecological transitions.