Leg Proportionality and a Possible Synapomorphy in Superorder Neuropterida with Discussion on Ambulation, Extreme Volantry, and Aerial Predation

Abstract

Clades in which adult insects have substantially diminished their ability to ambulate and instead move almost exclusively via flight are rare in nature. This phenomenon, which we call “extreme volantry,” is explored within the insect superorder Neuropterida, in which neuropterids have two modes of locomotion (ambulation and flight), but some neuropterids have lost ambulation and use flight alone. There is a transition from mixed use locomotory mode to a flight-dominated locomotory mode in Nemopteridae + Myrmeleontidae, and there is a second transition within Myrmeleontidae where extreme volantry (incapacity to walk and moving by flight) is coupled with aerial predation in the owlflies. Considering the evolutionary arc among these locomotory functional groups, we ask what the fate of legs is once the need for ambulation is removed. We measured the lengths of individual leg podites of 65 species of Neuropterida, and normalized them to compare proportions across the superorder. We used PERMANOVA and PERMDISP to test for differences. Impacts of phylogeny on variables was analyzed using phylogenetic ANOVA. Taxonomic groups were found to be significantly different when grouped functional group. Neuropterids with different locomotory modes exhibited notable differences in wing length, tarsomere 5, and pretarsal claw proportions. A novel joint type, the tricondylic joint complex (TJC) found in lacewings and is described and identified as a likely adult synapomorphy of Neuropterida. The TJC is a modification of the pleisiomorphic dicondylic hind leg metatrochanter/metafemur joint where the articulation has been modified to incorporate an additional sclerite that bridges the metafemur and metatrochanter while articulating with both. This sclerite, dubbed the tricondylic bar, articulates within a V-shaped notch, the “tricondylic notch.” 529 neuropterid species, representing all Neuropterida families, and 47 outgroup taxa from close holometabola lineages, were examined for the presence of the TJC. Found only in Neuropterida, the TJC was absent in Megaloptera and Myrmeleontidae. Two families, Coniopterygidae and Nymphidae, exhibited atypical TJCs. Tricondylic joint complexes of selected taxa were measured to examine variation within the trait and tested using PERMDISP and PERMANOVA. We present post hoc hypotheses for the origin, function, movement, and taxonomic/phylogenetic distribution of this novel joint.