Convergent evolution of hindwing flight in beetles and cockroaches

Polyneopteran insects fly only with their hindwings. Their thickened forewings protect the hind-wing—which is particularly important to clades that spend time in narrow spaces. Yet, this constrains the size of the hindwing, to be able to fit safely underneath the forewing. Numerous clades have solved this by evolving different folding mechanisms in the hind wing. In particular, cockroaches are unique in Polyneoptera in having an apically folding field. Outside of Polyneoptera, beetles (Coleoptera) have independently evolved a similar mechanism. In both groups, the surface area of the hindwing can be greatly expanded depending on the size of this field. To receive the maximum benefit from the wing to body size ratio, we hypothesized that the apical field size is positively correlated with body size.  We tested this hypothesis using cockroaches and beetles, two evolutionarily independent datasets. We took wing and body measurements from specimens representing more than 50 beetle families and cockroach genera, calculated the phylogenetic independent contrast, and analyzed them with a multiple linear regression.  We found that the apical field composes less of the wing as the hindwing size increases in both taxa. Ancestral state reconstruction indicates that the folded apical field evolved nine times in cockroaches but only once in the common ancestor of all beetles. Major questions remain about the morphological correlates of apical field size, and its functional constraints.