The surface-skimming hypothesis for the evolution of insect flight

The surface-skimming hypothesis for the evolution of insect flight poses that insects first used wings and aerodynamic locomotion to move in two dimensions across water surfaces. Here I present an overview of recent advances in our understanding of surface-skimming locomotion, and how these findings relate to phylogenetic origins of insects and developmental and anatomic origins of insect wings. Behavioral surveys show taxonomically widespread use of skimming by Plecoptera and more taxonomically restricted use of skimming by Ephemeroptera. Because these two orders arose near the root of the early split of pterygote insects into the Paleoptera and Neoptera, traits that appear in both groups are strong candidates for traits possessed by the first winged insects. Comparisons across plecopteran species show that skimming speed increases as contact with the water surface decreases, thereby providing a mechanical pathway over which directional selection may have acted to improve aerodynamic capability in early skimmers. Evolution along this route may have occurred within species in response to factors such as scramble competition and sexual selection. Phylogenetic analyses suggest that the common ancestor of modern Plecoptera was capable of both skimming and flying; such dual ability is widespread among extant stoneflies, including the most basal families. Both the mechanics and the semi-aquatic setting of skimming fit well with the growing understanding that insects and crustaceans are sister clades and that insect wings evolved from gills.