Molecular phylogenetic analysis of evolutionary trends in stonefly wing structure and locomotor behavior

Insects in the order Plecoptera (stoneflies) use a form of two-dimensional aerodynamic locomotion called surface skimming to move across water surface s. Because their weight is supported by water, skimmers can achieve effecti ve aerodynamic locomotion even with small wings and weak flight muscles. Th ese mechanical features stimulated the hypothesis that surface skimming may have been an intermediate stage in the evolution of insect flight, which h as perhaps been retained in certain modern stoneflies, Here we present a ph ylogeny of Plecoptera based on nucleotide sequence data from the small subu nit rRNA (18S) gene. By mapping locomotor behavior and wing structural data onto the phylogeny, we distinguish between the competing hypotheses that s kimming is a retained ancestral trait or, alternatively, a relatively recen t loss of flight. Our results show that basal stoneflies are surface skimme rs, and that various forms of surface skimming are distributed widely acros s the plecopteran phylogeny. Stonefly wings show evolutionary trends in the number of cross veins and the thickness of the cuticle of the longitudinal veins that are consistent with elaboration and diversification of flight-r elated traits, These data support the hypothesis that the first stoneflies were surface skimmers, and that wing structures important for aerial flight have become elaborated and more diverse during the radiation of modern sto neflies.