Phylogeny of the water strider genus Rheumatobates (Heteroptera : Gerridae)

The genus Rheumatobates comprises thirty-seven species and subspecies of Ne w World water striders belonging to subfamily Rhagadotarsinae. Among specie s, males vary dramatically in the degree and nature of modifications of the antennae, three pairs of legs and abdominal and genital segments. Characte rs describing this modification have traditionally been used to differentia te and group species. The general assumption has been that modified species belong to one group and unmodified species to another. These two 'species groups' are subdivided into 'subgroups', but little effort has been made to resolve relationships among them. We conduct the first numerical cladistic analysis of Rheumatobates using a data set comprised of 102 characters, pr imarily describing modification of male external morphology. To address con cerns about the inclusion of characters to be optimized on the phylogeny, c haracters describing modification of antennae and hind legs were included a nd then excluded in separate analyses. A preferred phylogeny was chosen fro m the four equally parsimonious cladograms found after successive reweighti ng of characters. There was good resolution at all levels of the phylogeny. Most of the major clades and terminal relationships were moderately to str ongly supported, whereas the basal relationships were less well supported. The general assumption that unmodified and modified species form two monoph yletic groups was not supported. However, traditionally recognized 'subgrou ps' within the modified species group were largely upheld. The analysis als o suggested several major clades and relationships among these clades that were not previously recognized. The exclusion of characters describing modi fication of antennae and hind legs did not change the resolved major clades of the reconstructed phylogeny.