DESCRIPTION OF LARVAE OF 17 NEARCTIC SPECIES OFHYDROPORUSCLAIRVILLE (COLEOPTERA: DYTISCIDAE: HYDROPORINAE) WITH AN ANALYSIS OF THEIR PHYLOGENETIC RELATIONSHIPS

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Abstract
The larvae of 17 species ofHydroporusClairville were described and characterized; a key for their discrimination and illustrations of structural features of first- and third-instar larvae are provided. Many structural features, especially those of chaetotaxy and porotaxy of head capsule, head appendages, legs, last abdominal segment, and urogomphi, were found to be useful for taxonomic and phylogenetic comparisons. Based on 21 larval characters, the phylogenetic relationships among the five main lineages ofHydroporus(s. lat.) occurring in North America were studied. Structurally, the species of the subgenusHydroporus(s. str.) and of thevilisspecies-group (H.planiusculusFall) were found to be the most generalized taxa whereas the subgeneraNeoporusGuignot andHeterosternutaStrand encompassed the species with the most derived condition. The subgenusHeterosternutawas closely related to the subgenusNeoporusand both subgenera were hypothesized to form a distinct monophyletic unit on the basis of five synapomorphies. The only representative of theoblitusspecies-group studied,H.paugusFall, was related toHydroporus(s. str.) and thevilisspecies-group by the shape of the siphon. This species was a very peculiar clement withinHydroporus(s. lat.) because it was the only known species of the genus without stemmaia in the first- and second-instar larvae, with stemmata greatly reduced in size in the third-instar larvae, and with a secondary dorsomedian seta on antennomere 2. The larval features ofHydroporus(s. lat.) were compared with those ofHygrotusStephens. The comparison suggested that both genera form a monophyletic unit, that each of them is a distinct monophyletic unit, and thatHydroporus(s. lat.) is the sister-group ofHygrotusStephens.