Larval morphology of Platambus Thomson, 1859: evidence of contrasting morphotypes and phylogenetic considerations (Coleoptera: Dytiscidae, Agabinae).

All larval instars of Platambus apache Young, 1981 and P. maculatus (Linnaeus, 1758), and the third instar larva of P. semivittatus (LeConte, 1852) are described, illustrated in detail and compared to larvae of other known Platambus Thomson, 1859 species. The descriptions use the now generalized larval descriptive format of aquatic Adephaga, which includes chaetotaxic characters. Based on larval characters, a cladistic analysis is conducted using the program TNT to test the monophyly of Platambus and its relationship with five other agabine genera (Agabus Leach, 1817, Agabinus Crotch, 1873, Hydrotrupes Sharp, 1882, Ilybiosoma Crotch, 1873, Platynectes Régimbart, 1879). The monophyly of Platambus as currently defined is not corroborated in our analysis, as the relationships with some species of Agabus are inconclusive. There is a basal split between Palearctic and Nearctic species of Platambus, whereas all five Palaearctic species studied are recovered as monophyletic with very strong support, the Nearctic P. semivittatus and P. apache are deemed to potentially represent distinct evolutionary lineages as reflected by their contrasting larval morphology.