Primary setae and pores on the legs, the last abdominal segment, and the urogomphi of larvae of Nearctic Colymbetinae (Coleoptera: Adephaga: Dytiscidae) with an analysis of their phylogenetic relationships

Through an analysis of the primary setae and pores (campaniform sensilla) of first-instar larvae of 32 species of Colymbetinae, 33 species belonging to other dytiscid subfamilies, and seven species belonging to other families of Hydradephaga, the ancestral system of primary setae and pores of the legs, the last abdominal segment, and the urogomphi of Colymbetinae is deduced. Seventy-four setae and 24 pores have been assigned to the generalized colymbetine larva: 51 setae and 18 pores on the legs, 15 setae and three pores on the last abdominal segment, and eight setae and three pores on each urogomphus. A comparison with the ground-plan pattern of the subfamily Hydroporinae is provided. Members of Colymbetinae differ from those of Hydroporinae by the presence of setae TR3, AB14, and AB15, the absence of pore ABd, and the presence of a variable number of additional setae on the femur. A tentative phylogenetic reconstruction of the colymbetine genera provided is based on 24 larval characteristics. The monophyletic origin of the subfamily Colymbetinae is confirmed based on the shared presence of seta AB14. Two monophyletic lineages unite basally: (1) [(Agabus Leach + Ilybius Erichson) + Agabinus Crotch], and (2) {[Colymbetes Clairville + (Rhantus Dejean + Neoscutopterus Balfour-Browne)] + Matus Aube}. The genera Agabinus and Matus are postulated as a sister-group of (Agabus + Ilybius) and [(Colymbetes + (Rhantus + Neoscutopterus))], respectively. Compared with other Colymbetinae, both Neoscutopterus and Matus stand out by the presence of several strikingly peculiar features. The subgenus Rhantus (Nartus) is postulated to be the sister-group of Neoscutopterus based on the shared presence of additional setae on the dorsal margin of the femur.