| Summary: | The larva of the whirligig beetle Enhydrus sulcatus (Wiedemann, 1821) is described and illustrated for the first time, including detailed morphometric and chaetotaxic analyses of the cephalic capsule, head appendages and legs. Larvae of Enhydrus Laporte, 1834 exhibit the characters traditionally recognized as autapomorphies of the family Gyrinidae: well developed cardo, completely divided prementum, presence of lateral abdominal tracheal gills, and presence of four terminal hooks on the pygopod. The egg bursters located on the parietal, the presence of an additional sensorial plate on the third antennomere, and a well developed lacinia may also represent autapomorphies of the family. Enhydrus larvae share with those of the other known Dineutini genera the presence of numerous minute additional setae on the mandible, the presence of additional setae on the cardo, the submedial position of the coxal seta CO12, the absence of the trochanteral seta TR2, and the presence of numerous pore-like additional structures on the ultimate maxillary and labial palpomeres. On the other hand, Enhydrus can be distinguished from the other known dineutine genera by the presence of pore-like additional structures on the basal maxillary and labial palpomeres, the presence of ventral spinulae on the pygopod, and the bare tracheal gills, among other characters. Keywords: Adephaga, Dineutini, Morphometry, Neotropics, Sensilla
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