The third instars of Coelhydrus brevicollis Sharp and Darwinhydrus solidus Sharp are described and illustrated for the first time, with detailed morphometric and chaetotaxic analyses of the cephalic capsule, head appendages, legs, last abdominal segment, and urogomphi. A key for the identification of the third instars of the genera of Hyphydrini known in detail is provided. A parsimony analysis based on 49 informative larval characteristics was conducted using the program TNT. The consensus tree supports a placement of Coelhydrus Sharp and Darwinhydrus Sharp within the tribe Hyphydrini. Within the Hyphydrini, Coelhydrus is postulated to share a monophyletic origin with Hyphydrus Illiger and Desmopachria Babington. The phylogenetic placement of Darwinhydrus, however, remains contentious as our study found it part of an unresolved polytomy with Andex Sharp and Primospes Sharp.