Larvae of the genus Megalopinus, of the monobasic subfamily Megalopsidiinae, are described for the first time. Larvae can be distinguished from all other known staphylinid larvae by the presence of a mandible with a subapical bladelike process, and are differentiated in detail from other larvae of the staphylinine group of subfamilies in a key. To confirm the identity of Megalopinus larvae, which have not been reared to adults, we compared partitions of 28 adult and 20 larval morphological characters derived from members of the stenine group (Megalopsidiinae, Euaesthetinae, Steninae) and three staphylinid outgroups (Oxyporinae, Pseudopsinae and Piestinae). Partitioned cladograms were similar with the only difference being the placement of Pseudopsinae as sister taxon to the stenine group (adult characters) or included within it (larval characters). Characters were combined and one tree was produced with the following relationships: Megalopsidiinae (Pseudopsinae (Steninae, Euaesthetinae)).

Megalopinus is a specialized genus occurring mainly under rotting logs where fungal growth occurs, a habit for which the new term hyphyledic is proposed. Adults digest their food preorally, using a rotary-mill method for extracting liquefied tissues from masticated prey items. A deeply bifurcate labrum bearing modified setae functions as a sieve, while hyaline processes on the labium may be used for tearing captured prey by Megalopinus adults. Implications for the evolution of the adult Stenus “stick-capture” method for prey capture, which involves extruding a rodlike labium that exudes sticky substances at the tip, are discussed based on new phylogenetic information.