The present study provides new data on the morphology and distribution of the labial tip sensilla of 41 species of 20 gerromorphan (sub) families (Heteroptera: Gerromorpha) obtained using a scanning electron microscope. There are eleven morphologically distinct types of sensilla on the tip of the labium: four types of basiconic uniporous sensilla, two types of plate sensilla, one type of peg uniporous sensilla, peg-in-pit sensilla, dome-shaped sensilla, placoid multiporous sensilla and elongated placoid multiporous subapical sensilla. Based on their external structure, it is likely that these sensilla are thermo-hygrosensitive, chemosensitive and mechano-chemosensitive. There are three different designs of sensilla in the Gerromorpha: the basic design occurs in Mesoveliidae and Hebridae; the intermediate one is typical of Hydrometridae and Hermatobatidae, and the most specialized design in Macroveliidae, Veliidae and Gerridae. No new synapomorphies for Gerromorpha were identified in terms of the labial tip sensilla, multi-peg structures and shape of the labial tip, but eleven new diagnostic characters are recorded for clades currently recognized in this infraorder. One synapomorphy is recorded for Hydrometridae + Hermatobatidae + Macroveliidae + Veliidae + Gerridae; five for Macroveliidae + Veliidae + Gerridae; one for Veliidae + Gerridae; and one for Gerridae alone. Within Gerridae, one possible autapomorphy is recorded for Trepobatinae, and two possible autapomorphies for Rhagadotarsinae. We also record a possible autapomorphy for Cylindrostethinae, which indicates that this taxon is monophyletic, as is indicated by other morphological characters. Finally, we record one putative diagnostic character for Macroveliidae + Veliidae, two families that are otherwise not considered sister groups. This new set of characters based on features of the mouthparts of gerromorphan bugs might provide new insights into the cladogenesis of the infraorder.
