Couplet 41

41a:  Elytra completely covering abdomen; antenna short, not reaching middle of pronotum, with 9 antennomeres, club composed of 5 pubescent antennomeres; ventral surface with very short, dense pubescence (Ochithebiinae).

Hydraenidae  p. 408

41b:  One or more abdominal terga usually exposed beyond elytra; antenna short to long, reaching beyond middle of pronotum in those species with long elytra; antennal club, if present, not involving 5 antennomeres; underside of body without short, dense pubescence (Omaliinae).