1920] Richmond, Studies on the Biology of the Aquatic Hydrophilidoe 85 



Ocular areas in groups of six, aggregated; epicranial suture entirely aosent, 

 each mandible with a terminal seta, inner tooth and lacinia mobilis; palpifer 

 with the inner rudimentary lobe not claw-like; abdominal segments with 

 well-developed chitinous plates Hydrochous. 



8. Type pseudo-apneustic. Seven pairs of very prominent tracheal gills present 



on the abdomen. Ninth and tenth abdominal seghients very much reduced 



and no stigmatic atrium present Berosus. 



Type pseudo-metapneustic. Tracheal gills not nearly as prominent or absent. 

 Ninth and tenth abdominal segments reduced, forming a stigmatic atrium . . 9. 



9. Ocular areas round, usually small and more closely aggregated; legs reduced or 



absent; pleural lobes not prominent ; abdomen truncate 19. 



Ocular areas oval, larger, aggregated but more distant; legs well developed, 

 visible from above except in Paracymus; pleural lobes usually prominent; 

 abdomen not noticeably truncate, more or less elongate 10. 



10. First segment of antennae not distinctly longer than the following two taken 



together; finger-like antennal appendage present; labro-clypeus with teeth 

 usually well defined; mouth-parts stouter; mandibles not grooved internally; 

 stipes large and swollen, usually with an inner row of five stout setse; externo- 

 frontal angles of mentum not prominent, rounded; legs much shorter, femora 



without fringes of swimming hairs; gills and prostyles absent 11. 



First segment of antennae distinctly longer than the following two taken to- 

 gether; finger-like antennal appendage absent ; labro-clypeus with teeth small 

 or absent; mouth-parts more slender; mandibles grooved internally; stipes 

 not swollen, with seta? arranged differently; externo-frontal angles of mentum 

 prominent and acute; legs very long; femora with fringes of long swimming 

 hairs; gills present (Hydrophilus, Tropisternus) or absent (Hydrous); pro- 

 styles present 17. 



11. Frontal sutures parallel and not uniting to form an epicranial suture; left ex- 



pansion of epistoma much more prominent than the right and with a row of 

 stout seta?; ligula absent; reduced sclerites of meso- and metathorax widely 



separated; tarsus well developed, about as long as tibia Laccobius. 



Frontal sutures not parallel and may or may not unite to form an epicranial 

 suture; lateral expansions of epistoma similar and usually in line with the 

 anterior margin of the labro-clypeus, no rows of stout setse except in Hydro- 

 bius; ligula present and longer than the first segment of the palpus ; sclerites 

 of meso- and metathorax reduced but not so widely separated; tarsus less 

 developed usually much shorter than the tibia 12. 



12. Antenna? shorter and antennal appendage more prominent, especially in the 



first stage larva; epicranial suture absent; legs reduced; abdomen more 



truncate; cercus with long terminal seta 13. 



Antennae longer and antennal appendage less prominent; epicranial suture 

 present but usually short; legs fairly long, not reduced; abdomen narrowed 

 caudally; cercus with a shorter terminal seta 14. 



