WATER BEETLES IN RELATION TO PONDFISH CULTURE. 



269 



11. Prothorax two or three times as long as wide; cerci short and naked; labium with a large ligula; 



stipes of maxilla as long as the palp 12 



11. Prothorax a little longer than wide; cerci half the length of the abdomen and densely plumose; 



labium without a ligula Laccophilus. 



11. Prothorax considerably wider than long ; cerci long and plumose; labium without a ligula; maxillary 



palp much longer than stipes 13 



12. Ligula tipped with two long, jointed spines Thermonectes. 



12. Ligula without terminal spines Acilius. 



13. Larger, 15 to 20 mm.; cerci densely plumose 14 



13. Smaller, 8 to 12 mm.; cerci with scattered hairs only 15 



14. Maxillary stipes slender, with concave sides, armed on the inner margin with a long seta and a 



single spine Colymbetes. 



14. Maxillary stipes stout, with convex sides, armed on the inner margin with two or three spines 



and a row of setae Rhantus. 



15. Maxillary stipes armed on the inner margin with two long curved claws and many small spines and 



setae Ilybius. 



15. Maxillary stipes armed on the inner margin with small spines and setae only; no curved claws. 



Agabus. 



16. Body segments with filiform tracheal appendages as long as the body itself; tip of abdomen 



bluntly rounded Peltodytes. 



16. Body segments with short triangular appendages; tip of abdomen acuminate, ending in two 



filiform, jointed setae Haliplus. 



17. Epicranial suture wholly lacking; each mandible with a terminal seta, a simple lobe or lacinia 



mobilis, and an inner tooth Hydrochous. 



17. Epicranial suture present; mandibles without a terminal seta or a lacinia mobilis, but with one or 



more inner teeth 18 



18. Abdomen with seven pairs of long lateral filaments, which serve as tracheal gills; mandibles 



with two or three teeth 19 



18 . Tracheal gills reduced to mere tubercles or wanting ; mandibles with only one or two teeth 20 



19. Second joint of antenna with finger appendage outside of the terminal joints; no stigmatic atrium at 



tip of abdomen Berosus. 



19. No finger appendage on antenna; ninth and tenth abdominal segments forming a stigmatic 



atrium H ydrophilus . 



20. Second joint of antenna with finger appendage outside of the terminal joint; mandibles not 



grooved internally ; femora without a fringe of swimming hairs 21 



20. No finger appendage on the antenna; mandibles definitely grooved internally; femora with a 



fringe of long swimming hairs 22 



21 . Labium without a ligula ; tarsus as long as the tibia Laccobius . 



21. Labium with a ligula, which is longer than the basal joints of the palpi; tarsus much shorter than the 



tibia Philydrus. 



22 . Mandibles each with a single inner tooth ; ligula not as long as the basal joints of the palps. 



Hydrous. 



22. Mandibles each with two or more inner teeth; ligula distinctly longer than the basal joints of 

 the palps Tropisternus. 



KEY FOR IDENTIFICATION OF PVPJE. 



1. Tibiae of first two pairs of legs folded tightly against the femora and the two inclined forward at an 

 angle of 45° with the body axis; knees projecting strongly and armed with spines; antennae folded 

 across the eyes; no posterior cerci HALIPLIDiE 2 



1. Femora at right angles to the body axis, tibiae not folded against them; knees neither projecting nor 

 armed with spines; antennae not crossing the eyes; a pair of stout posterior cerci, jointed or bifid; 

 styli long and jointed, with an enlarged basal portion HYDROPHILID^ 3 



1. Femora at right angles to body axis, tibiae not folded against them; first pair of knees projecting but 

 not armed with spines; each compound eye divided into a dorsal and ventral half, separated some- 

 what; noposterior^erci; etyli in the form of short bristles, not jointed GYRINID^ 8 



