86 Bulletin American Museum of Natural History [Vol. XLII 



13. Frons truncate behind; labrum tridentate, the lateral teeth bifid; mandibles 



symmetrical, each with two inner teeth; palpiger enlarged in front; ligula 

 about as long as palpi, apparently two-segmented; anterior margin of pro- 

 notum without a fringe of stout setse; legs not visible from above . . Paracymus. 

 Frons rounded behind; labrum quadridentate ; mandibles symmetrical, each 

 with three inner teeth; palpiger not enlarged in front; ligula not as long as 

 the palpi, one-segmented; anterior margin of pronotum with a fringe of stout 

 setae; legs barely visible from above Ancama. 1 



14. Mandibles symmetrical, each with two or three inner teeth; abdomen without 



prolegs 15. 



Mandibles asymmetrical, the right with two inner teeth, the left with only 

 one; abdomen with prolegs on the third to seventh segments. . . .Philydrus. 



15. Labro-clypeus with five distinct teeth, the outer left tooth a little distant from 



the rest; each mandible with three inner teeth; mentum sub quadrangular. 



Hydrobi us. 



Labro-clypeus with at least six teeth; each mandible with two inner teeth; 



mentum cordiform 16. 



16. Labro-clypeus with six distinct teeth, placed in two groups, two on the left and 



four on the right, mentum covered with small spines; anterior sclerites of the 



metathorax with caudal projections Helochares. 



Labro-clypeus with more than six teeth, those towards the right not clearly 

 defined and with several smaller teeth; mentum with small spines only towards 

 the base; anterior sclerites of the metathorax without caudal projections, 

 rectangular Cymbiodyta. 



17. Head subspherical ; labro-clypeus without teeth; each mandible with a single 



inner tooth; ligula not longer than first palpal segment; gills absent; pro- 

 notum not entirely chitinized Hydrous. 



Head subquadrangular, narrowed behind; labro-clypeus with inconspicuous 

 teeth; each mandible with more than one, usually with two inner teeth; 

 ligula distinctly longer than first palpal segment; pronotum entirely chitin- 

 ized, gills present but more or less rudimentary 18. 



18. Mentum transverse; sides only slightly rounded; fronto-external angles very 



prominent; pleural gills rudimentary but indicated by tubercular projections, 



each with several terminal setse Tropisternus . 



Mentum, convex, with its sides strongly rounded towards its basal half, its 

 sides serrate in the last instar ; fronto-external angles less prominent ; pleural 

 gills fairly well developed and pubescent Hydrophilus. 2 



19. Head ovate or subspherical; antennal appendage shorter; mandibles asym- 



metrical; stipes broadened and depressed on its outer side; ligula 3 exceeding 

 the palpi and pointed; median line on ventral side of the head impressed and 

 with a small pit mid-way; legs incomplete; abdomen without ventral suckers. 



20. 



^The characters found in Ancwna infuscata are incorporated here. It seems possible that' Anca>na 

 limbata, the larva of which d'Orchymont, 1913, described, may really belong to the genus Paracymus, 

 as its characters more closely resemble the latter genus. 



2 D'Orchymont says in his key that the tarsus of Hydrophilus is toothed on the inner side, but only 

 the two inner seta; are found in H. obtusatus. 



3 The specimens at hand do not show the ligula formed as described here, but more larvae will havr> 

 to be examined before Schiddte's figure is discredited. 



