268 
BULLETIN OF THE BUREAU OF FISHERIES. 
“water tigers” indiscriminately; but it is not a difficult task to learn the three 
generic names, and there is no need of distinguishing species among the larvae any 
more than among the adults. Use the photographs and figures of larval heads, 
therefore, for identification and the text for habits and food. 
The descriptions of the larvae and pupae have been made as scientifically accurate 
as possible, in order that they may serve as a contribution to economic entomology. 
As was stated in the introduction, our knowledge of the life histories of the North 
American water beetles is still deplorably limited. It is hoped that the present 
paper will contribute a little to that desirable knowledge. 
The keys that follow include the larvae and pupae of those genera found in the 
Fairport ponds. Richmond (1920, p. 83) gave more detailed keys of the larvae and 
pupae of the Hydrophilidae and also one of the egg cases. His keys contained 22 
genera of the single family included, and may be used to supplement those here 
given. 
KEY FOR IDENTIFICATION OF LARWE. 
1. Tarsi with two claws, mandibles suctorial 2 
1. Tarsi with only a single claw, mandibles manducatory 3 
2. Abdomen with slender lateral filaments, which serve as tracheal gills, its apex armed with four 
prehensile hooks GYRINIDdS 4 
2. Abdomen with neither filaments (except Coptotomus) nor hooks; larvae elongate, slender and 
agile DYTISCIDiE 5 
3. Eyes in groups of five; larvae vegetable eaters HALIPLIDjE 16 
3. Eyes in groups of six; larvae carnivorous HYDROPHILIDdS 17 
4. Lateral filaments of abdomen all plumose; mandibles with fine teeth on the inner mar- 
gin Gyrinus. 
4. First two pairs of lateral filaments not plumose in mature larva; mandibles without teeth on the 
inner margin Dinetjtes. 
5. Abdomen with slender lateral filaments which serve as tracheal gills; cerci long, slender, and densely 
plumose Coptotomus. 
5. No lateral filaments on the abdomen; cerci shorter and less plumose 6 
6. Mandibles attached diagonally to ventral surface of head and shutting up against the tip of the 
proboscis, visible both dorsally and ventrally .7 
6. Mandibles attached beneath the frontal margin of the head, toothed on their inner margin, not 
visible dorsally 8 
6. Mandibles attached to the anterior margin of the head and shutting past each other like a pair of 
shears, visible dorsally and ventrally 9 
7. Proboscis broad, with a notch on either lateral margin; labial palps two-jointed, seven times as long 
as the labium Hydroporus. 
7. Proboscis narrow, club-shaped, not notched; labial palps three-jointed and not longer than the 
labium itself Hyphydrus. 
8. Mandibles slender, narrowly sulcate at the tip, simple; third joint of antennae more than twice as 
long as the fourth Hydrocanthus. 
8 . Mandibles stout, bifid at the tip ; third joint of antennae no longer than fourth . . Canthydrus . 
9. Maxillae filiform, eight- to ten-jointed, not enlarged at the base 10 
9. Maxillae only five or six jointed, distinctly enlarged at the base, with a well defined four-jointed 
palp 11 
10. Larger, 50 to 75 mm. long; antennae 7-jointed; frontal margin of head evenly rounded; cerci 
long and densely plumose Dytiscus. 
10. Larger, 50 to 75 mm. long; antennae 9-jointed; frontal margin of head very uneven; no 
cerci Cybister. 
10. Smaller, 20 to 25 mm. long; antennae 7-jointed; cerci long with scattered hairs only. 
Hydaticus. 
