270 
BULLETIN OF THE BUREAU OF FISHERIES. 
I . Femora at right angles to body axis, first pair of tibise folded against them, but not the other two pairs ; 
knees neither projecting nor armed; posterior cerci in the form of stout spines, not jointed nor 
bifid ; styli in the form of short spines, also not jointed D YTISCIDiE 9 
2. Styli in the form of long, curved, linear spines, not jointed; wings folded tightly against the body 
and smooth Peltodytes. 
2. Styli in the form of short triangular spines; wings loosely held against the body and very 
rough - Haliplus. 
3. Styli on dorsal surface of abdomen in transverse rows of four on each segment; spiracles partly 
concealed by lateral tubercles 4 
3. Styli on dorsal surface of abdomen in transverse rows of six on each segment; spiracles not concealed, 
no tubercles 7 
4. Two supraorbital styli over each eye; basal portion of pronotal styli not annulate; no meta- 
sternal spine; cerci long, slender, moniliform 5 
4. One supraorbital stylus over each eye or none; metastemal spine prominent; basal portion of 
pronotal styli multiannulate; cerci short, stout, and bluntly bifid at the tip 6 
5. 24 pronotal styli; no inner spur on mesotibia; tarsi all tipped with stout spines; eighth abdominal 
segment with a pair of dorsal appendages resembling cerci Laccobius. 
5. 26 pronotal styli; inner spur on mesotibia prominent; no spines at tips of tarsi; no appendages on 
eighth abdominal segment Berosus. 
6. Larger, 25 to 30 mm. long; one supraorbital stylus; posterior cerci superficially annulate and 
bifid Hydrous. 
6. Smaller, 12 mm. long or less; no supraorbital stylus; cerci not annulate, but bifid and acute. 
Tropisternus. 
7. Larger, 13 mm. long or more; 32 pronotal styli; eighth abdominal segment with two small dorsal 
tubercles, each tipped with a stylus; cerci bifid and acuminate Hydrophilus. 
7. Smaller, 8 mm. long or less; 24 pronotal styli; no dorsal tubercles on the eighth abdominal segment, 
but with a pair of styli; cerci not bifid but long and linear Philydrus. 
8. Larger, 10 mm. long or more; bristles arranged as in Figures 92 and 93 (p. 307); labrum prominent 
and well defined Dineutes. 
8. Smaller, 6 mm. long or less; bristles not definitely arranged; labrum less prominent. 
Gyrinus. 
9. Posterior margins of first six or seven abdominal segments raised into prominent transverse ridges 
dorsally; cerci short, stout, and straight 10 
9. Dorsal surface of abdomen smooth, without transverse ridges; cerci long, narrow, and crooked, or 
club-shaped 13 
10. Eighth abdominal segment much narrower than the seventh; metatibial spur large and promi- 
nent 11 
10. Eighth abdominal segment nearly as wide as the seventh; metatibial spur scarcely visible. .12 
II. Larger, 30 mm. long or more; styli in the form of stiff bristles; cerci simple and acute. 
Cybister. 
11. Smaller, 15 mm. long or less; styli with a stout basal portion and a slender terminal seta; cerci 
bifid and blunt Thermonectes. 
12. Larger, 10 mm. long or more; styli with a short enlarged basal portion; anterior margin of 
pronotum three-quarters as wide as the posterior Ilybius. 
12. Smaller, 6 mm. long or less; styli with a short enlarged basal portion; anterior margin of 
pronotum only one-third as wide as the posterior Agabus. 
12. Smaller, 4 to 5 mm. long; styli in the form of stiff bristles, without enlarged basal portion; 
anterior margin of pronotum three-quarters as wide as posterior. Laccobius. 
13. Anterior margin of pronotum much narrower than posterior; styli in the form of slender bristles, 
without enlarged basal portion 14 
13. Anterior margin of pronotum nearly as wide as posterior; styli with a short enlarged basal portion 
and a long terminal seta 15 
14. Larger, 35 to 40 mm . long; pronotum with two raised processes on the anterior margin; cerci 
club-shaped and densely plumose Dytiscus. 
