270 



BULJLETIN OF THE BUREAU OF FISHERIES. 



I. Femora at right angles to body axis, first pair of tibiae 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 YTISCID^ 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; metasternal 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. 



