938 



FRESH-WATER BIOLOGY 



7(4) 

 8(3) 



Last abdominal segment elongate pyramidal, and furnished with a mid- 

 ventral line of hairs. . Gvretes. 



Hind legs longer than the fore; eyes 

 two 9 



9 (14) Base of the hind legs covered by broad 



overlapping coxal plates. 



Family HALIPLIDAE . . 10 



10 (1.3) Coxal plates concealing only the three 



basal segments of the abdomen; 

 last segment of palpi shorter 

 than the preceding segment. 1 1 



11 (12) Pronotum widest before the middle. 



Brychius. 



12 (11) Pronotum widest at the rear end. 



Haliplus. 



13 (10) Coxal plates concealing all but the last 



of the ventral abdominal seg- 

 ments; last segment of palpi 

 longer than the preceding seg- 

 ment Peltodytes. 



14(9) Base of the hind legs exposed . ... 15 



15 (48) Antennae shorter than the palpi; legs 



usually with swimming fringes. 

 Family HYDROPHILIDAE . . 16 



16 (23) Scutelium wanting or indistinct or very 



small and scalelike. Posterior 

 femora subcylindrical and not 

 noticeably widened in the mid- 

 dle; pro thorax narrowed behind, 

 narrower than the elytra. The 

 species are small, elongate, 

 roughly sculptured, greyish or 

 nearly black, and usually tinged 

 with bronze and metallic col- 

 ors 17 



17 (20) Scutelium indistinct, or apparently want- 



ing; if at all evident it is 

 distinctly triangular and acute. 

 Species from 1-2 mm. long. 18 



18 (19) Pronotum without striae; maxillary palpi 



as long as the head and thorax 

 together, the ultimate segment 

 longer than the penultimate; 

 elytra with more than ten rows 

 of punctures. . . . Hydraena. 



ig (18) Pronotum bearing from one to five longitudinal striae or abbreviated 

 grooves; the maxillary palpi much shorter than the head 

 and thorax together, the ultimate segment shorter than the 

 penultimate; elytra with only ten rows of punctures. 



Octhebius. 



FIG. 1392. Diagram of the ventral 

 aspect of a diving beetle, Coptotomus 

 inlerrogatus: a, antenna; b, mouth; 

 c, c, coxal cavities for the fore and 

 middle legs; d, labial palpi; e, eye; 

 J, maxillary palpi; g, lateral margin 

 of the pro thorax; ~h, epipleuron of 

 the wing cover (elytron); i, pros- 

 ternal process; /', metasternal fork; 

 k, hind coxa with I, the inner lam- 

 ina; p, the coxal process and q, the 

 coxal notch; r, trochanter of the 

 hind leg; 5, femur; /, tibia; u, hind 

 tarsus of five joints; 1,2,3, 4, 5> 6, 

 ventral abdominal segments, st l , sP, 

 sP, sterna of pro-, meso-, and meta- 

 thoi ax, respectively; w, wing of the 

 metasternum; m, episternum, and 

 n, epimeron of the successive thor- 

 acic segments. The coxal line is the 

 line extending between / and k in 

 the figure, and the coxal border is the 

 part of the coxal process, p, that is 

 marked off laterally by the posterior 

 end of the coxal line. 



