938 FRESH-WATER BIOLOGY 



7 (4) Last abdominal segment elongate pyramidal, and furnished with a mid- 



ventral line of hairs Gyretes. 



8 (3) Hind legs longer than the fore; eyes 



two 9 



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



overlapping coxal plates. 



Famxily 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 conceaHng 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) Scutellum wanting or indistinct or very 



small and scalelike. Posterior 



femora subcylindrical and not 



noticeably widened in the mid- pic.1392. Diagram of the ventral 



die ; pro thorax narrowed behind, aspect of a diving beetle, Coptotomus 



narrower than the elytra. The fX'i^,'c,^ii^1""li'\^°''S:i 



species are small, elongate, middle legs; d, labial palpi; e, eye; 



roughly sculptured, greyish or {.rthe'pSfoti^; 5;. 'epipt "'?!," 

 nearly black, and usually tinged the wing cover (elytron); f, pros- 



- - - ,11. 1 temal process; /', metasternal fork; 



k, hind coxa with /, the inner lam- 

 ina; p, the coxal process and q, the 

 coxal notch; r, trochanter of the 

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

 tarsus of five jcints; /, 2, j, 4, 5- <5, 

 ventral abdominal segments, 5/^ sf-, 

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

 thoi ax, respectively; u', wing of the 

 metasternum; m, episternum, and 

 n, epimeron of the successive thor- 



with bronze and metalHc col 

 ors 17 



17 (20) Scutellum indistinct, or apparently want- 

 ing; if at all evident it is 

 distinctly triangular and acute. 

 Species from 1-2 mm. long. 18 



18 (iq) Pronotum without striae; maxillary palpi ,., .»_...... „. -..- -_-^^ ^ . 



as long as the head and thorax ^ -^^^ S^rV'nd ViS 



together, the ultimate segment the figure, and the coxal border IS the 



longer than the penultimate; ^^S^^^\^^%, 

 elytra with more than ten rows end of the coxal line. 

 of punctures. . . . Hydraena. 



19 (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. 

 *^ Octhehius. 



part of the coxal process, p, that is 

 marked off laterally by the posterior 



