AQUATIC INSECTS 939 



20 (17) Scutellum appearing as a very small but distinct scale, shaped like 



the last joint of one's thumb; the ultimate segment of labial 

 palpus longer than the penultimate. Species from 3-6 mm. 

 long 21 



21 (22) The pronotum bearing five longitudinal striae; labial palpi moderately 



large Helophorus. 



22 (21) The pronotum coarsely punctured but without longitudinal striae; 



labial palpi short Hydrochus. 



23 (16) Scutellum distinct, moderately large. Posterior femora flattened 



and distinctly widened at their middle; pro thorax not 

 narrowed posteriorly, as wide as the elytra at their base. 

 The species are small or large, of oval, elliptical or even 

 hemispherical form, with not very coarse sculpture, and 

 are commonly pitchy black, often more or less testaceous, 

 very occasionally with metallic tinges 24 



24 (43) Metasternum not prolonged into a spine; tarsi not compressed. 



The species of this group are all smaller than those of the 

 next, less than 9 mm. long 25 



25 (26) Fifth ventral segment with a deep notch in the middle of its apical 



border; middle and posterior tibiae and tarsi bearing a close- 

 set fringe of long silky setae; scutellum elongate, acute. 



Berosus. 



26 (25) Fifth ventral segment not notched; tibiae and tarsi not fimbri- 



ate 27 



27 (28) First two ventral segments concealed by whitish translucent plates, 



one on each side, upon which are a number of long appressed 

 setae. The species are very convex, have a tendency to 

 partially roll up like a sow-bug, and are from 1-2 mm. in 

 length .-.-. Chaetarthria. 



28 (27) No such plates over the first ventral segments 29 



29 (30) Posterior tibiae incurved, small at base and considerably enlarged at 



their apex. ............. \ ..... Laccobius. 



30 (29) Posterior tibiae straight; little or not at all thickened at their apex. 31 



31 (32) Abdomen with apparently eight ventral segments. The single 



species is i^ mm. long; of a black color with pale legs. 



Limnebius. 



32 (31) Abdomen with five ventral segments, the tip of the sixth often 



visible ;. . . 33 



33 (40) Terminal segment of the maxillary palpus rarely as long as, usually 



shorter than, the preceding segment 34 



34 (35) Elytra deeply longitudinally striate; tarsi of the middle and pos- 



terior legs with only four segments Helocombus. 



35 (34) Elytra not striate 36 



36 (39) Mesosternum with a feeble transverse carina or simple 37 



37 (38) Mesosternum with a feeble transverse carina; tarsi of the middle 



and posterior legs with four segments Cymbiodyta. 



38 (37) Mesosternum simple; all tarsi with five segments, the first usually 



triangular Helochares. 



