22 



Fresh-water Biology 



{Continued from page 20) 



7 — Claws of at least front tarsi distinctly anteapical, with terminal tarsal segment more 



or less cleft 8 



— Claws all apical, last tarsal segment entire 9 



8 — Hind femur extending beyond apex of abdomen; intermediate and hind pairs of 



legs approximated, very distant from front pair. Beak 4-segmented. Gerridae. . 17 



— Hind femur not extending much beyond apex of abdomen; intermediate pair of legs 



about equidistant from front and hind pairs (except in Rhagovelia). Beak 3-seg- 



mented — Veliidae 23 



9 — Antennae 4-segmented. Membrane of wing without cells. Mesovelliidae 



Tarsi 3- jointed (1 genus Mesovelia) 

 — ^Antennae 5-segmented: Tarsi 2-jointed (Hebridae: length 2 mm.) Hebrus 



10 — Legs quite similar Plea 



— Legs dissimilar, hind legs flattened and fringed for swimming 11 



11 — Last segment of antennae much shorter than penultimate Notonecta 



— Last segment of antennas longer than the penultimate Buenoa 



12 — Front margin of prothorax deeply excavated for the reception of the head . . Ambysus 



— Front margin of prothorax not deeply excavated for the reception of the head 



Pelocoris 



13 — Body broadly oval and fiat; legs not extremely long and slender; prothorax much 

 broader than the head ; anterior femora but little longer than tibiae Nepa 



— Body elongate oval; legs not extremely long and slender; prothorax little broader 

 than head; anterior femora considerably longer than tibias Curicta 



— Body very elongate; legs long and slender; prothorax narrower than head; anterior 

 femora considerably longer than tibiae Ranatra 



14 — Mesothorax with strong midventral keel, membrane of hemelytra reduced. . .Abedus 

 — Mesothorax without midventral keel, membrane of hemelytra not reduced 15 



15 — First or basal segment of the beak longer than the second; furrow of the wing mem- 

 brane nearly or quite straight. (Length about 1 inch or less) Belostoma 



— First segment of beak shorter than the second. Furrow of the wing membrane shal- 

 lowly S-shaped. (Length more than 1| inches) 16 



16 — Anterior femora grooved for the reception of the tibiae Lethocerus 



— Anterior femora not grooved for the reception of the tibiae Benacus 



17 — Inner margin of the eyes arcuately sinuate behind the middle. Body comparatively 



long and narrow, abdomen long 18 



— Inner margin of the eyes convexly rounded; body comparatively short and broad, 

 abdomen very short in some forms 21 



18 — Pronotum sericeous, dull; antennae comparatively short and stout 19 



— Pronotum glabrous; shining; antennae long and slender Tenagogonus 



19 — First segment of antennas shorter than the second and third taken together 20 



— First segment of the antennae longer than the second and third taken together. .Gerris 



{Continued on page 24) 



