AQUATIC INSECTS 933 



KEY TO AQUATIC AND SEMI-AQUATIC HEMIPTERA 



1 (8) Antennae longer than the head, free. Forms that walk on the water. 2 



2 (s) Last segment of tarsi split, claws inserted before the apex. ... 3 



3 (4) Beak four-jointed ; intermediate and posterior legs extremely long and 



slender; body widest back of the pro thorax. 



Family Gerridae. 



4 (3) Beak three-jointed; none of the legs extremely long and slender; body 



widest across the prothorax Family Veliidae. 



5 (2) Last segment of tarsi entire, claws inserted at the apex 6 



6 (7) Body linear; head as long as thorax; legs extremely long and slender; 



beak not reaching anterior coxae. Family Hydrometridae. 



7 (6) Body oval ; head shorter than thorax ; legs not extremely long and slender ; 



beak reaching intermediate coxae. . Family Acanthiidae. 



8 (i) Antennae shorter than the head, nearly or quite concealed beneath the 



margin of the head, or in a cavity beneath the eyes. . 9 



9(12) Ocelli two; anterior coxal cavities open behind; antennae four-jointed, 



simple. (Live near the water) 10 



10 (11) Fore legs slender, fitted for running; eyes triangular. 



Family PELOGONmAE. 



11 (10) Fore legs stout, fitted for grasping; eyes projecting, subglobose. 



Family Nerthrhdae. 



12 (9) Ocelli none; anterior coxal cavities open or closed behind; antennae 



three or four-jointed, simple or with some of the segments 

 produced into a lateral hook. (Live in the water) . . . 13 



13 (32) Anterior coxal cavities closed behind; antennae four-jointed, simple 



or hooked 14 



14 (15) Antennae simple; no caudal appendages; fore legs fitted for grasping, 



middle and hind legs for walking. . . Family Naucoridae. 



15 (14) Second and third (sometimes fourth) joint of antennae produced into 



lateral hooks; end of abdomen with a pair of caudal append- 

 ages; fore legs fitted for grasping, middle and hind legs for 



walking or swimming 16 



i6 (29) Antennae foiur-jointed; caudal appendages short, strap-shaped, re- 

 tractile; middle and hind legs flattened, fitted for swimming; 

 tarsi two-jointed. . . . Family Belostomatidae . . 17 



17 (18) Fore tarsus with two claws Hydrocirius. 



18 (17) Fore tarsus with a single claw 19 



19 (22) Mesothorax with a strong mid-ventral keel *. 20 



20 (21) An internal tooth borne upon both joints two and three of the an- 



tenna; all the ventral surface of the body hairy. Serphus. 



21 (20) An internal tooth borne upon joints two, three and four of the 



antennae; venter hairy only in the middle. . . . Abedus. 



22(19) Mesothorax without mid-ventral keel; antennae four-jointed. '. . 23 



23 (26) Furrow of the membrane of the fore wing regularly curved; an acute 



internal tooth on antenna! segments two and three, the 



fourth simple and pointed 24 



«4 (25) Membrane of the fore wing small; the reentrant angle seen at either 

 side of the front of the head when viewed from above 13 

 wholly in the front Pedinocoris. 



