934 FRESH-WATER BIOLOGY 



25 (24) Membrane large; reentrant angle bordered externally by the eye 



itself {Zaitha of most of our literature) .... Belostoma. 



26 (23) Furrow of wing membrane S-shaped; a recurved internal tooth 



borne on antennal segments two, three and four. ... 27 



27 (28) Front femora grooved internally for the reception of the tibia {Belos- 



toma oi most oi our UtersLtuie) Amorgius. 



28 (27) Front femora not grooved internally Benacus. 



29 (16) Antennae three-jointed; caudal appendages long, j&hform, grooved; 



middle and hind legs fitted for walking; tarsi one-jointed. 



Family Nepidae . . 30 



30 (31) Body oval; legs not extremely long and slender; prothorax much 



broader than head; anterior femora but little longer than 

 tibiae Nepa. 



31 (30) Body linear; legs extremely long and slender; prothorax but Uttle 



broader than head; anterior femora more than twice as 

 long as tibiae Ranatra. 



32 (13) Anterior coxal cavities open behind; antennae three or four-jointed, 



without lateral hooks t,t, 



33 (38) Head inserted in the prothorax; antennae four-jointed; beak three 



or four-jointed, not retractile; anterior tarsi one or two- 

 jointed, of the usual form, with two claws. 



Family NoTONEcrroAE . . 34 



34 (37) Antennae inserted in cavity beneath eyes, second joint thickest; 



hind legs flattened, ciliated, fitted for swimming; abdomen 

 keeled and hairy. (Size larger) 35 



35 (36) Last joint of antenna less than half as long as the third, which is 



fringed with capitate hairs. Hind tarsi without claws. 

 (Body stouter) Notonecta. 



36 (35) Last joint of antenna fringed with capitate hairs, and many times 



longer than the third, which is very small and inconspicuous. 

 Hind tarsi with claws. (Body more slender). . . Buenoa. 



37 (34) Antennae inserted beneath the margin of the head, third joint longest 



and thickest; hind legs like the middle legs, tarsi with 

 claws; abdomen not keeled or hairy. (Size smaller, not 

 over 3 mm. long) Plea. 



38 {$^) Head overlapping the prothorax; antennae three or four- jointed; 



beak short, unjointed, retractile; anterior tarsi one-seg- 

 mented, flattened, with a fringe of hairs on the edge, and 

 without claws Family Corixjdae. 



KEY TO NORTH AMERICAN AQUATIC NEUROPTERA 



Adults 



1 (4) Veins of the wing disc all ending in a succession of symmetrical forks, 



the terminal forks forming a distinct peripheral zone; 

 antennae moniliform. . . Family Hemerobiidae . . 2 



2 (3) The median vein repeatedly forked; some of the branches of vein Cui 



again forked. . Sisyra. 



