12 THE UNIVERSITY SCIENCE BULLETIN. 
A. Antenne shorter than head—concluded. 
BB. Ocelli absent; aquatic. 
C. Hind tarsi with indistinct setiform claws. (Save Plea, 
which is less than 3 mm. long.) 
D. Head overlapping thorax dorsally. Front tarsi 1- 
segmented, paleform. Corixvide. 
DD. Head inserted in thorax. Front tarsi normal. 
Notonectide. 
CC. Hind tarsi with distinct claws. 
D. Membrane of hemelytra reticulately veined. 
KE. Apical appendages of abdomen long and slen- 
der; tarsi 1-segmented. Nepide. 
EE. Apical appendages of abdomen short and flat, 
retractile. Tarsi 2-segmented. 
Belostomatide. 
DD. Membrane of hemelytra without veins. 
Naucoride. 
AA. Antenne as long or longer than head, exposed. 
B. Head as long as: entire thorax; both elongated. Length about 
10 mm. Hydrometride. 
BB. Head shorter than thorax, including scutellum. 
C. Claws of at least the front tarsi distinctly anteapical, 
with terminal tarsal segment more or less cleft. 
D. Hind femur extending much beyond apex of abdo- 
men; intermediate and hind pairs of legs approxi- 
mated, very distant from front pair. Beak 4- 
segmented. Gerride. 
DD. Hind femur not extending much beyond apex of 
abdomen; intermediate pair of legs about equi- 
distant from front and hind pairs (except in 
Rhagovelia). Beak 3-segmented. Veltide. 
CC. Claws all apical, last segment entire. 
D. Antenne 5-segmented (save Merragata, which has 
4). First and second segments of antenne thicker 
than the others; clavus similar in texture to the 
membrane, which is without veins; head and thorax 
suleate beneath. Hebride. 
DD. Antennz 4-segmented. Hemelytra not as above. 
E. Membrane of wing without cells,ov aptews 
Mesoveliidz. 
EE. Membrane of wing with 4 or 5 long closed 
cells. Saldidz. 
