12 THE UNIVERSITY SCIENCE BULLETIN. 



A. Antennae shorter than head — concluded. 

 BB. Ocelli absent; aquatic. 



C. Hind tarsi with indistinct setiform claws. (Save Plea, 

 M'hich is less than 3 mm. long.) 

 D. Head overlapping thorax dorsally. Front tarsi 1- 

 segmented, palaeform. Corixidse. 



DD. Head inserted in thorax. Front tarsi normal. 



Notonectidse. 

 CC. Hind tarsi with distinct claws. 



D. Membrane of hemelytra reticulately veined. 



E. Apical appendages of abdomen long and slen- 

 der; tarsi 1-segmented. Nepidse. 

 EE. Apical appendages of abdomen short and flat, 

 retractile. Tarsi 2-segmented. 



Belostomatidse. 



DD. Membrane of hemelytra without veins. 



Naucoridx. 



AA. Antennae as long or longer than head, exposed. 



B. Head as long as entire thorax; both elongated. Length about 

 10 mm. Hydrometridse. 



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. Gerridx. 



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. Veliidse. 



CC. Claws all apical, last segment entire. 



D. Antennae 5-segmented (save Merragata, which has 

 4). First and second segments of antennae thicker 

 than the others; clavus similar in texture to the 

 membrane, which is without veins; head and thorax 

 sulcate beneath. HebHdse. 



DD. Antennae 4-segmented. Hemelytra not as above. 

 E. Membrane of wing without cells,ord,pfe>'pi/S. 



Mesoveliidse. 

 EE. Membrane of wing with 4 or 5 long closed 

 » cells. Saldidas. 



