HUNGERFORD: AQUATIC HEMIPTERA. 107 
the Diptera was probably applied to some of these bugs. The Gerrids are 
allied to the Veliids and have often been placed together, with others, in 
the family Hydrometridz. 
KEY TO THE GENERA OF GERRID&. 
A. Inner margin of the eyes accurately sinuate behind the middle. 
Body comparatively long and narrow, abdomen long. 
B. Pronotum sericeous, dull, antenne comparatively short and 
stout. 
C. First segment of the antenne shorter than the second, 
and third taken together. 
D. Antenne as long as half the body; sixth abdominal , 
segment of the male roundly emarginate. 
Limnoporus. 
(1 species. ) 
DD. Antennae not as long as half the entire length of 
the insect, not extending beyond the thorax; sixth 
abdominal segment of male doubly emarginate. 
Gerris. 
(9 species. ) 
CC. First segment of the antenne longer than the second 
and third taken together. Gerris. 
(subgenus Aquarius.) 
(4 species. ) 
BB. Pronotum glabrous, shining; antennz long and slender. 
Tenagogonus. 
(3 species. ) 
AA. Inner margin of the eyes convexly rounded; body comparatively 
short and broad, abdomen so short as to appear almost nymphal in 
some forms. 
B. First antennal segment much shorter than the other three 
taken together; not much longer than the second and third 
taken together, and sometimes shorter. 
C. Fourth (apical) segment of antenne longer than the 
third. 
D. Eyes larger, fairly prominent; colors black and 
yellow. Trepobates. 
(1 species. ) 
DD. Eyes smaller, widely separated; plumbeous forms, 
entirely sericeous. Halobates. 
2 species. Ocean dwellers. ) 
CC. Fourth segment of antenne never more than equal to 
third; basal segment of anterior tarsi much shorter than 
second; hind femur equal to or much shorter than hind 
tibia and tarsus taken together. Rheumatobates. 
(3 species. ) 
BB. First antennal segment nearly equal to the remaining three 
taken together, much longer than second and third; antenne 
almost as long as entire body; hind femora twice as long as 
hind tibia. . Metrobates. 
(1 species.) 
