108 THE UNIVERSITY SCIENCE BULLETIN. 
Genus GERRIS Fabr. 1794. 
This genus is our largest in the family. It contains 9 species, placed 
in the two subgenera Aquarius and Gerris. The former with four 
species, G. remigis, G. orba, G. conformis and G. robusta. The latter con- 
tains G. gillettei, G. marginatus, G. argenticollis, G. buenoi, and G. can- 
aliculatus. 
KEY TO GERRIS. 
A. First segment of antenne shorter than second and third together. - 
Subg. Gerris. 
B. Sixth abdominal segment acutely produced into a spine, com- 
paratively slender species (mostly apterous). 
G. canaliculatus. 
BB. Sixth abdominal segment not spinously produced, stouter 
species (mostly winged). 
C. Anterior margin of pronotum with a flavous line; sec- 
ond emargination of sixth male segment roundly rec- 
tangular, female genital segment comparatively broad; 
connexivum dark. G. buenoi. 
CC. Anterior margin of pronotum yellow clothed with thick 
silvery pubescence. G. argenticollis. 
CCC. Anterior margin of pronotum concolorcus; second 
emargination of sixth male segment narrow and 
rounded; female genital segment comparatively nar- 
row, connexivum flavus. G. marginatus. 
(Note) Gerris gillettei L & S is omitted from this series 
pending further study. 
AA. First segment of antenne as long or longer than the second and 
third together. Subg. Aquerius. 
B. Comparatively stout bugs. The spines of sixth abdominal 
segment not reaching end of genital plate. 
C. Male venter, with two large blackish spots on each seg- 
ment; male venter suleate down the middle. : 
CC. Male venter without conspicuous black patches, shal- 
lowly sulcate down the middle. G. robusta. 
BB. Comparatively slender, generally winged, spines of sixth seg- 
ment reaching end of genital segment. G. conformis. 
(Note.) G. orba omitted for present. Van Duzee says he is 
not sure he can tell it from remigis. 
Gerris remigis Say 1832. 
Say, Het. N. Harm., p. 35, 1832; Compl. writing Le Conte, vol. I, p. 362. 
“Dark olivaceous; thorax rounded behind, without an elevated line. 
“Tnhabits United States. 
“Body dark brownish-olivaceous; thorax transversely rugulose without 
much appearance of a dorsal raised line; a dull ochraceous and indented 
line before; posterior margin regularly and obtusely rounded with but a 
very narrow depressed margin; tergum with a black line on the middle, 
in which is a series of obsolete gray lines; lateral margin with a series 
of grayish points or short transverse lines; beneath with a silvery seri- 
ceous reflection, an impressed line on the anterior part of the pectus 
behind the anterior feet. 
“Length, half an inch. 
