lo University of Michigan 



Family Gerridae, the Water-striders 

 Five species, belonging to two genera, have been taken at 

 Douglas Lake. 



Key to the Genera of Genidae of the Douglas Lake Region 

 Inner margins of eyes convexly rounded; bod}'^ comparatively 



paratively long and slender (Gerrinae) ; length 7 mm. 



or more Gerris 



Inner margins of eyes convexly rounded; body compartivcly 



short and broad (Halobatinae) ; length less than 6 mm 2 



2. Basal segment of antennae nearly as long as other three to- 



gether; antennae nearly as long as body (not yet taken at 



Douglas Lake) Metrobates 



Basal segment of antennae much shorter than other three to- 

 gether, sometimes shorter than segments 2 and 3 together 3 



3. Fourth antennal segment longer than third ; color black, dors- 



al surface varied with j'ellow ; occurs on sheltered parts 



of lakes (not yet found at Douglas Lake) Trei'OBATES 



Fourth antennal segment not more than equal to third ; anten- 

 nae and hind legs of males curiously distorted ; color 

 black above, more or less marked with yellow; usually 

 found on slow-flowing -streams RhEumatobaTES 



Genus Gerris Fabricius 

 The four species of Gerris occuring at Douglas Lake may 

 be separated by the following key. 



1. Basal segment of antennae longer than second and third to- 



gether; usually wingless; length 12-13 mm.; occurs on 



running water G. reinigis 



Basal segment of antennae shorter than second and third 



together ; usually winged 2 



2. Antennae as long as half the body; 6th abdominal segment 



of male roundly emarginate below; length about 15 



mm G. riifiisciitellatits 



Antennae less than half as long as body, not longer than 

 head and thorax together ; 6th ventral segment of male 

 doubly emarginate; length 10 mm. or less; dorsal surface 

 black 3 



3. Lateral border of anterior lobe of pronotum with a short 



longitudinal yellow line behind eye ; median emargination 

 of 6th ventral segment of male roundly rectangular ; length 



7-8 mm G. bitenoi 



Lateral border of pronotum concolorous ; median emargina- 

 tion narrow and rounded; length 9-10 mm G. iiiargi)iafus 



