THE AMERICAN NATURALIST. [Vol. XXXV II. 



decrease regularly in size posteriorly. Fig. 3, a represents one 

 of the first pair and Fig. 3, b one of the last pair. 



Total length about 9 mm. ; antennae 5-6 mm. ; caudal setae 

 7 mm. 



Caudal setae abundantly fringed, the lateral ones on the 

 inner side only; fringe three or four times as long as setae is 

 wide, regularly abundant, not confined to nodes except towards 

 tip, where it is short and sparse and almost imperceptible ; 

 fringe widest on the middle third, which is strikingly dark 

 colored (Fig. 3, d) ; lateral setae slightly longer than terminal, 



bringing their tips in a line ; angle between outer seta? habitu- 

 ally about 30 ; active darting swimmers, as might be imagined 

 from the oarlike caudal setae, and correspondingly poor walkers 

 and climbers. When at rest the abdomen is held much elevated, 

 as in Fig. 3, c. 



The specimens taken on June 14 (Fig. 4) differ somewhat 

 from the preceding, and I imagine the former might have been 

 females. One of the latter which I bred was a male, but I 

 cannot be sure that it is the individual from which I wrote the 

 description. 



Total length 10 mm.; antenna: 5.25 mm.; setae 7 mm ; 

 greatest width 2.25 mm. The habitual angle at which the 

 outer caudal setae was held was slightly greater than in the 



