1 40 Illinois State Laboratory of Natural History. 



about half as long as the first, having on the inner margin 

 two cushion-like processes (the upper, smaller one hairy, and 

 the lower densely tuberculate), and being armed at the tip 

 with two processes forming a forcipate structure, the outer 

 broad, plowshare-shaped, the inner a long and narrow spine, 

 hairy within. 



Inner ramus of left fifth foot one- segmented, of the same 

 width throughout, with a broadly rounded tip ; inner margin 

 rugose. 



Second basal segment of the fifth foot of the female with 

 the usual delicate hair at the outer margin. First segment 

 of outer ramus short and broad. Second segment large, 

 about one and a half times as long as the first, armed on the 

 middle third of the inner margin with seven or eight very 

 large, strong, pointed spines, and on the outer margin and 

 opposite the upper spines of the inner margin with three or 

 four spines. Third segment distinct, armed with two spines, 

 the outer one short, thick, sharp, smooth, the inner one about 

 twice as long and armed with a few rather strong spinules. 

 Just without these spines, on the second segment, is a shorter, 

 smooth spine. 



Inner ramus of the fifth foot of the female distinctly two- 

 segmented, the first segment subquadrate, the second as wide 

 as the first and nearly twice as long, and armed at the tip 

 with two thick heavy spines reaching to the end of the second 

 segment of the outer ramus. These spines are armed with 

 heavy spinules. Disregarding the spines, the ramus reaches 

 just to the end of the first segment of the outer ramus. 



Length of female 4.0-4.5 mm. ; of male 3.5-4 mm. 



This Diaptomus is the largest of the American species and 

 a very beautiful one. Dr. Forbes states in his original des- 

 cription ('82a) that "all were red throughout." Specimens 

 taken in April, 1897, from ponds south of Urbana, 111., when 

 they were in the height of sexual activity, were colored as 

 follows: thorax and anterior appendages (all but the first 

 pair of antenna') blue ; first pair of antenna, fifth pair of 

 legs (in the male), and abdomen red. In the female all the 

 legs were blue. 



