56 Illinois State Laboratory of Natural History. 



crease rapidly in length at the posterior end and lade away 

 in front. The male does not have this character, and adult 

 females are very rarely found without it. Of the four apical 

 bristles the middle two arc well developed. The inner one 

 of this pair is much longer than the outer. The innermost 

 of the four is a very slender seta. The outermost varies in 

 character from a long seta bordered externally by harbules 

 and internally by long slender cilia, to a long strong spine 

 serrate on both sides. It may be anywhere from one half 

 to four fifths as long as the stylet. 



The first antennae are twelve-segmented. Dr. Schmeil 

 notes the presence of a minute sensory bristle on the ninth 

 segment in the place occupied by the sense-club on the 

 twelfth segment of seventeen-segmented antenna?. It is 

 very inconspicuous indeed. The last three segments are 

 usually very long and are armed on the inner side by hyaline 

 plates. 



The usual armature of the swimming feet is as follows : — 

 First pair : outer ramus, three spines, live seta* ; inner 

 ramus, one seta, one spine, four seta?. Second and third 

 pairs : outer ramus, four spines, five seta j ; inner ramus, 

 two spines, four seta?. Fourth pair : outer ramus, three 

 spines, five seta?; inner ramus, one seta, two spines, two 

 seta. 



The fifth foot (PI. XVIII., Fig. 2) is one-segmented and 

 plate-like. On the inner side is a very strong serrate spine. 

 At the tip, borne on a cone-shaped projection, is a very long 

 slender seta, parallel to the spine. On the outer side i> a 

 delicate little seta projecting outward at a considerable angle. 



The receptaeulum semi7iis (PL XVIII., Fig. 3) is almost com- 

 pletely divided into an anterior and a posterior part, by a 

 median constriction. The porus is situated in the middle of 

 this narrow connecting part. The lower half extends down 

 into the narrow part of the first abdominal segment. The 

 spermal canals are attached to the receptaeulum at the outer 

 angles of the lower part. 



The egg-sacs are usually long, with many eggs, but some- 

 times contain only a few, arranged in a spherical mass. 



