NO. 1788. NORTH AMERICAN ERGA8ILID.^— WILSON. 373 



Total length, 1 mm. Ceplialothorax, 0.4 mm. long, 0.45 mm. wide. 

 Length of free segments, 0.19 mm.; of genital segment, 0.22 mm.; of 

 abdomen, 0.24 mm.; of anal setae, 0,65 mm. 



(concinnus, beautiful, elegant in appearance.) • 



Young female, a developmental stage. General bod}?- form long 

 and narrow, similar to that of the male, but witli marked differences 

 in detail. Cephalothorax circular in outline, about as long as wide, 

 narrowed and projecting a little between the antennee, and squarely 

 truncated posteriorly. 



Free and genital segments all about the same length, but dimin- 

 ishing regularly in width. Each of the free segments carries a 

 rounded projection on either margin, covering the base of the swim- 

 ming leg. The genital segment is almost a perfect rectangle, and the 

 abdomen contains but a single joint, which is deeply cut posteriorly 

 by the anal fissure. Each anal lamina is armed with three setse, one, 

 short and slender, on the outer margin near the base, and two at the 

 tip, the inner of which is nine times the length of the outer and more 

 than half the length of the entire body. 



The first antennse are distinctly six-jointed, the three terminal 

 joints much narrower than the basal and armed throughout with 

 densely plumose setae. The large seta given off from the distal end 

 of the third joint is itself jointed near the base and thus forms a sort 

 of ramus. 



The mouth-parts are similar to those of the adult but with the 

 following differences. The first maxillae project well from the ventral 

 surface and each carries four plumose setse diverging but little. 



The basal joints of the second maxillae are considerably elongated 

 and the cutting blade is covered with long bristles. The maxillipeds 

 are distinctly three-jointed; the proximal joint is in normal position 

 behind the second maxillae, the second joint turns forward outside 

 the base of the second maxillas but is not long enough to reach their 

 anterior margin, while the terminal claw is comparatively small and 

 weak. Of the swimming legs each ramus of the fourth pair contains 

 but two joints, while the fifth legs are represented by mere spines on 

 the sides of the fifth segment like those in the adults of some Erga- 

 silus species. 



Total length, 0.5 mm. Cephalothorax, 0.2 mm. long, 0.21 mm. wide. 

 Length of free segments, 0.18 mm.; of anal setae, 0.28 mm. 



This developmental stage helps to fix the anatomy of several 

 appendages in this genus which have been heretofore somewhat in 

 doubt. The first antennae are really six-jointed and the three basal 

 joints become fused in later development. The fifth thorax segment 

 is at first of equalimportance and corresponding size with the other 

 segments, but it loses its size and importance as the sexual products 



