NORTH AMERICAN SPECIES OF THE GENUS ATAX 195 



anterior margin no concavity; the posterior margin evenly 

 rounded or with a shallow median concavity. The first pair 

 of legs distinguished by an unusual thickness and in the non- 

 parasitic species provided with long, stout, movable, sword- 

 shaped spines, inserted into prominently projecting papillae. 

 Second pair usually exceeding the third in length. The pro- 

 portion between the length of the body and legs quite variable, 

 but generally the legs of the parasitic species shorter than those 

 of the non-parasitic. The maxillary shield is not fused with 

 the neighboring epimera. In ventral view it resembles in 

 shape a broad chalice. The palpi are long; in the parasitic 

 forms they equal or exceed in thickness the first pair of legs. 

 The next to the last segment possesses on its ventral side three 

 papillae varying in size in different species; of these the one 

 at the outer end ends in a chitinous spur, while the two others 

 placed somewhat farther posteriorly are each crowned with a 

 little hair. The fifth palpal segment is short and provided with 

 chitinous claws. Among the epimera those of the fourth pair 

 are distinguished by their size and more or less rectangular 

 form. Third epimeron imperfectly separated from the fourth. 

 Genital area at the extreme end of the body. The chitinous 

 plates surrounding the genital cleft from either side, bear to- 

 gether 10, 12, or numerous, acetabula. In the females charac- 

 teristic sword-like spines appear in the vicinity of the genital 

 opening, which are employed in oviposition. 



This diagnosis is not strictly applicable to all of our species, 

 even excluding Atax {Najadicold) ingens (Koenike), since both 

 A. abnormipes mihi and A. indistinctus mihi are deeply emargi- 

 nate posteriorly, A. pectinatus mihi is a non-parasitic form yet 

 lacks the movable spines on the first pair of legs, and the 

 genital area is not in all forms at the extreme end of the body, 

 though usually approaching that position. 



The sub-genus Nojadicola differs in the following respects. 

 The fore legs are not thickened, and all are short, with few 

 short spines and simple claws. The genital area is not at the 

 end of the body but immediately behind the last epimera and 

 as Koenike's figures show, bears a certain resemblance to 



