686 



THE AMERICAN NATURALIST [Vol. XL 



usually exceed 20 nun. in length, while the Xaididie on the con- 

 tnu'v are rarely more and usually much less than 1 ') mm. in length. 



The form and position of the setie are the chief characters 

 relied upon for the separation of the genera and species. These 

 may be long and hair-like (capilliform), short and straiglit (needle- 

 like), or S-shaped (sigmoid), and may terminate simply or in two 

 hooks fbiuncinate). A slight enlargement (nodulus) is usually 



represent inu' a tyj)ic:il Naid (Fig. 1) together with the ditferent 



