184 



it does not differ at all in the female in superficial characters, every 

 effort baviug failed to dis(;over any feature whereby specimens of this 

 sex might be distinguished from each other. As the genital structure 

 is so distinct this is rather surprising, and the distinguishing feature 

 will no doubt be still discovered. 



The primary characters of tbe female genitalia are of the same type 

 shown in dubia, but the distinction is yei obvious. Tbe pubic process, 



while divided at tip much 

 as in tbe preceding species, 

 is only about half as long, 

 and does not divide tbe 

 upper plates as in the pre- 

 ceding species. It resem- 

 bles tbe upper i)art of tbe 

 dubia structure set upon 

 the superior plates ; tbese 

 latter are large and nearly 

 quadrate, in marked con- 

 trast with tbe narrow, 

 linear structures of dubia. 

 Tbe inferior plates differ as 

 markedly, as can be readily 

 seen by a comparison of 

 tbe figures. 



Tbe males also offer no 

 babital or other differences 

 from dubia, except in the 

 ventral characters, but these are obvious and easily recognized. Tbe 

 ridge in this species is very much curved, ver}' mucli ov*erbanging, the 

 ends reaching tbe apical margin of tbe segment, while the arch, com- 

 bined with the depression of tbe last segment, forms a ])erfect oval. 

 In this species tbe space included by tbe arch of the ridge is smooth; 

 in dubia it is punctured. 



This species seems rather more southern than the preceding. It is 

 practically the only form taken at Washington, many thousands being- 

 taken while only one specimen of the other forms was discovered. 

 Other localities are New York, Kew Jersey, Central Missouri, Iowa, 

 Georgia. Tbe specimens from New York and New Jersey are from my 

 collection, and form the small minority of the specimens taken. The 

 specimens from Central Missouri are from Professor Riley's collection, 

 and the figures in the Missouri Reports, so extensively copied, probably 

 represent this species. 



Finally, these forms represent a series of si)ecies, evidently derived 

 from the same stock, and which have differentiated in physiological 

 rather than superficial or babital characters. They have become dif- 



FiG. 43.— Lachnosterna ARCUATA. 1, clasper of male from 

 front and above; 2, clasper of male (right) from side; 3, 

 clasper of male (left) from side; 4, ventral cliaracters of 

 male ; 5, genital structure of female ; 5a, pubic process ; 

 5b, superior plates ; f)c, inferior plates — enlarged (original). 



