146 ENTOMOLOGICAL NEWS. [May, 'o6 



accomplished, the pair come to rest in tree-top, on the ground, 

 or where not, and copulation takes place." — Williamson, 

 Proc. Ind. Acad. Sci., 1901, p. 125. 



In the final act of copulation still other harmonious relation- 

 ships of parts are necessary. The abdomen of the female 

 must be curved forward and upward and its apex brought 

 into proper position to the accessory genitalia of the second 

 abdominal segment of the male. Often this is accomplished 

 only after repeated trials, and sometimes there is failure to 

 copulate. Any deformity in the grasping apparatus of 

 the male which would tend to hold the female out of 

 proper position, though admitting of securely retaining her, 

 or any malformation of the female's thorax or head, ha\> 

 ing the same effect, might prevent copulation. Abdominal 

 length or brevity of abdomen of either sex might also prevent 

 it. Crooked abdomens or the absence of free articulation of 

 abdominal segments might have a prohibitive effect. And, if 

 the apex of the female abdomen is brought into position with 

 the accessory genitalia of the male, it yet remains, before co- 

 pulation can take place, that these parts should be mutually 

 adapted to each other. So, to sum up, the Odonata, in the 

 securing of the female, the filling of the seminal vesicle and 

 the final act present an example of copulation possibly unique 

 in the complexity of its accomplishment. This complexity 

 explains at once the isolation which certain species enjoy 

 though associated with numerous congeners of similar habits. 

 That such species with their complex sexual adaptations ap- 

 peared by mutation is almost inconceivable. Geographical 

 isolation and gradual modification offer a more plausible ex- 

 planation. 



Various factors contribute to the preservation of related 

 species when brought in contact with each other in a restricted 

 area. The factor above discussed, i. e., complexity of the 

 sexual act, is almost inoperative probably in the genus Libel- 

 lida for example. Here there is considerable uniformity in 

 size and great uniformity in the structure of all parts con- 

 cerned in copulation. But the males fight for the females in 

 the open, over the water or above the reeds ; and it is possible 



