98 AMERICAN SPIDERS 



The very close correspondence between the male and female 

 genitalia of insects ted Leon Dufour to the formulation of the so- 

 called "lock and key" principle. It was his belief that the crossing 

 of species was impossible for physical reasons, and that the male 

 organ could not be introduced into that of a strange female because 

 of differences in length, shape, and size. The female organ was re- 

 garded as an unyielding lock that could be opened only by a key 

 that corresponded exactly with its form. Whereas we must reject 

 the theory that these organs are adaptations that exclude the cross- 

 ing of species, and instead assign that function to fundamental in- 

 stinctive patterns probably based on chemical stimuli, it must be 

 admitted that in spiders the differences between the genitalia of 

 allied groups are usually sufficiently great to make pairing impos- 

 siblein effect a "lock and key" presenting an impassable barrier to 

 all but the most closely related species. 



It must be kept in mind that the secondary genitalia of spiders 

 are extremely ancient organs probably fully evolved long before 

 the late Paleozoic Era, where we find fossil spiders. Both primitive 

 true spiders and living tarantulas, discretely separated even at that 

 time, have similar palpi, indicating that the general features of their 

 organs antedate the separation of the two suborders. It is little 

 wonder then that in the palpi and epigyna are clues to the general 

 phylogeny of the whole group. These organs have undergone 

 changes corresponding closely with the specialization of spiders 

 themselves. Indeed, sexuality and the araneid mode of copulation 

 are adaptations that have probably contributed more to spider evo- 

 lution than have any other features. 



