The Evidence from Sexual Cliaracters. 189 



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In the fiddlei' crabs, one of the claws of the male is 

 enormously developed, so that it compares with the 

 other about as a base-viol does with its bow. In the 

 female both claws are alike, and both small. There are 

 a number of species of fiddler crabs, forming together 

 the genus Gelassimus, and the big chnv of the male, in 

 each species, has certain points of difference from all 

 the other species. 



The fact that the features wdiich characterize males as 

 distinguished from females, are also the features which 

 distinguish species from each other, certainly indicates 

 that the origin of specific difference is to be sought in 

 some peculiarity of the male organism. 



Insects. — Many insects have stridulating organs, by 

 which, as in the house-cricket, they produce their sharp 

 music. In many cases these organs are exclusively con- 

 fined to tlie males; in others they are present but rudi- 

 mentary in the female, while they are perfectly developed 

 in both sexes of certain others. In all cases we find that 

 the organs for this purpose differ greatly in closely re- 

 lated forms, and thus show that they are of compara- 

 tively recent acquisition. 



In the Cicadas the females are mute, and the sound is 

 produced in the male, by the vibration of the lips of the 

 spiracles, which are set into motion by a current of air 

 discharged from the trachese. It is increased by a 

 wonderfully complex resonating apparatus, consisting 

 of two cavities covered by scales. This apparatus is 

 jiresent, very much less developed, in the female, but it 

 is never used for producing sound. 



The males of the crickets, grasshoppers, and Locus- 

 tidae, are all remarkable for their musical powers, 

 which are absent in the females. Although these three 

 groups of insects are pretty closely related to each other. 



