256 ANIMAL COLORATION. 



are supposed also to have been produced by the a3sthetic 

 preferences of the female, not of course such characters as 

 the accessory grasping limbs of male Crustacea — for these 

 have an obvious use in seizing the female, — but structural 

 characters, such as the jjresence of wattles and plumes among 

 birds, which have no such use. 



Among the Crustacea sexual diversity of coloration is 

 not very common. Darwin* only mentions two examples — 

 Squilla stylifera and a fiddler crab belongiog to the genus 

 Gelasimus. 



Another example of this phenomenon is a common American 

 edible crab — Neptunus hastata ; but the difference of colour is 

 limited to the large claws — the " chelas " as they are technically 

 termed : these appendages are much bluer in the male than 

 in the female. This rather small diff'erence is, according to 

 Prof. Conn,t quite constant. 



Among vertebrates, we have sexual differences in coloration 

 even so low down in the scale as fishes. Mr. Cunningham | 

 has lately shown that the two species Arnoglossus lanteriia and 

 A.Cophotes (plaice) are in reality males and females respectively 

 •of but one species. Sexual differences of this kind are met 

 with in the Amphibia — for instance, in the notched crests and 

 lurid colouring of the large newt. The blackened index 

 finger of the male edible frog is associated with a structural 

 modification which enables him to seize the female at pairing 

 time ; this character, like the stag's antlers, only reaches its 

 full development at the actual breeding season. 



Sexual differences are not common among reptiles ; it is a 



•curious fact that in tortoises the colour of the iris sometimes 



differs in the two sexes, as it does commonly among birds. 



* " Descent of Man," chap. ix. 



t Johns Hopkins Univ. Circulars, iii., p. 5. 



X Proc. Zool. Sac, 1890, p. 640. 



