15 



There is an interesting group of butterflies, comprised in the 

 genera Catopsilia and Callidri/as, not very far removed in point of 

 relationship from our own " brimstone butterfly," members of 

 which group are to be found in the tropics of both Eastern and 

 Western Hemispheres. These butterflies vary much on the under 

 surface, but many of them present, when their wings are closed over 

 their back in the ordinary position of rest, an appearance which is 

 strongly suggestive of a dead and partly decayed leaf. The usual 

 coloration in these specimens is yellow, with irregular spots and 

 blotches of brownish crimson ; and in the midst of one or more of 

 these blotches are generally to be seen a few pale spots, which look like 

 the holes or " windows " that are apt to occur in withered leaves. 

 These spots are not really transparent, but are formed by glistening 

 white scales which produce the effect of transparence. There is little 

 doubt that a butterfly showing these features, when settled with 

 closed wings among withered leaves, would escape detection from all 

 but the very sharpest-sighted enemies. They form a good example 

 of what is called " protective resemblance." 



Now the point to be observed is that those individuals which 

 show the protective resemblance in greatest perfection are females. 

 The males under certain circumstances exhibit the same kind of 

 likeness to dead leaves, but always to a far less degree than their 

 mates. Here then we have a group of cases in which the females 

 are better protected than the males, the protection coming into play 

 not during the flight of the insect, but when the butterfly is settled 

 and at rest. 



We may now take into consideration a form of dimorphism 

 which is of a specially interesting character, and which throws 

 fresh light on the bionomic significance of this divergence between 

 the sexes. 



There is a group of butterflies called Xepherouia, the males of 

 which are of a pale glossy blue, bordered and streaked with black. 

 In the females the blue is replaced by a creamy white, which may 

 sometimes in places become yellow ; the streakiness is greatly 

 enhanced, and a submarginal row of dark spots usually makes 

 itself visible. All these features render the aspect of the females 

 ver}'^ dissimilar from that of the males ; but in proportion as the 

 females depart in appearance from the males of their own species, 

 they approach the characteristic aspect of an entirely different group 

 of butterflies, found in the same regions, viz., India, Burma, and 

 the islands of the Malay Archipelago. These latter butterflies 

 belong to the sub-family of Danaines, with which Pierines like the 

 Nepheronia have nothing but a remote affinity. We have here 

 what is evidently a case of mimicry. There is a strong presumption 

 that the Danaines are avoided by insect-eating birds, and there is 

 every reason to think that the Nepheronia females derive benefit 

 from the protection afl'orded by the characteristic aspect which they 

 share with the Danaines. 



