194 DARWINISM AND THE PROBLEMS OF LIFE 



resembles the bee, and the hornet is closely imitated by 

 the bee hawk-moth, which has assumed the transparent 

 wings, the shape, and the yellow abdominal bands of 

 its model. This imitation of living models is called 

 mimicry, and we find innumerable instances of it in the 

 tropics.^ 



The light colours that many insects show when they 

 are flying, and that are of great importance to them, 

 as it is by means of these that the sexes find each 

 other and maintain the species, are often different in 

 male and female. In the case of the dragon-flies the 



' The imitation of objects is very common among tropical insects. 

 There is a leaf-butterfly, the callima, the wings of which, when folded, 

 not only have the form of a leaf with a stalk, but even a long central 

 rib with side branches — looks, in fact, so much like a dried leaf that 

 it takes an expert to recognise the animal at rest. There are also 

 locusts that have wings most strikingly like leaves, and other locusts 

 that are almost indistinguishable from twigs; many naturalists have 

 thought them to be twigs when the natives brought them. 



Mimicry also is wonderfully developed in the tropics. In South 

 America there are black, yellow, and red butterflies, the heliconides, 

 which are not eaten by birds and reptiles on account of their repulsive 

 smell and taste. Other butterflies, originally white, have adopted both 

 the appearance and the habits — such as slow flying — of these malodor- 

 ous butterflies, and constantly mix amongst their models, which always 

 fly in swarms. Thus the white are protected as well as the heliconides, 

 though they have not the same nasty taste. 



In this case selection has coloured the females earlier and more 

 thoroughly than the males, some of which still have the pure white of 

 their ancestors on the hind wing, and some on the upper side of both 

 the front wings. Selection does not act as powerfully in the males as 

 in the females, and their transformation is slower. The males are 

 always more numerous than the females, as a matter of fact, besides 

 that one of them suffices for the fertilisatipn of a number of females ; 

 moreover, their death does not involve the destruction of a number of 

 eggs. If the species is to be preserved, it is the females especially 

 that must be cared for. 



