246 SEXUAL DIMOEPHISM 



shady places reflected from the bark of trees or such dull- 

 coloured surroundings. 



Darwin mentions the fact that in the Bombycidae and 

 Noctuidre when at rest the fore wings overlap and conceal 

 the hind wings, and these are exposed only during flight. In 

 accordance with this the hind wings are often brightly coloured. 

 The yellow under- wing, Tryphama pronuba, is an illustration 

 of this. It often flies about during the day or early evening 

 and its yellow hind wings are bright and conspicuous. 

 Species of Catocala offer other examples of the same condi- 

 tion. The upper surface of the hind wings are coloured red 

 and black, and are therefore very conspicuous, but when the 

 insect settles the hind wings are entirely covered with the 

 fore wings, the dorsal surface of which is protectively coloured 

 with zigzag markings of various shades of gray, so that they 

 are indistinguishable on surfaces of bark or rock. This is 

 shown in the figures of Catocala electa. 



In some butterflies a similar result is attained by a 

 method exactly opposite. The wings are held vertically 

 when the insect is at rest, and the fore wing is covered by 

 the hind wing : the under surface of the hind wing is then, 

 protectively coloured, and the insect at rest is only to be 

 detected with difficulty. An interesting account of this 

 protective resemblance in Satyrus semele will be found in 

 Barrett's Lepidoptera of the British Islands, vol. i. p. 223. 

 This insect settles habitually on the ground, and the under 

 side of its hinder wings varies in colour in some degree 

 according to the colour of the soil on which it usually rests ! 

 In its usual haunts on heaths and hill-sides this surface of 

 the wings is always dark, like the earth or rock ; in chalky 

 districts the colour is mottled with white, producing in an 

 extraordinary degree the mottled whitish appearance of 

 weathered and stained chalk. The selectionist explains all 

 the above facts by the utility of the different colorations. 



