332 THE COLOURS OF ANIMALS 



Sexual Colours are not developed on parts of the body 

 which move so rapidly that they become invisible 



Another fact of the same kind has only suggested 

 itself to me lately. The bright colours of courtship 

 are especially characteristic of two groups of animals, 

 birds and insects, and it may not unreasonably be sup- 

 posed that this fact is related to the convenient frame- 

 work afforded by the surface of the wings. In each 

 group we may distinguish two kinds of flight : in one 

 it is produced by an excessively swift vibration of the 

 wings, in the other by a relatively slow flapping move- 

 ment. In the former, including the humming-birds 

 and the majority of insects, the wings are quite 

 invisible, owing to their rapid motion ; in the latter, 

 including the majority of birds and butterflies and 

 many moths, they can be easily seen. We find, as a 

 general rule, that the colours distinctive of sex are 

 displayed on the wings in the latter group, but are 

 absent from the wings in the former. Facilities for 

 female observation are thus afforded by the distribu- 

 tion of colour. 



When colours are best seen from one direction, this 

 corresponds with the position in which the female 

 would see them 



Again, the magnificent iridescent colours on the 

 wings of certain butterflies, due to interference of light, 



