OTHER THEORIES OF SEXUAL COLOURING 333 



are best seen when the insect is looked at from in front, 

 as it would be by a female when the male is approaching 

 her. Mr. Wallace, however, argues that the msile follows 

 the female and hovers over her, so that she can hardly 

 see the upper side of his wings at all. We know but 

 little of the way in which an insect sees, but the struc- 

 ture of the eye as a large rounded mass made up of 

 radiating elements renders it probable that any object 

 which comes within the area obtained by prolonging 

 the radii will be seen, provided it is at the right dis- 

 tance. Hence the male would be seen approaching 

 the female from behind, in front, or the side, and the 

 only requisite for producing the best impression upon 

 her is that his head shall be towards her, and that the 

 upper side of the wings shall be seen. The courtship 

 of a butterfly usually passes through three stages : in 

 the first, the male sees the female and approaches 

 her ; in the second, they fly together for a variable 

 distance, fluttering around and about each other, 

 although the male is probably the more active and 

 the pursuer ; in the third, the female has been over- 

 come by the attentions of the male, she no longer 

 flies, but settles on the ground or a leaf, while the- 

 male flutters over her and finally settles also. In 

 each of these phases the planes of both body and 

 wings are ever shifting, and the upper side of the 

 latter is certainly visible to the female from time to 

 time. It is therefore most significant that the irides- 

 cent colours of Diadema holina should be seen from 



