SEXUAL SELECTION 



217 



is an indication that similar colours in their fellows may awaken 

 similar agreeable sensations. 



This conclusion is furthermore confirmed by the fact that, in the 

 male sex, numerous species of buttei-fly possess another means of 

 exciting the females, namely, by pleasant odours. Volatile ethereal 

 oils are secreted by certain cells of the skin, and exhale into the 

 air through specially constructed scales. Usually the apparatus for 

 dispersing fragrance occurs on the wing in the form of the so-called 

 scent-scales {Duftschuppen), peculiar modifications of the ordinary 

 colour-scales of the wing, but sometimes they take the form of 

 brush-like hair-tufts on the abdomen, and they are in all cases so 

 arranged that the volatile perfume from the cells of the skin 



Fig. 53. Scent-scales of diurnal butterflies, a, of Pieris. h, of Argynnis 

 2iaphia. f, of a Satj-rid. d, of Lyetena. All highly magnified. 



penetrates into them, and then evaporates through very thin spots 

 on the surface of the scale, or through brush-like, expanded fringes 

 on their tips. Many of these have long been known to entomologists, 

 because their divergence in form from the ordinary scales attracted 

 attention ; and it was also observed that they never occurred on the 

 females, but only on the males. Their significance, however, remained 

 obscure until, by a happy chance, Fritz Miiller, in his Brazilian 

 garden, discovered the fact that there are butterflies which give off 

 fragrance like a flower, and then close investigation revealed to him 

 the connexion between this delicate odour and the so-called 'male 

 scales.' One can convince oneself of the correctness of the observation 

 even in some of our own butterflies by brushing the finger over the 

 wing of a newly caught male Garden White [Pieris napi). The 

 finger will be found covered with a white dust, the rubbed-ofl" wing- 



