TRACHEATES I 95 



female body is usually of a greenish shade, while the 

 males prefer bluish colours ; in the Libellula depressa 

 this passes into white, which is the colour of the thick 

 abdomen in this species. In others the males have 

 beautiful dark blue wings, which give them a fairy-like 

 appearance, while those of the females are colourless. 

 Among the butterflies the argus and small copper butter- 

 flies have their pretty light colouring only in the male 

 sex ; the females have an indifferent appearance. 



Other " masculine characteristics" are developed 

 among the insects. In many butterflies the males 

 give out a strong scent ; it is a charming coincidence 

 in nature that the butterflies that seem, in form and 

 colour, to be the flowers of the animal world, have also 

 very often the perfumes of their models. One can 

 easily perceive this odour by holding to one's nose a 

 male cabbage-butterfly, which differs from the female 

 by the absence of the black spots and borders on the 

 wings ; it gives out an agreeable pungent odour. The 

 argus butterflies, the mother-of-pearl, the convolvulus- 

 sphinx, and many others, have a male scent. In all 

 cases the odour is restricted to special scales. The dust 

 of the butterflies, which is so easily brushed off with 

 the hand, consists of very delicate and tiny scales, which 

 are for the most part connected with small glands. 

 Some especially large glands secrete an odorous matter, 

 and let it pass into a peculiarly shaped scale which is 

 called a scent-scale. These scales are often closed by 

 special structures, so that they only pour out their 

 perfume at the will of their owner. It appears, therefore, 

 that in this case the perfume is really meant for the 



