Sexual Modifications 325 



plexity, are developed on the pronotum of certain 

 Chafers in the male only. In cases where one sex is 

 without wings, it is almost always the female which 

 has lost them, and degeneration in this sex has gone 

 still farther in many insects. A female Psychid moth 

 in outward form resembling a grub spends her life 

 within the pupa-case, and the female Stylops — a legless 

 and headless maggot — passes her whole existence with- 

 in the body of a bee ; while the winged males of both 

 insects fly freely through the air. Among Butterflies 

 and Dragonflies the wings of the male insect are often 

 more brilliantly coloured than those of the female ; 

 many male Lycaenidae for example, as described in 

 Chapter III (p. 132, etc.), have bright blue wings 

 while their females are dark brown. 



Various suggestions have been made as to the cause 

 of these sexual differences. By some naturalists they 

 are believed to be the direct result of the essential 

 natures of the sexes. The small and active sperm- 

 cell with a more abundant vitality than the passive 

 egg-cell, dissipates energy while the egg-cell stores it 

 up. So it is suggested that the male animal is, in his 

 whole nature, more vigorous than the female. The 

 acute senses, swift movements, and special adornments 

 of form and colour found in male insects, are believed 

 to be simply the necessary expression of masculine 

 activity, while the wingless grub-like females of 

 psychid moths and bee-parasites show feminine 

 passivity in an extreme form (191). 



The colours and ornaments of male animals are 

 also explained as due to the selection through many 

 generations by the females of the most attractive 

 mates, those most richly and beautifully adorned thus 

 transmitting their qualities to their offspring (190). 

 Among some animals — birds and spiders for instance — 

 in which a prolonged courtship and a display by the 



