28 



HEREDITY AND SEX 



of the mallard duck has a green head and a reddish 

 breast (Fig. 16), while the female is streaked with brown. 

 In insects the males of some species of beetles have 

 horns on the head that are lacking in the female (Fig. 

 17). The males of many species of butterflies are col- 

 ored differently from the females. 



FIG. 14. White-booted humming bird, two males 

 and one female. (After Gould.) 



The phosphorescent organ of our common firefly, 

 Photinus pyralis, is a beautiful illustration of a second- 

 ary sexual character. On the under surface of the male 

 there are two bands and of the female there is a single 

 band that can be illuminated (Fig. 18). At night the 

 males leave their concealment and fly about. A little 

 later the females ascend to the tops of blades of grass 



