40 BUTTERFLIES 



to determine the principal facts in regard to the subject. 

 Our common Graylings and numerous species of Angle- 

 wings are so abundant and easily observed that they offer 

 splendid opportunities to any one who will undertake a 

 serious study of the subject. 



Parasitic Enemies 



All three of the earlier stages of butterflies — egg, larva, 

 and chrysalis — are subject to attack by various parasitic 

 insects which develop at the expense of the host. Such 

 parasites are probably the most important check upon the 

 increase of butterflies, and along with birds, mammals, 

 and bacterial diseases, they help to keep up that balance of 

 nature which in the long run maintains a surprising uni- 

 formity in the numbers of each kind of butterfly. 



For the most part these insect parasites are small four- 

 winged flies, although many of them are two-winged flies. 

 In either case the life stages show a series of changes much 

 like those of the butterflies themselves. Each parasite 

 exists first as an egg, second as a larva, third as a pupa, and 

 fourth as an adult fly. The larval stage, however, is 

 simply that of a footless grub which Hves within the body 

 of its victim absorbing its life blood and gradually kilhng it. 



The parasites of butterfly eggs are legion. They are 

 tiny flies whose life-story in briefest summary is this : The 

 butterfly lays an egg. The parasite fly finds this egg soon 

 after it is laid, and pierces the shell with her tiny, sharply 

 pointed ovipositor and deposits inside of the shell her own 

 microscopic egg. This egg within the egg soon hatches 

 into a curious little larva that develops at the expense of 

 the contents of the butterfly egg shell, and soon absorbs 



