THE STRUGGLE FOR LIFE 



217 



sit ism: the external parasite, as the sheep-tick, or the 

 Lird-louse, external parasites spending the whole exist- 

 ence upon the host; the ichneumon-fly, depositing its 

 egg- upon the back of some caterpillar wherein the ich- 

 neumon larva 1 will dwell until pupation, then to emerge 

 as a winged insect, an internal parasite for part of its 

 existence. The advantages gained by the parasite are 

 great, abundant food, safety, and warmth, all neces- 

 sities of its natural well-being. There are, however, 

 disadvantages. The parasite tends to degenerate 

 through disuse of organs. The sheep-tick was once a 

 winged fly, but since it spends its whole life upon the 

 same animal its wings were no longer used, and conse- 

 quently were less and less developed. The female scale 

 insect after settling upon plants becomes simply a 

 living sac, a footless, headless grub, capable of digestion 

 and reproduction. Parasitic insects that depend ex- 

 clusively upon certain forms for existence lay them- 

 selves liable to great reduction in numbers, even to ex- 

 tinction. This is likely to occur should their host 

 become greatly reduced in numbers, either through the 

 attacks of the parasites themselves, or through other 

 causes. 



( 'hief among parasitic insects are the Hymenoptera. 

 These prey largely upon vegetable-feeding insects, by 

 dwelling as footless grubs in the bodies of the hosts, sub- 

 sisting upon the so-called blood of the insect. 



Among Hymenoptera there are some parasitic upon 

 plants, and others are parasitic upon eggs of other in- 

 sects. This subject of insect parasitism is of so great 

 biologic importance as to be of vital interest to man 



