PLANT LICE 



its gruesome chamber a lining of silk web; which done, it 

 lies down to rest and soon changes to a pupa. After a 

 short time it again transforms, this time into the adult 

 of its species, and the latter cuts with its jaws the hole in 

 the back of the aphid and emerges. 



In other cases, the dead aphid does not rest flat on 

 the leaf but is elevated on a small mound (Fig. no A). 

 Such victims have been inhabited by the grub of a re- 

 lated species, which, when full-grown, spins a flat cocoon 

 beneath the dead body of its host, and in this inclosure 

 undergoes its transformation. The adult insect then 

 cuts a door in the side of the cocoon (B), through which 

 it makes its exit. 



Insects that usurp the bodies of other insects for their 

 own purposes are called parasites. Parasites are the 



Fig. i 10. Aphids parasitized by a parasite that makes a cocoon beneath 



the body of the aphis, where it changes to a pupa and, when adult, 



emerges through a door cut in the side of the cocoon 



worst enemies that insects have to contend against; but 

 really they do not contend against them in most cases, 

 except in the way characteristic of insects, which is to 

 insure themselves against extermination by the number 

 of their offspring. The aphid colonies are often, how- 

 ever, greatly depleted during a season favorable to the 

 predacious and parasitic insects that attack them; but no 

 species is ever annihilated by its enemies, for this would 

 mean starvation to the next year's brood of the latter. 

 The laws of compensation usually maintain a balance 



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