218 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



development of the host mechanism which elaborates specific pro- 

 tective substances called antibodies, and secondly, through "the 

 adaptation of certain normally phagocytic cell groups to the intern- 

 ment and gradual destruction of the parasite." ^ 



Sometimes the introduction of a parasite has a visible effect upon 

 the host. In the case of certain gall insects, such as the Cynipoidea, 

 the deposition of an egg by the female in a plant tissue, or the subse- 

 quent movement of the larva, furnishes the stimulus which causes 



Wright Pierce 



Unopened oak gall beside one which has been opened to show the enclosed 



larvae. 



abnormal proliferation of tissue, resulting in the enclosing of the insect 

 larva and the production of a so-called gall. The type of gall pro- 

 duced on a given plant appears to be specific, whether it occurs in 

 root, leaf, twig, or stem. Usually a gall ceases to grow about the 

 time when the enclosed larva finishes feeding. In such instances it 

 dries and forms a protective covering inside of which the insect 

 pupates, ultimately gnawing its way out. 



Effects of a Parasitic Life 



Parasitism as a biological phenomenon probably has a more far- 

 reaching effect upon the structure of the parasite than upon the host. 

 In the first place the former no longer has to worry about locomotion 

 or the securing of food because these two important functions are 

 taken care of by the host. Consequently a gradual simplification 

 of the organs of a parasite takes place, until in forms like the tape- 



> Smith, T., Parasitism and Disease, Princeton Univ. Press, 1934, p. 111. 



