4 



Insects and Disease 



parasitic life has been one of degeneration. While 

 they may be specialized to an extreme degree in 

 one direction they are usually found to have some 

 of the parts or organs, which in closely related 

 forms are well developed, atrophied or entirely 

 wanting. As a rule this is a distinct advantage 

 rather than a disadvantage to the parasite, for 

 those parts or organs that are lost would be use- 

 less or even in the way in its special mode of life. 



Then, too, the parasite often gives up all its inde- 

 pendence and becomes wholly dependent on its 

 host or hosts not only for its food but for its dissem- 

 ination from one animal to another, in order that 

 the species may not perish with the host. But in 

 return for all this it has gained a life of ease, free 

 from most of the dangers that beset the more inde- 

 pendent animals, and is thus able to devote its 

 whole time and energy to development and the 

 propagation of the species. 



We are accustomed to group the parasites that 

 we know into two classes, as harmful or injurious 

 and as harmless, the latter including all those 

 kinds that do not ordinarily affect our well-being in 

 any way. But such a classification is not always 

 satisfactory or safe, for certain organisms that to- 

 day or under present conditions are not harmful 

 may, on account of a great increase in numbers or 



