216 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



difference between the methods of a carnivore and a parasite is simply 

 the difference between Hving upon capital and upon income ; between 

 the habits of the beaver, which cuts down a whole tree a hundred 

 years old, and the bark beetle, which levies a daily toll from the tissues 

 of the tree ; between the burglar and the blackmailer. The general 

 result is the same, although the methods employed are different." ^ 



Who Qualifies? 



Parasites vary greatly in their types of relationships. Some may 

 be classified as either internal or external, according to their location in 

 or upon the host. They may be otherwise classified as temporary, or 

 free-living during a part of their life cycles ; yermanent, or parasitic 



throughout their life span ; 

 and 'periodic, only visit- 

 ing their hosts to obtain 

 nourishment. Actually 

 there are almost as many 

 gradations and variations 

 in the degree of parasitism 

 among animals and plants 

 as there are kinds of par- 

 asites. Mosquitoes and 

 some fleas visit their hosts 

 just long enough to sat- 

 isfy their appetites. The 

 cattle tick, Boophilus an- 

 nulatus, never leaves its 

 host except when ready to 

 lay eggs. Scab mites and 

 some lice are permanent, 

 living upon the same host 

 from one generation to the next, only leaving or being transferred 

 by direct contact. In between these extremes occur such well-known 

 forms as the hookworm, which has a free-living larval stage, and 

 the botflies that pass their larval existence as parasites. 



Among plants, the large and heterogeneous group of bacteria exhibit 

 many varieties of parasitism, while higher in the plant scale such forms 

 as dodder and broomrape exemplify true parasitism. Other groups 

 are partially parasitic during their life cycle. 



■ From Elton, C, Animal Ecology. By permission of The Macmillan Company, 1935. 



Wright Pierce 



Dodder, an example of a plant parasite which 

 starts life as a self-respecting plant growing in 

 soil. 



