296 Nutrients 



strate" is highly restricted in its occurrence, the distribution of the 



saprophyte will be correspondingly curtailed. 



Many illustrations of the control of distribution by the nutrient 

 factor for these various types of organisms will occur to the reader, 

 but perhaps the richest source of examples is found in the insect 

 world, as fully discussed by Brues (1946). Our common North 

 American walking stick, Diapheromera femorata, feeds generally on 

 the leaves of oaks, whereas a giant East Indian member of this insect 

 group eats the leaves of only a single species of Eugenia found in 



Sumatra. 



The degree to which insect pests on crop plants are monophagous 

 is an important consideration in the introduction of new crop species 

 and in the procedures to be adopted in pest control. The Colorado 

 potato beetle, which spread northward from its native home in 

 Mexico, feeds almost entirely on the foliage of the potato plant, and 

 it sooner or later moves into every new area in which potato farming 

 is begun. The cotton boll weevil crossed the Rio Grande from 

 Mexico about 1892, and by 1894 it had spread to six counties in south- 

 ern Texas. Advancing 40 to 160 miles a year, the weevil had infested 

 more than 85 per cent of the Cotton Belt of the United States by 

 1922. In view of the terrific destruction caused by this pest to the 

 cotton crop, it is fortunate that the insect is prevented by its mono- 

 phagous feeding habit from spreading to other crops or to native 



vegetation. 



Insects also provide examples of the high degree of specificity of 

 many parasite-host relationships-a matter in which nutrition is prom- 

 inently involved. The Mallophaga (biting or bird lice) generally re- 

 strict themselves to hosts of one, or of closely related, species. The 

 physiological basis for this specificity is strikingly illustrated by the 

 taxonomic relations of the lice that feed upon the bodies of the cow- 

 bird. This bird lays its eggs in the nests of other birds belonging to 

 no less than 158 species, where the young cowbirds would have every 

 opportunity to acquire the lice of the foster species if the infestation 

 depended primarily upon chance contact. Actually, however, the 

 lice of the cowbird are not those of the foster species but belong to 

 genera found on other blackbirds taxonomically related to the cowbird. 



The absence of certain minor constituents from the diet may have 

 far-reaching ecological consequences. Lack of sodium chloride in 

 the food causes deer, elk, and other ruminants to travel long distances 

 to salt licks. Big Horn sheep, driven by the advance of civiHzation 

 into parts of the Rocky Mountains where water is soft, must be pro- 

 vided with extra salt. A rich supply of lime in the food is required 



