514 ZOOLOGY 



caused by the unsegmented worms. The "hot" diseases of 

 horses, sheep, and cattle are produced by the larval stages of 

 flies. Mange, itch, etc., as found in various domestic animals, 

 are due to the action of mites. These also attack poultry. 



508. Animals Hurtful to Plants and Plant Products. — At 

 this point animals, more particularly the insects, do great 

 damage to man's interests. The story is too long to tell here, 

 but insects attack plants at every stage of their history from 

 the time they germinate until the time they are stored. They 

 devour foliage, fruits, timbers, seeds, stored grain, and manu- 

 factured products. It is said that "the elm has eighty species 

 of insects that are more or less supported by it; birches and 

 maples, over one hundred; com is attacked by about two 

 hundred species, of which fifty do notable injury and some 

 twenty are pests; apple insects number some four hundred 

 species." 



It is estimated that the farmers and fruit growers of America 

 alone lose above 500,000,000 dollars annually from the ravages of 

 insects on crops and forests. Man has done much to control 

 most animals except the insects, and something to control them. 

 The most effective means thus far found include spraying plants, 

 to keep away the females when about to lay, or to kill the young; 

 the rotation of the crops in such a way that the insects may hatch 

 without finding the kind of food on which they depend ; and the 

 introduction of animals, either predatory or parasitic, that attack 

 the hurtful species. 



Occasionally through a period of years such animals will 

 multiply to such an extent as to become a plague, when in 

 ordinary years they merely reduce the returns of the agricultur- 

 alist. Such are the outbreaks of the Rocky Mountain locust, 

 the Hessian fly, the San Jos6 scale, the chinch bug, and the like. 

 Less frequently similar things happen in respect to other animals, 

 as the plague of rabbits in Australia, and of the rats in the 

 Bermudas. 



509. Domestication of Animals. — Occasional reference has 

 been made in the preceding paragraphs to the domestic animals. 

 This means something quite different from the merely useful 

 animals. This term refers to the control by man of the life 



