46 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



case of the mature flying individuals. Indeed even simpler 

 and more effective means can be brought to bear against the 

 eggs of the grasshoppers. But a knowledge of the times, 

 places and peculiar manner of the egg-laying of grasshoppers 

 was necessary as a basis for devising these remedies. Butter- 

 flies and moths take only plant nectar and water for food, and 

 are harmless in the adult stage. But in their immature 

 stage, as strong-jawed biting larvae (caterpillars) many kinds 

 are extremely injurious. The mosquito is annoying as a 

 blood-sucking pest and dangerous as a breeder and disseminator 

 of yellow and malarial fevers only as a full-developed flying 

 adult, but it is only in its immature or larval and pupal stages 

 passed in quiet water that it can be successfully fought. The 

 student of economic zoology then should give a special atten- 

 tion to the study of animal multiplication and development. 



Sex and the Fertilization of the Egg. Among the one-celled 

 animals we found that before an individual divided into two, 

 that is, multiplied, it sometimes met another individual with 

 which it exchanged body substance. Among most of the many- 

 celled animals the germ cell or fertilized egg cell which develops 

 into a new individual is produced by the fusion of two so-called 

 reproductive cells from two distinct individuals of the same 

 species or kind of animal. 



The reproductive cells produced by the females are known 

 as eggs or ova, and are usually produced in the ovaries; those 

 produced by the male are called spermatozoa and are produced 

 in the spermaries, or testes. Before the ova can begin their 

 development they must be fertilized by the spermatozoa. 

 There are a few exceptions to this general rule, young being 

 produced by some kinds of animals from unfertilized eggs. 

 But these cases are comparatively rare and in most of them 

 fertilization of some of the eggs, at least, takes place also. 



We shall find among the Metazoa various devices to aid in 

 bringing the ova and spermatozoa of two individuals of a kind 

 together. Many of the aquatic animals simply cast their 

 reproductive cells into the water where they meet by chance. 

 Of course many of the ova thus thrown out are never reached 

 by the spermatozoa and so no development takes place, but 



