6 CIRCULAR 241, TJ. S. DEPARTMENT OF AGRICULTUBE 



All the species of leaf hoppers pass through a series of molts, 

 usually four or five in number, and in these various instars they show 

 a gradual progression toward the adult form. In some cases these 

 early forms are sufficiently like the later and adult stages to be recog- 

 nized as of the same species, but more commonly there is such a differ- 

 ence in their appearance that it is only by rearing them from stage 

 to stage or by careful comparison of the different stages that it is 

 possible to make out the correct life history. The molting occurs at 

 uniform periods and consists in the shedding of the entire epidermal 

 covering, tins usually remaining attached to the surface of the plant 

 as a thin, transparent film. The insects increase in size and soon 

 change from the light color of recent emergence to the darker or more 

 distinctive color common to the species. All. of course, pass through 

 the egg and nvmphal stages, and the last nymphal stage may be con- 

 sidered as corresponding to the pupal stage of insects in general. 



The number of generations in each season is also a variable matter. 

 but there are commonly two generations each season, in some species 

 probably three, and in a few it is known that only a single generation 

 occurs. This is, of course, an important factor in the economic im- 

 portance of the species, since each additional generation provides for 

 an immense increase in the numbers of individuals and also makes 

 much less effective the special conditions of culture required for the 

 crop on which it feeds. 



RELATION TO ENVIRONMENT 



The leaf hoppers constitute one element in a very complex associa- 

 tion of plants and animals, including birds, mammals, reptiles, toads, 

 insects, and spiders, and it is only by the recognition of this relation 

 that any very adequate appraisal of their proper place in nature and 

 of their importance in the economy of cultivation can be suggested. 

 Primarily they are associated with certain kinds of plants upon which 

 they depend for their sustenance, and the abundance of leaf hoppers 

 will be affected, necessarily, by the abundance of the food plant and 

 its availability as food material. An undue increase of the leaf hop- 

 pers, which should result in the diminution of the food supply, must 

 necessarily affect the possibilities of multiplication and cause a cer- 

 tain reduction in the number of insects. This is, however, by no 

 means the only relationship, for a large number of other organisms, 

 both plant and animal, will affect the problem. The occurrence of 

 different birds, spiders, and insects which prey upon the leaf hoppers 

 will naturally reduce their numbers and to that extent favor the 

 plants which serve as their food, whereas the presence of herbivorous 

 animals, grasshoppers, cutworms, etc.. serves to reduce the available 

 food supply. 



Aside from these dominant forms there are also various fungous 

 parasites which attack both insects and plants and which play their 

 part in the complex of which the leaf hoppers are such a conspicuous 

 element. Furthermore, the minute insect parasites which attack the 

 leaf hoppers add their part, tending to keep the latter reduced in 

 numbers. The relation of these and other direct parasites will be 

 considered under the general head of natural enemies. 



