HEMIPTERA 1 57 



als that reproduce in this way. Usually, the agamic generation pro- 

 duced by the winter eggs is wingless. The agamic female which 

 hatches from a winter egg, being the starting point from which arise 

 the generations that intervene between this egg and the production 

 of other true eggs, is termed the stem-mother. 



The offspring of the stem-mothers are wingless or winged or 

 both, and are agamic. In many cases they are born alive. This 

 can be seen by examining almost any colony of plant-lice during the 

 summer-time. While an agamic mother is unconcernedly feeding 

 or walking about, it may be giving birth to a young louse ; the lat- 

 ter can be seen with the unaided eye, but better with a lens, emerg- 

 ing from the caudal end of its mother, tail first, and kicking vigor- 

 ously, even before its head has been delivered. In other cases, the 

 agamic form produces egg-like bodies, which are termed pseudoi'a, to 

 distinguish them from the fertilized or true eggs. And, in still other 

 cases, they produce living young, which are enveloped in a pellicle, 

 from which they emerge in the course of a few minutes ; such an 

 enveloping pellicle with its enclosed young is also termed a pseudo- 

 vum. 



The number of agamic generations that may follow without the 

 intervention of sexual forms varies with different species, and, in 

 some cases at least, varies in the same species, depending upon tem- 

 perature and other conditions. Thus Kyber, in the earl) - part of 

 this century, succeeded, by keeping the insects in a warm room, in 

 raising a series of agamic generations of two species of Aphids, 

 which extended through four years without the intervention of sex- 

 ual forms. 



As already indicated, the agamic generations are of two forms, 

 wingless and winged. Each of these has a peculiar function in the 

 economy of the species. The wingless generations, which are usu- 

 ally the more numerous, by their great fecundity provide for the 

 enormous and rapid multiplication of individuals, which is so charac- 

 teristic of these insects. But this great increase of individuals would 

 be disastrous to the species, by the destruction of the infested 

 plants and the consequent starving of the insects, were it not sup- 

 plemented by other powers. We find, therefore, interspersed among 

 these wingless sedentary generations, generations which are winged 

 and migrating. Thus the spread of the species is provided for. 



Generally on the setting in of cold weather, or in some cases on 

 the failure of nourishment, the weather being still warm, there is 

 produced a generation including individuals of both sexes. The 

 ii 



