582 HEMIPTERA-HOMOPTERA CHAP. 



production of young by parthenogenetic females, the young so 

 produced becoming rapidly (sometimes in the course of eight or 

 ten days, but more usually in about twenty days) themselves 

 devoted to a similar process ; so that in the comparatively short 

 period of a few months the progeny resulting from a single 

 individual is almost innumerable. This remarkable state of 

 affairs is accompanied by other peculiarities of physiology, with 

 the result that the life-histories of successive generations become 

 very diverse, and complex cycles of series of generations differing 

 more or less from one another are passed through, the species 

 finally returning to l;)i-sexual reproduction, and thus inaugurat- 

 ing another cycle of generations. The surprising nature of 

 these facts has in the last 150 years caused an immense 

 amount of discussion, but no satisfactory light has yet been 

 thrown on the conditions that really give rise to the exceptional 

 phenomena. These phenomena are (1) parthenogenesis ; (2) 

 oviparous and viviparous reproduction : (.3) the production of 

 generations of individuals in which the sexes are very unequally 

 represented, males being frequently entirely absent ; (4) the pro- 

 duction of individuals differing as to the acquirement of wings, 

 some remaining entirely apterous, while others go on to the 

 winged form; (5) the production of individuals of the same sex 

 with different sexual organs, and distinctions in the very early 

 (but not the earliest) stages of the formation of the individual ; 

 (6) differences in the life-habits of successive generations; (7) 

 differences in the habits of individuals of one generation, giving 

 rise to the phenomenon of parallel series. All these phenomena 

 may occur in the case of a single species, though in a very 

 variable extent. 



The simple form of Aphid life may be described as follows : — 

 eggs are laid in the autumn, and hatch in the spring, giving 

 rise to females of an imperfect character having no wings ; these 

 produce living young parthenogenetically, and this process may be 

 repeated for a few or for many generations, and there may be in 

 these generations a greater or less number of winged individuals, 

 and perhaps a few niales.^ After a time when temperature falls, 



' There is some doubt on this point, as tlie earlier observers seem to have supposed 

 tliat a winged individual appearing in a generation oliiefly apterous was ipso facto, 

 a male ; it seems, however, to be certain that perfect Avinged males appear in some 

 species in generations producing no perfect sexual females. Speaking generally. 



