THE APHIDES OR PLANT-LICE. 417 



ance of the Aphides in the spring till near the time 

 of their final disappearance in the autumn. At this 

 period, however, instead of these viviparous indivi- 

 duals, true males and females make their appearance 

 and copulate, when the females deposit eggs, which, 

 after lying dormant through the winter, reproduce 

 viviparous individuals in the spring. In this way 

 Bonnet traced the development of nine, and Duvau, 

 of eleven generations of viviparous Aphides, whilst 

 Kyber, by keeping a colony of these insects in a warm 

 room, continued this process of asexual reproduction 

 for no less than four years without the appearance of 

 a single male insect. For a long while these obser- 

 vations appeared so paradoxical that their accuracy 

 was doubted, and when there was no longer any ques- 

 tion as to the occurrence of the phenomena, phy- 

 siologists were sorely puzzled to account for such an 

 anomalous mode of generation. I need not refer to 

 the various theories which have been propounded with 

 this view, as it is now generally admitted to be an 

 example of what Steenstrup calls the " Alternation of 

 Generations," of which we have so many instances 

 amongst the lower animals. In this view, the vivi- 

 parous individuals are not females, but sexless nurses, 

 and the observations of various anatomists have shown 

 that they possess no ovaries, but that the living young 

 are produced by a process analogous to gemmation 

 in the interior of the abdomen. 



Besides the true Aphides, the tribe of the Plant-lice 

 includes a small group of insects in which the hinder 

 legs are formed for leaping, some of which are found 

 in great abundance upon various plants, and one of 

 them, the Psylla Pyri, is exceedingly injurious to 

 pear-trees. The larvae and pupae are generally clothed 



