BUGS. 221 



change which took place in their daily lives through all that time. They were his 

 pets, and absorbed his attention as thoroughly as if they belonged to some higher 

 group. He informs us that he took precautions to isolate a single female plant-louse 

 at the beginning of her young life, and to follow her through to the end. He found 

 that she changed her skin four times within a period of nine days, and was then a full- 

 grown insect. On the tenth day she deposited a living young one, and each day con- 

 tinued to add others until, at the expiration of three weeks, ninety-five little ones had 

 been brought forth. Pursuing the subject, he found that these young ones grew up 

 and also brought forth living young, and these in turn others, until by the end of the 

 third month he had secured five distinct generations of plant-lice, each like the original 

 mother. He was not satisfied to stop there, but repeated the experiments with other 

 and widely different species which lived upon dissimilar plants. This he continued to 

 do until the return of cold weather, in November, when he found that both sexes were 

 finally developed, which united for the production of fertilized eggs. 



Later investigators have pursued this subject still farther, and have shown that the 

 fertilized egg laid in the autumn hatches out, about the time of leaf-budding, in the 

 following spring, a female stem-mother which starts anew the life-cycle of her species 

 for the remainder of the year. 



The three stages of existence, usually called larva, pupa or nymph, and imago, the 

 adult insect, are scarcely as distinctly marked here as in the higher families of the 

 Homoptera, and in the unwinged forms there is no appreciable difference but size to 

 separate the intermediate stage from the adult. 



From recent verifications of the investigations of older authorities, we are enabled 

 to state with confidence that there are winged females which produce only living 

 young ; that very rarely has the winged female been found to lay eggs ; that un- 

 winged females produce living young, which are either winged or unwinged, and that 

 at the return of cold weather a brood of wingless females arises, which unites with 

 winged males and deposits fertile eggs. The male appears late in the season, and in 

 the species which live above the earth is almost always winged. Exceptions to these 

 statements will be found in some of the lower forms, such as the Chermesina and their 

 near allies, where the oviparous females are either winged or unwinged in the same 

 cluster. 



Not only do we find such differences of nature among the sexes, but besides these 

 there is also sometimes an important modification of form, as well as color, within the 

 limits of a cycle of generations. Thus in the European Chaitophorus aceris two kinds 

 of larvae are developed and brought forth at nearly the same time. The one is a nor- 

 mal Aphis, similar to its mother, of a brown color, and garnished with little tufts of 

 hair ; but the other is bright green, with a broad head, lobate behind and fused with 

 the thorax, the back and sides decorated with four series of plate-like attachments, 

 which give the insect an appearance somewhat like the carapace of a tortoise. In- 

 stead of the hairs seen in the normal young, the margins of the body and the limbs are 

 furnished with a series of flat transparent leaflets, in which ramify a system of radia- 

 ting vessels. Similar folioles also arise from the basal joint of the antennae, and more 

 slender ones sprout from tubercles on the end of the venter. Not being gregarious 

 like its sisters, it remains solitary, fixed to the inner angles of the leaf-ribs of the syca- 

 more or maple, and so continues for the four months or more allotted to its existence. 

 Dimorphism of such an abnormal character is scarcely to be met with in any other 

 group of the insects ; and it is especially striking here, occurring as it does in the 



