TOL. LXI.] PHILOSOPHICAL TRANSACTIONS. 'i^ 



soonest to their growth; which Dr. R. has sometimes observed to happen in 10 

 days, where warmth and plenty of nourishment have mutually conspired. From 

 which considerations, he is thoroughly convinced, that these various coverings 

 are not connate with the insect ; but that they are, like the scarf-skin, successively 

 produced. 



Early in the month of June, some of the 3d generation, which were produced 

 about the middle of May, after casting off their last covering, discover4 erect wings, 

 much longer than their bodies : and the same is observable in all the succeeding 

 generations, which are produced during the summer months; without however 

 distinguishing any diversity of sex, as is usual in several other kinds of insects. 

 For some time before the aphides come to their full growth, it is easy to disco- 

 ver which of them will have wings, by a remarkable fullness in the breast, which 

 in the others is hardly to be distinguished from the body. When the last cover- 

 ing is rejected, the wings, which were before folded up in a very narrow compass, 

 gradually extend themselves in a most surprizing manner, till their dimensions 

 are at last very considerable. But these winged ones have this further peculiarity, 

 that the number of them does not seem so much to depend on their original 

 structure, as on the quantity or quality of the nourishment with which they are 

 supplied : it being frequently observable, that those on a succulent shoot have 

 few or none with wings among them ; while others of the same generation, on a 

 less tender branch, are most of them winged: as if the first rudiments of the 

 wings were composed in the former, while nature thought proper to expand them 

 in the latter, that they might be more at liberty to supply their wants. 



The increase of these insects in the summer time is so very great, that by 

 wounding and exhausting the tender shoots, they would frequently suppress all 

 vegetation, had they not many enemies which restrain them. To enumerate the 

 variety of other insects, that in their worm and fly state are constantly destroying 

 them, would exceed the bounds of the present design : there is one however so 

 singular in the manner of executing its purpose, that he cannot pass it by with- 

 out soine further notice. This is a very small black ichneumon fly, with a slender 

 body, and very long antennae ; which darts its pointed tail into the bodies of the 

 aphides, at the same time depositing an egg in each. This egg produces a worm, 

 which feeds on the containing insect, till it has acquired its full growth ; when 

 it is usually changed to that kind of fly from which it had its origin. In this 

 however it is sometimes prevented by another sort of small black fly, which 

 wounds this worm through its pearl-like habitation ; and by laying one of its eggs 

 in it, instead of the former fly, produces its own likeness. 



Dr. R. however further observes that notwithstanding these insects have many 

 enemies, they are not without friends ; if we may consider those as such, who are 

 very officious in their attendance, for the good things they expect to reap by 



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