vol. LXl.] PHILOSOPHICAL TRANSACTIONS. ]21 



even double that number: nor can he altogether agree with this ingenious 

 author, that there are a greater variety of plants producing aphides, than there 

 are different sorts of this insect. Where plants are of a like nature, they are 

 usually frequented by the same insects ; but many of these plants will be found 

 to support two or more quite different sorts. On the peach and nectarine indeed 

 the aphides are the same, and he did not find on these trees more than one sort. 

 The plum-tree, on the other hand, has 2 sorts, very distinct from each other ; 

 one of a yellowish green, with a round short body ; the other of a blueish green, 

 as it were enamelled with white, and the shape more oblong. On the gooseberry- 

 bush and currant the same aphides may be found ; but each of these is inhabited 

 by 2 verj' different sjiecies ; one being of a dusky green, with a short plump body ; 

 the other of a paler green, the body more taper, and transversely wrinkled. To 

 these instances he further adds, that the rose-tree supports not less than 3 dis- 

 tinct species : the largest of which is of a deep green, having long legs of a brown- 

 ish cast, with the joints of a very dark brown, as are also the horns and antennae ; 

 a 2d sort is paler green, has much shorter legs and a more flat body ; the 3d sort 

 is of a pale red, its body transversely wrinkled, and is most frequently on the 

 sweetbriar. 



The great variety of species, which occur in the insects now under consider- 

 ation, may indeed make an inquiry into their particular natures seem not a little 

 intricate and perplexed; having them however skilfully redticed under their proper 

 genus, the difficulty is by this means considerably diminished. All the insects 

 comprehended under any distinct genus, we may reasonably suppose to partake 

 of one general nature ; and by diligently examining any of the particular species, 

 may thence gain some insight into the nature of all the rest. With this view Dr. 

 R. has chosen, out of the various sorts of aphides, the largest of those found on 

 the rose-tree ; not only as its size makes it the more conspicuous, but as there 

 are few others of so long a duration. This sort, appearing early in the spring, 

 continues late in the autumn ; while several are limited to a much shorter term,' 

 in conformity to the different trees and plants whence they draw their nourishment. 



Sect. I. If, at the beginning of February, the weather happens to be so warm, 

 as to make the buds of the rose-tree swell and appear green; small aphides are 

 fi-equently to be found upon them, not larger than the young ones in summer, 

 when first produced. But there being no old ones to be found at this time of 

 the year, which in summer he had observed to be viviparous ; he was formerly 

 not a little perplexed by such different appearances, and was almost induced to 

 give credit to the old doctrine of equivocal generation. That the same kind of 

 animal should at one time of the year be viviparous, and at another oviparous^ 

 was an opinion he could then by no means entertain. This however frequenti 

 observation has at last convinced him to be fact; having found those aphidesy. 



VOL. XIII. R 



