The effect of the puncture of aphides on growing plants is strikingly
illustrated in the shoots of the lime-tree and several other plants,
which become bent and contorted on the side attacked by the insects, in
the same way that a shoot might warp by the loss of its juices on the
side exposed to a brisk fire. The curvings thus effected become very
advantageous to the insects, for the leaves sprouting from the twig,
which naturally grow at a distance from each other, are brought close
together in a bunch, forming a kind of nosegay, that conceals all the
colour of the sprig, as well as the insects which are embowered under
it, protecting them against the rain and the sun, and at the same time
hiding them from observation. It is only requisite, however, where
they have formed bowers of this description, to raise the leaves, in
order to see the little colony of the aphides,--or the remains of those
habitations which they have abandoned. We have sometimes observed
sprigs of the lime-tree, of a thumb's thickness, portions of which
resembled spiral screws; but we could not certainly have assigned the
true cause for this twisting, had we not been acquainted with the
manner in which aphides contort the young shoots of this tree.[GG] The
shoots of the gooseberry and the willow are sometimes contorted in the
same way, but not so strikingly as the shoots of the lime.
[Illustration: Shoot of the Lime-tree contorted by the punctures of the
_Aphis tiliæ_.]
Pseudo-Galls.
It may not be out of place to mention here certain anomalous
excrescences upon trees and other plants, which, though they much
resemble galls, are not so distinctly traceable to the operations of
any insect. In our researches after galls, we have not unfrequently
met with excrescences which so very much resemble them, that before
dissection we should not hesitate to consider them as such, and
predict that they formed the nidus of some species of insects. In more
instances than one we have felt so strongly assured of this, that we
have kept several specimens for some months, in nurse-boxes, expecting
that in due time the perfect insect would be disclosed.
One of these pseudo-galls occurs on the common bramble (_Rubus
fruticosus_), and bears some resemblance to the bedeguar of the rose
when old and changed by weather. It clusters round the branches in the
form of irregular granules, about the size of a pea, very much crowded,
the whole excrescence being rather larger than a walnut. We expected
to find this excrescence full of grubs, and were much surprised to
discover, upon dissection, that it was only a diseased growth of the
plant, caused (it might be) by the puncture of an insect, but not for
the purpose of a nidus or habitation. (J. R.)
[Illustration: Pseudo-gall of the Bramble, drawn from a specimen.]
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