440 On an Insect which is occasionally 
very considerable, as it was clear that they every year greatly 
enlarged the extent of the canker, not merely by devouring the 
neighbouring alburnum, but by forming numerous cells in it, 
which when quitted by the chrysales are filled with water by 
every shower, and thus become the source of more speedy and 
extensive decay. Many of the cankers in the tree above alluded 
to, have eaten half-way through the small trunk and branches, 
which if not sheltered bya wall must have been long ago broken 
off by the wind. 
This tree is a remarkable example of the effect of partial de- 
‘cortication, as recommended by Dr. Darwin (Phytologia, p.378), 
jn inducing the production of flower instead of leaf-buds. Not 
only the bark, but half the trunk, as above observed, is eaten 
through in many places; yet though a new twig is scarcely ever 
put forth, it never fails to be laden with blossom and fruit. Here 
‘I may observe that a similar result, as to the increased produce 
of fruit, and the paler green of the leaves, with that above re- 
ferred to by Dr. Darwin, I have myself seen on a branch of a 
‘pear-tree, from which nearly a complete cylindei of bark had 
been gnawed by cattle. It was filled with fruit, while not a pear 
‘was to be seen on the rest of the tree. ’ 
Narrower examination, however, has shown me, that their 
attacks are by no means confined to the diseased parts of fruit- 
trees; nor directed, as I at first conjectured, against the apple- 
tree only. Being more anxious to ascertain the ceconomy of an — 
injurious insect, than desirous of preserving the tree which they 
chiefly attacked, I took no steps for extirpating them; and they 
have, in consequence, seemed to increase every year since I first 
observed them, and last year carried on their operations so ex- 
tensively, as to threaten more serious injury in return for my 
forbearance, than I had calculated upon. Not only were they 
more than usually abundant near the margins of all the old can- 
‘kers, but J observed their masses of excrement adhering, in every 
‘direction, to the surface of the healthiest pear- and apple-trees 
‘in the garden; and wherever these indications appeaied, the 
application of the knife aways detected the caterpillar beneath. 
It is thus evident that, where they abound, no other cause is 
wanting to generate canker and disease. Though their attacks 
upon the bark ond alburnum should not at first be extensively 
injurious, the admission of water inta their empty cells, and fre- 
quent repetitions of the mischief, must, in the end, cause rotten- 
ness; and it is perhaps not improbable that to these insects 
should be often primarily attributed the canker laid to the charge 
of the soil, or the mode of cultivation. } 
After these prefatory remarks, I shall proceed to describe the 
insect in its different states, adding such observations as have 
occurred 
