OAK-Leaf Gall. These are of several kinds; the remarkable species called the mushroom gall is never found on any other vegetable substance but these leaves: and beside this there are a great number of other kinds.
The double gall of these leaves is very singular, because the generality of productions of this kind affect only one side of a leaf or branch, and grow all one way: whereas this kind of gall extends itself both ways, and is seen on each side of the leaf, in form of two protuberances, opposite the one to the other. These are of differently irregular shapes, but their natural figure seems that of two cones, with broad bases, and very obtuse points, though sometimes they are round, or very nearly so.
These make their first appearance on the leaf in April, and remain on it till June or longer. They are at first green, but afterwards yellowish, and are softer to the touch than many other of the productions of this kind: they are usually about the size of a large pea, but sometimes they grow to the bigness of a nut. When opened, they are found to be of that kind which are inhabited each by one insect only, and each contains one cavity. The cavity in this is, however, larger than in any other gall of the size, or even in many others of three times the size; the sides of it being very little thicker than the substance of the leaf.
It is not easy to ascertain the origin of the several species of flies which are at times seen in this manner to come out of the same species of galls. It seems the common course of nature, that only one species of insect forms one kind of gall; yet it may be, that two or three kinds may give origin to the same kind. There is, however, another occasion of our seeing different species come out of different galls of the same kind: and this is the effect of the enemies of the proper inhabitants.
It might appear that the parent fly, when she had formed a gall for the habitation of her worm offspring, had placed it in an impregnable fortress; but this is not the case; for it frequently happens, that a fly, as small perhaps as that which gave origin to the gall, produces a worm which is of the carnivorous kind, as the other feeds on vegetable juices. This little fly,
well knowing that where there is one of these protuberances on a leaf, there is a tender and defenceless insect within, pierces the sides of the gall, and deposits her egg within it. This, when it hatches into a worm, feeds upon the proper inhabitant; and finally, after devouring it, passes into the chrysalis state, and thence appears in the form of its parent fly, and is seen making its way out of the gall, in the place of the proper inhabitant.
On opening these leaf-galls, which are properly the habitation only of one animal, it is common to find two, the stronger preying upon the body of the other, and sucking its juices as it does those of the leaf: often it is found wholly employed in devouring its unoffending neighbour at once: this is probably the case when its time of eating is nearly over: and, in fine, when we find the gall inhabited by only one insect, or containing only one chrysalis, as it ought in its natural state to do, we are never certain that this is the proper inhabitant, as it may be one of these destroyers who has eaten up the other, and supplied its place. See APHIS, ENTOMOLOGY INDEX.
OAK Saw-dust is now found to answer the purposes of tanning, as well, at least, as the bark. See TANNING.
OAK of Jerusalem. See CHENOPODIUM, BOTANY INDEX.