Oar Apple, the popular name applied to certain large excrescences or galls found upon the leaf, stems, or tender twigs of different oaks, produced by the action of insects. The oak apple of Europe, to which the term more particularly applies, is an object familiar to every English school boy, and is produced by cynips terminalis. It is of the size of an ordinary apple, and is found quite commonly about Easter time on the tender shoots and twigs of the common European oak (quercus robur). At this time it presents the appearance of an ordinary codling that has been roasted, being of a pale, dingy buff color, of spongy consistence, and having an irregular and wrinkled surface. The American oak apple, which is its analogue, is produced on the leaf stem of the black oak (Q. tinctoria) by cynips q. spon-gifica. In both these instances the gall is produced in the same manner as the well known gall nut of commerce. (See Galls.) With her ovipositor, admirably adapted to the purpose, the female pierces the plant tissues, and therein consigns an egg together with a small quantity of a peculiar poisonous fluid. Under the influence of this fluid the gall rapidly develops, and is generally fully formed before the egg hatches. The egg is whitish in color and soft.
It invariably swells more or less by en-dosmosis of the surrounding juices, and the outer pellicle is so delicate that no shell is left in hatching; but the larva, or young gall insect, seems rather to be gradually transformed from the egg. This larva is whitish, very soft, and has an inconspicuous head and no legs. The body is more or less cylindrical, tapering to both ends, but more especially behind, and lies in a curved position within its cell. As the larva grows the gall substance around its cell hardens into a cream- or buff-colored shell, which partially separates from its surroundings. This separation may perhaps be in part explained by the absorption of digested matter, as no fasces are found in the cavity, and, if excreted and absorbed, they would naturally cause increased hardening, and lessen the influence of the plant immediately around the cavity. The pupa state is gradually assumed, and the fly attains perfection and remains in its cell for some time before eating its way out to liberty through the walls of its gall; all the transformations being less sudden than in the majority of insects, on account of the delicacy of the successive skins to be thrown off.
The American oak apple begins to develop as soon as the leaves put forth in spring, and when mature has a shiny, rather smooth, dingy buff-colored exterior, the space between the central cell and the external rind being filled with a drab-colored spongy mass, which becomes more solid and paler toward the centre. The insects issuing from this gall in early summer are of both sexes, and have been described as cynijis q. sjionr/ifica; those issuing from it in the fall, and which have been described as C. q. acicu-lata, are larger and otherwise different, and are all females. - There is another large gall found exclusively on the red oak (Q. rubra), which is called the bastard oak apple. It differs from the genuine American oak apple in having the central cell connected with the rind by slender radiating filaments instead of spongy matter. The insects produced from this gall, and described as cynips q. inanis, are undis-tinguishable from the bisexual flies produced in early summer from the genuine American oak apple; a fact of great biological significance, which indicates that these galls, though so very different in structure, may be specifically related. - These oak apples are (as are indeed all galls) the result of the combined action of an animal and a vegetal organism, and must necessarily cease to exist if either of the organisms which cooperate to produce them were swept from existence; yet the study of galls belongs to the entomologist rather than to the botanist, and those of this country have been investigated especially by Baron Osten-Sacken, H. F. Bassett, Prof. 0. V. Riley, and the late Dr. Harris and Mr. B. D. Walsh, whose discoveries present some of the most remarkable facts in insect biology, and afford striking examples of dimorphism, of parthenogenesis, and of alternation in generation.
Not all the insects found in oak apples are gall flies; parasitic insects deposit their eggs in the forming apple, and their larvae live at the expense of the grub of the true gall maker. Several species, called wquilines, devour the gall substance at the peril of the young of the true architect; while others take possession of the old and deserted galls.
Gall Fly magnified. The lines below show the natural length of body and wings.
American Oak Apple, showing- internal structure, the grub in the central cell, and the hole on the side through which the perfect fly issues.
Bastard Oak Apple.