(890). In many insects, especially of the Hymenopterous order, the generative apparatus is terminated externally by peculiar instruments provided for the purpose of introducing the eggs into a proper situation. This is particularly remarkable in the Ichneumons, which deposit their ova in living caterpillars; and in the Saw-flies (Tenthredo), whose eggs are insinuated into the substance of the leaves, or even of the branches of trees. To describe all the contrivances employed for this purpose would lead us far beyond our prescribed limits: one example of an organ of this description must suffice.

(891). In Sireoc gigas (fig. 171) the ovipositor consists apparently of three pieces of considerable length, seen in the figure to project from the inferior margin of the abdomen. Of these pieces, two form a sheath enclosing a third, called the terebra, or borer, which in the Tenthredo contains two saws of extremely beautiful construction, as we learn from an account of them given by Professor Peck, and quoted by Kirby and Spence*. The original description, which it would be unpardonable to abbreviate, is as follows. "This instrument," says Professor Peck, "is a very curious object; and, in order to describe it, it will be proper to compare it with the tenon-saw used by cabinet-makers, which, being made of a very thin plate of steel, is fitted with a back to prevent its bending: the back is a piece of iron, in which a narrow and deep groove is cut to receive the plate, which is fixed. The saw of the Tenthredo is also furnished with a back; but the groove is in the plate, and receives a prominent ridge of the back, which is not fixed (to the saw), but permits the saw to slide forward and backward as it is thrown out and retracted.

The saw of artificers is single; but that of the Tenthredo is double, and consists of two distinct saws with their backs: the insect, in using them, first throws out one, and, while it is returning, pushes forward the other; this alternate motion is continued till the incision is effected, when the two saws, receding from each other, conduct the egg between them into its place".

Female generative organs of the Queen Bee: a a, ovigerous tubes; bb, oviducts; c, spermatheca; e, vagina, or common excretory duct.

Fig. 170. Female generative organs of the Queen Bee: a a, ovigerous tubes; bb, oviducts; c, spermatheca; e, vagina, or common excretory duct.

* Introd. to Entom. vol. iv. p. 161.

Sirex gigas.

Fig. 171. Sirex gigas.

(892). With respect to the number of eggs laid by insects, it varies in different species: the Flea, for example, lays about twelve, and many Diptera and Coleoptera average perhaps fifty; but others are far more prolific: among moths, for example, the Silkworm produces 500, and some from 1000 to 2000; the Wasp ( Vespa vulgaris) deposits 3000; the Ant (Formica), from 4000 to 5000. The Queen-bee is said by Burmeister to lay from 5000 to 6000; but Kirby and Spence consider that in one season the number may amount to 40,000 or 50,000, or more. Yet, surprising as this latter statement may appear, the fecundity of the Queen-bee is far inferior to that of the White Ant (Termes fatalis); for the female of this insect extrudes from her enormous matrix innumerable eggs at the rate of sixty in a minute, which gives 3600 in an hour, 86,400 in a day, and 2,419,200 in a lunar month. How long the process of oviposition continues in the termite is unknown; but if it were prolonged through the entire year, the amazing number of 211,449,600 eggs would proceed from one individual; setting, however, the number as low as possible, it will exceed that produced by any known animal in the creation.

(893). The Aphides, or plant-lice, furnish a remarkable instance of fecundity. In these insects it has been satisfactorily ascertained, by Bonnet, Lyonet, and Reaumur, that a single sexual intercourse is sufficient to impregnate not only the female parent, but all her progeny down to the ninth generation! The original insect still continues to lay when the ninth family of her descendants is capable of reproduction; and Reamur estimated that, even at the fifth generation, a single Aphis might be the great-great-grandmother of 5,904,900,000 young ones.

(894). The impregnated ova of the Aphis* are deposited at the close of summer, in the axils of the leaves either of the plant infested by the species, or of some neighbouring plant; and the ova, retaining their latent life through the winter, are hatched by the returning warmth of spring, giving birth to a wingless hexapod larva. This larva, if circumstances, such as warmth and food, be favourable, will produce a brood (or indeed a succession of broods) of eight larva? like itself, without any connexion with the male. In fact, no winged females have, at this season, appeared. If the virgin progeny be also kept from any access to the male, each will again produce a brood of the same number of aphides; and careful experiments have shown that this procreation from a virgin mother will continue to the seventh, the ninth, or the eleventh generation, before the spermatic virtue of the ancestral coitus be exhausted. In the last larval brood, individual growth and development proceed further than in the parent, and some individuals become metamorphosed into winged males, others into oviparous females.

By these the ova are developed, impregnated, and oviposited, and thus provision is made for disseminating the individuals, and for continuing the existence of the species over the severe famine-months of winter 1.

(895). This mode of reproduction is evidently referable to the nursing system of Steenstrup (§ 383); and inasmuch as, in the system of nursing, the whole advancement of the welfare of the young is effected only by a still and peaceful organic activity - is only a function of the vegetable life of the individual, so also all those forms of animals in whose development the nursing system obtains actually remind us of the propagation and vital cycle of plants. For it is peculiar to plants, and as it were their special characteristic, that the germ, the primordial individual in the vegetation or seed, is competent to produce individuals which are again capable of producing seeds or individuals of the primary form, or that to which the plant owed its origin, only by the intervention of a whole series of generations. It is certainly the great triumph of morphology that it is able to show how the plant or tree (that colony of individuals arranged in accordance with a simple vegetative principle or fundamental law) unfolds itself through a frequently long succession of generations into individuals becoming constantly more and more perfect, until, after the immediately precedent generation, it appears as Calyx and Corolla, with perfect male and female individuals, stamens (Staubblatter) and pistils (Fruchtblatter), and, after the fructification, brings forth seed which again goes through the same course.