CHAPTER XVII.

THE NATURAL HISTORY OF VARIOUS SPECIES OF SAWFLIES.

An account of the Ovipositor, and Sawing Apparatus-An account of the Rose Sawfly-Description of their Eggs, Larve, the Effect of Rain-Description of their Cocoon-their Perfect State-its ravages on Turnips.

Or all the admirably adapted instruments with which insects are furnished to enable them to accomplish the various ends of their existence, we know of none which can be compared, in the beauty of their construction, to the organs which are employed for the purpose of depositing their eggs by the females of a large family of insects, to which the French have very appropriately given the name of mouches-d-scie, and which we term sawflies; although, as we shall subsequently see, the formation of the organs in question is, as indeed might naturally be expected, much more complex and interesting in a mechanical point of view, than the tool from which the flies derive their name.

These insects belong to the same order (Hymenoptera) as the wasp and the bee, which they somewhat resemble; instead, however, of having the hind part of the body separated from the thorax by a very narrow waist, as we may term it, it is merely a continuation of the body without any visible contraction. Moreover, instead of being provided with a sting like these insects, it is plainly perceived that that instrument of terror is here replaced by the much more harmless, but not less effective instrument, to which we have already alluded.

Many of these insects are of a tolerably large size; and as there are at least two hundred and

fifty different species of them to be found in Great Britain, opportunities for investigating their habits and economy are not wanting to those who may be desirous of verifying our account of them. The perfect insects are generally found upon various kinds of plants, upon or within which the females deposite their eggs, and which they are instructed by instinct to place in small grooves formed in the bark, wood, or leaves of the plant, upon which the future progeny are to be nourished. It is necessary, therefore, that the female should be furnished with an instrument capable of making a proper incision, and we accordingly find the ovipositor admirably formed for this purpose. The under surface of the body exhibits near its extremity a narrow slit, formed of two horny, jointed grooves or sheaths, between which, when unemployed, the ovipositor or auger itself lies concealed; when, however, the insect wishes to deposite her eggs, she bends the extremity of the body downwards and inwards, protruding the instrument at an angle with the body from the base of the slit formed by the two grooves.

The ovipositor itself is composed of two broad but very thin saws, which are projected and withdrawn by the female during the act of cutting the channel in which the eggs are to be placed, by the assistance of strong muscles at the base of each saw. These saws do not act in concert together, but while one is pushed forward the other is withdrawn from the incision made in the stem or bark. This appears to us to be owing to the circumstance of the saw, which remains in the slit, serving the purpose not only of a saw, but also of a wedge, keeping the mouth of the channel open until the other saw is thrust deeper into the wound. These two saws are not straight, like handsaws used by carpenters, but are of a curved form, somewhat like a flattened S. Like the common handsaw, each of these saws is lodged in a distinct piece, formed of

two membranaceous plates, similar to the handle of a pocket-knife; which must, doubtless, be of great service, by giving support to the saws when in motion; but it is here observable, that instead of being fixed in these back plates, like the handsaw, each saw works backwards and forwards in its own groove. Moreover, the teeth of these saws, instead of being simple, like the carpenter's saw, are themselves still more finely toothed; each tooth, in fact, under a high-powered lens, considerably resembling the entire saw as seen with a lower power. It is in the last place to be noticed, that each of these saws has its outer flattened surface transversely scored and channelled; so that, as Messrs. Kirby and Spence well observe, while the vertical effect of this double instrument is that of a saw, it at the same time acts laterally as a rasp; and when, by the alternate motion of these saws, the incision or cell is made sufficiently deep, the two saws, receding from each other, conduct the egg between them into it. This is effected, according to Valisnieri, by means of two canals between the component membranes of the back-supports of the plates of the saws. When in action these saws do not, as might be supposed, cut two parallel courses, but work in the same cut; and, consequently, though the teeth are extremely fine, the effect is similar to a saw with a wide set.

The following is an account of the proceedings of the rose sawfly (Hylotoma rosa):-"In the fine days of summer, towards about ten o'clock in the morning, the female is seen traversing with eagerness all the branches of this shrub, one after the other. She usually rests on that one which is nearest to the extremity of the principal stem, and there makes an aperture with her saw: when she has judged that the hole is of suitable dimensions, she deposites an egg in the cavity; she then remains quiet for a few moments, always having her ovi

positor engaged in the branch; a moment after, she draws away quickly the largest part of it, and emits at the same time a frothy liquid, which rises as far as the external edges of the notch, and sometimes beyond them. Some authors have thought that the use of this liquid was to bedew and moisten the eggs, but Valisnieri believes that it serves to prevent the aperture from closing. Be this as it may, after the female has emitted this fluid, she withdraws her ovipositor, and proceeds to the fabrication of another hole. Sometimes she makes but four in a line, one after another; most frequently, however, she makes about a score. The part of the branch which is notched in so many places presents nothing remarkable the first day of the operation, and it is not until the following day that it begins to assume a brown colour; and in the sequel all the wounds become raised, and acquire more and more convexity every day. This growth is owing to the augmented volume of each egg, as it daily grows larger. It forces the skin of the branch upwards, and the aperture to grow larger. This last finally becomes considerable enough to give passage to the larva, which, in coming out of the egg, quits its retreat to seek the leaves of the shrub on which it is nourished."

The singular fact of the increase in size of the eggs after deposition, and previous to the bursting forth of the young, is quite at variance with the ordinary circumstance that the eggs of oviparous animals attain their full size previous to their being deposited. An analogous case, however, exists, not only among the gallflies and ants, but also in the eggs of fishes, whose size in like manner is said to increase previous to the exclusion of the young. Reaumur considered that it was in consequence of imbibing nutriment in some unknown manner through the membranous covering from the vegetable juices which surround them; the eggs at their

full size being nearly twice as large as when first deposited. We, however, suggest, that the growth of the enclosed larva is attributable rather to fluid nutriment contained within the egg than to any imbibition of fresh matter. The subject is, nevertheless, highly worthy of attention.

The larvæ which are hatched from these eggs are often to be noticed in great quantities, and are the most destructive to the vegetable world of any of the insects in the order to which they belong. The gooseberry, currant, raspberry, cherry, rose, &c. being often entirely stripped of leaves by various species; this is especially the case when several females, each of which deposites a very considerable number of eggs, have selected the same tree. When this takes place, and the leaves are, in consequence, entirely consumed, the appearance of these grubs is, of course, attributed to blight: the willow and birch are no less subject to their attacks, and even the wheat and barley are often seriously injured by them.

These larvæ greatly resemble in form the caterpillars of butterflies and moths, being of a cylindrical form, with a scaly head, furnished with very powerful jaws, and having three pairs of scaly legs attached to the first three rings of the body, and several other pairs of fleshy membranous legs (varying in number in the different species) attached to the hinder segments of the body. They may, however, be readily distinguished from the caterpillars of the Lepidoptera, by the peculiar attitude which they assume when disturbed, coiling themselves up in a spiral form, with the head in the centre of the coil, and lying motionless upon their sides. When feeding upon a leaf, they commence their attack on it upon that side which they hold between their six forelegs; they are extremely voracious, feeding very fast, and without intermission. The larvæ of many of the smaller species live