You are doubtless aware that the little berries which

GALL-FLY, AND MECHANISM OF OVIPOSITOR.

look like bunches of green currants, often seen growing on

insects: stings and ovipositors. the oak, are not the proper fruit of the tree, but diseased developments produced by a tiny insect, for the protection and support of her young. Kirby and Spence tell us that the parent fly introduces her egg into a puncture made by her curious spiral sting, and in a few hours it becomes surrounded with a fleshy chamber. Mr. Yirey says the gall-tubercle is produced by irritation, in the same way as an inflamed tumour in an animal body, by the swelling of the cellular tissue, and the flow of liquid matter, which changes the organisation, and alters the natural external form.

Perhaps a still more charming example of animal mechanics is that' furnished to us by the Saw-flies (Tenthredinidce). These are very common four-winged insects of rather small size, many species of which are found in gardens and along hedges in summer, produced from grubs which are often mistaken for true caterpillars, as they strip our gooseberry and rose bushes of their leaves; but which may be distinguished from them by the number of their pro-legs, and by their singular postures; for they possess from eight to fourteen pairs of the former organs, and have the habit of coiling up the hinder part of their body in a spiral ring, while they hang on to the leaf by their six true feet.

These saw-fly caterpillars are produced from eggs which are deposited in grooves made by the parent fly in the bark of the tree or shrub whose future leaves are destined to constitute their food ; and it is for the construction of these grooves, and the deposition of the eggs in them, that the curious mechanism is contrived which I am now bringing under your notice.

Almost all our acquaintance with this instrument and the manner of its employment, we owe to the eminent French naturalist, Reaumur, and to his Italian contemporary, Valisnieri. Their details I shall first cite, as they have been put into an English dress by Kennie, and K then show you a specimen dissected out by myself, and point out some agreements and some discrepancies between it and them.

" In order to see the ovipositor, a female saw-fly must be taken, and her belly gently pressed, when a narrow slit will be observed to open at some distance from the anus, and a short, pointed, and somewhat curved body, of a brown colour and horny substance, will be protruded. The curved plates which form the sides of the slit are the termination of the sheath, in which the instrument lies concealed till it is wanted by the insect.

"The instrument thus brought into view is a very finely-contrived saw, made of horn, and adapted for penetrating branches and other parts of plants where the eggs are to be deposited. The ovipositor-saw of the insect is much more complicated than any of those employed by our carpenters. The teeth of our saws are formed in a line, but in such a manner as to cut in two lines parallel to and at a small distance from each other. This is effected by slightly bending the points of the alternate teeth right and left, so that one half of the whole teeth stand a little to the right, and the other half a little to the left. The distance of the two parallel lines thus formed is called the course of the saw, and it is only the portion of wood which lies in the course that is cut into sawdust by the action of the instrument. It will follow, that in proportion to the thinness of a saw there will be the less destruction of wood which may be sawed. When cabinet-makers have to divide valuable wood into very thin leaves, they accordingly employ saws with a narrow course ; while sawyers who cut planks use one with a broad course. The ovipositorsaw, being extremely fine, does not require the teeth to diverge much, but from the manner in which they operate, it is requisite that they should not stand like those of our saws in a straight line. The greater portion insects: stings and ovipositors. of the edge of the instrument, on the contrary, is towards the point somewhat concave, similar to a scythe, while towards the base it becomes a little convex, the whole edge being nearly the shape of an italic f.

" The ovipositor-saw of the fly is put in motion in the same way as a carpenter's hand-saw, supposing the tendons attached to its base to form the handle, and the muscles which put it in motion to be the hand of the carpenter. But the carpenter can only work one saw at a time, whereas each of these flies is furnished with two, equal and similar, which it works at the same time —one being advanced and the other retracted alternately. The secret, indeed, of working more saws than one at once is not unknown to our mechanics ; for two or three are sometimes fixed in the same frame. These, however, not only all move upwards and downwards simultaneously, but cut the wood in different places : while the two saws of the ovipositor work in the same cut, and, consequently, though the teeth are extremely fine, the effect is similar to [that of] a saw with a wide set [or course].

" It is important, seeing that the ovipositor-saws are so fine, that they be not bent or separated while in operation ; and this also nature has provided for by lodging the backs of the saws in a groove, formed by two membranous plates, similar to the structure of a clasp-knife. These plates are thickest at the base, becoming gradually thinner as they approach the point which the form of the saw requires. According to Valisnieri, it is not the only use of this apparatus to form a back for the saws, he having discovered between the component membranes two canals, which he supposes are employed to conduct the eggs of the insect into the grooves which she has hollowed out for them.

" The teeth of a carpenter's saw, it may be remarked, are simple, whereas the teeth of the ovipositor-saw are k 2 themselves denticulated with fine teeth. The latter also combines at the same time the properties of a saw and of a rasp or file. So far as we are aware, these two properties have never been combined in any of the tools of our carpenters. The rasping part of the ovipositor, however, is not constructed like our rasps, with short teeth thickly studded together, but has teeth almost as long as those of the saw, and placed contiguous to them on the back of the instrument, resembling in their form and setting the teeth of a comb. " *

Now look at this object which I have just extracted from the abdomen of a rather large female Saw-fly, of a bright green hue spotted with black. The first portion of the apparatus that protruded on pressure was this pair of saws of an y^-like figure. These agree in general with those described ; here is in each the double-curved blade, the strengthened back, the rasp-like jagging of the lateral surfaces, the teeth along the edge, and the secondary toothlets of the latter. All these essential elements we see, but there is much discrepancy in the detail, and many points not noticed ;—in part, doubtless, owing to its being another species which was under observation, and partly to the inferiority of the microscopes employed a hundred and fifty years ago to those we are using.

In the first place, the curve of the f is different, the convexity of the edge being towards the point and the concavity nearest the base. Then the strengthening does not appear to me a groove in which the saw plays, but a thickening of the substance of the back. Each main tooth of the saw in this case is the central point in the edge of a square plate, which appears to be slightly concave on its two surfaces, being thickened at its two sides, at each of which, where it is united to the following plate, it rises, and forms with it a prominent ridge running trans- * "Insect Architecture," 153.

versely to the course of the saw. Each of these ridges then forms a second tooth, as stout as the main edge-tooth, which, with the rest of the same series, forms a row of teeth on the oblique side of the saw, in a very peculiar manner, difficult to express by words. It is singular that this side of the saw should be studded with minute hairs, since those would seem to interfere with the action of the saw, or at least be liable to be themselves rubbed down and destroyed in its action. But their existence is indubitable ; there they are, pointing at a very acute angle towards the tip of the saw. The back edge of the implement bristles with many close-set hairs or spines, forming a sort of brush, but pointing in an opposite direction.

Each main tooth of the edge-series is cut into one or two minute toothlets on its posterior side (next the base

OUTER SAW OF SlW-Hl. a. A portiou more enlarged. of the saw) and about half-a-dozen on its opposite side (next the tip). The texture is clear and colourless where thin ; but in the thickened parts, as the main teeth, the transverse ridges, and the back, it is a clear amberyellow ; the strengthening back-piece deepening to a rich translucent brown.

There is, however, in this species of mine a second set of implements, of which the French naturalist, observant

as he was, takes not the slightest notice ; and his English commentator appears to have as little suspected its presence. This pair of saws that we have been looking at is hut the sheath of a still finer pair of lancets or saws, which you may see here. These are much more slender than the former, and are peculiar in their construction

I2TNEB SAW OF SAW-FLY.

Their extreme tip bears only saw-teeth, and these are directed backwards ; but one side of the entire length presents a succession of cutting edges, as if a number of short pieces of knife-blade had been cemented on a rod, in such a manner as that the cutting edges should be directed backwards, and overlap each other. The other lateral surface is plain, and both are convex in their general aspect. The appearance of these implements is very beautiful, for the texture is of a clear pale amber ; but the structure is strengthened by a band which runs along each edge, and by transverse bands crossing at regular intervals, of a denser tissue ; and these are of a rich golden translucent brown.

From the construction of this implement I should infer that its force is exerted by pulling and not by pushing ; the direction of the teeth and of the cutting plates shows this. The sharp horny point is probably thrust a little way into the solid wood or bark, and then a backward pull brings the teeth and cutting plates to act upon the material, and so successively. And probably these points are the first parts of the whole apparatus that come into operation j the blunter saws of the sheath serving mainly

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