The instrument thus brought into view is a very finely-contrived saw, made of a horny substance, 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 ovipositor-saw 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 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_. [Illustration: Ovipositor-saw of Saw-fly, with rasps shown in the cross lines.] 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 a saw with a wide set.