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 saws requires. According to Vallisnieri, 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 it has hollowed out for them. [Illustration: Portion of a Saw-Fly's comb-toothed rasp, and saw.] The teeth of a carpenter's saw, it may be remarked, are simple, whereas the teeth of the ovipositor-saw are 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, as may be seen in the figure. Of course, such observations are conducted with the aid of a microscope.