I need scarcely say that some of the best examples of natural saws are furnished by those insects which are known to entomologists as Tenthredinidæ, and to the general world as Saw-flies. These insects are supplied by Nature with a pair of most remarkable saws, which aid them in depositing their eggs. Indeed, without these instruments, the whole race of Saw-flies would long ago have become extinct. They haunt almost every kind of tree and many plants, and one valuable plant, the Turnip, is so devastated by them, that whole crops are sometimes swept away. As, therefore, the knowledge of the life-history of any insect will tell us whether to protect or destroy it, and the best method of adopting either course, we will cast a hasty glance at some of our commonest Saw-flies, the instruments which they employ, the mode in which they use them, and the analogies between them and the saws made by the hand of man. In the first place, it must be observed that the use of these saws is to cut grooves in young bark, these grooves being the depositories of their eggs. It follows, therefore, that as a tolerably wide groove is needed, the saw-blade is a tolerably thick one, and the teeth set on the same principle as that which is employed in the saw-sword of the pioneer. When the microscope is applied to the cutting instrument of the Saw-fly, it reveals the fact that there are two horny saws, which work alternately in their grooves, and that they are strengthened by a thick plate of horn on their backs. The system of toothing is very complicated. Not only are the sides as well as the edges of the saws toothed, but each tooth is furnished with smaller teeth, after the fashion of the shark’s wonderfully effective cutting apparatus. These subsidiary teeth vary greatly in shape and size according to the species, and in some cases each tooth is quite a complicated structure. In _Trichiosoma lucorum_, for example, a bee-like insect, very common upon hawthorn, the teeth are extremely beautiful. It is difficult to describe them without diagrams, but I will try to give the reader an idea of them. Each tooth is somewhat of a lancet shape, but is not terminated by a single point. At the tip comes the secondary tooth, which is conical and stands on a footstalk. The cone, however, is not simple, but is made of some seven or eight cutting plates, each smaller than its predecessor, and the last being a sharp conical point. The reader may imagine how effective such a saw would be in cutting green wood, the toothed sides and the subsidiary teeth alike preventing the blades from clogging, while the alternate movement of the saws enables them to do double work in the same time.