242 NATURE'S TEACHINGS. 



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 termi- 

 nated 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 



