150 EVENINGS AT THE MICEOSCOPB. 



cordingly employ saws with a narrow course ; wMle 

 sawyers who cut planks, use one with a broad course. 

 The ovipositor-saw being extremely fine, does not re- 

 quire the teeth to diverge much, but from the manner 

 from 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 

 contraiy, is towards the point somewhat concave, sim- 

 ilar to a scythe, while towards the base it becomes a 

 little convex, the whole edge being nearly the shape of 

 an italic/". 



" 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 

 cai-penter. 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 al- 

 ternately. The secret, indeed, of working more saws 

 than one at once is not unknown to our mechanics ; for 

 two or three are sometimes flxed in the same frame. 

 These, however, not only all move upwards and down- 

 wards 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 flne, the effect is similar to [that of] a saw 

 with a wide set. 



" 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 



