150 EYENTN-GS AT THE MICEOSCOPE. 



cordingly emploj saws witli a narrow course ; while 

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

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

 quire the teeth to diverge much, but from the mannei 

 from which they operate, it is requisite that thej should 

 not stand like those of our saws in a straio-ht 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 y. 



" 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 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 fixed in the same frame. 

 Tliese, 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 fine, the effect is similar to [that of] a saw 

 with a wide set. 



" It is important, seeing that the ovipositor-saws 

 arc 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 



