SAW-FLIES. 155 



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 forai 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 alternately. The secret, indeed, 

 of working more saws than one at once is not un- 

 known to our mechanics; for two or three are some- 

 times fixed in the same frame. These, liowever, not 

 only all move upwards and downwards simultane- 

 ously, but cut the wood m 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 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 a clasp-knife. These plates are thickest at the 

 base, becoming gradually thinner as they approach 

 the point which the form of the saws require. Ac- 

 cording 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.* 



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, 



* Reaumur, Mem. des Insectes, v. p. iii. 



