248 NATURAL HISTORY. [CH. XVII. 



two membranaceous plates, similar to the handle 

 of a pocket-knife ; which must, doubtless, be of 

 great service, by giving support to the saws when in 

 motion ; but it is here observable, that instead of 

 being fixed in these back plates, like the handsaw, 

 each saw works backwards and forwards in its own 

 groove. Moreover, the teeth of these saws, instead 

 of being simple, like the carpenter's saw, are them- 

 selves still more finely toothed ; each tooth, in fact, 

 under a high-powered lens, considerably resembhng 

 the entire saw as seen with a lower power. It is 

 in the last place to be noticed, that each of these 

 saws has its outer flattened surface transversely 

 scored and channelled; so that, as Messrs. Kirby 

 and Spence well observe, while the vertical effect 

 of this double instrument is that of a saw, it at the 

 same time acts laterally as a rasp ; and when, by 

 the alternate motion of these saws, the incision or 

 cell is made sufficiently deep, the two saws, receding 

 from each other, conduct the egg between them into 

 it. This is efiected, according to Valisnieri, by 

 means of two canals between the component mem- 

 branes of the back-supports of the plates of the 

 saws. When in action these saws do not, as might 

 be supposed, cut two parallel courses, but work in 

 the same cut; and, consequently, though the teeth 

 are extremely fine, the effect is similar to a saw 

 with a wide set. 



The following is an account of the proceedings of 

 the rose sawfly {Hylotoma rosa) : — " In the fine 

 days of summer, towards about ten o'clock in the 

 morning, the female is seen traversing with eager- 

 ness all the branches of this shrub, one after the 

 other. She usually rests on that one which is 

 nearest to the extremity of the principal stem, and 

 there makes an aperture with her saw : when she 

 has judged that the hole is of suitable dimensions, 

 she deposites an egg in the cavity ; she then remains 

 quiet for a few moments, always having her ovi- 



