INTERNAL ANATOMY OF INSECTS. 161 



which is fixed : the saw of the Tenthredo\s also furnished 

 with a back, but the groove is in the plate, and receives 

 a prominent ridge of the back, which is not fixed, but 

 permits the saw to slide forward and backward as it is 

 thrown out or retracted. The saw of artificers is single, 

 but that of the Tenthredo is double, and consists of two 

 distinct saws with their backs : the insect in using them, 

 first throws out one, and while it is returning pushes for- 

 ward the other ; and this alternate motion is continued 

 till the incision is effected, when the two saws receding 

 from each other, conduct the egg between them into 

 its place. In the artificial saw the teeth are alternately 

 bent toward the sides, or out of the right line, in order 

 that the fissure or kerf may be made sufficiently wide 

 for the blade to move easily. To answer this purpose 

 in some measure, in that of the Tenthredo the teeth are 

 a little twisted, so as to stand obliquely with respect to 

 the right line, and their point of course projects a little 

 beyond the plane of the blade, without being laterally 

 bent ; and all those in each blade thus project a little 

 outwards : but the kerf is more effectually made, and a 

 free range procured for the saws, by small teeth placed 

 on the outer side of each ; so that while their vertical 

 effect is that of a saw, their lateral effect is that of a rasp. 

 In the artificial saw the teeth all point outward (towards 

 the end) and are simple ; but in the saw of the Tenthredo 

 they point inward, or toward the handle, and their outer 

 edge is beset with smaller teeth which point outwards 

 (towards the end) a ." Valisnieri, Reaumur, and De Geer 

 describe the groove as being in the back ; but in Mr. 

 Peck's insect, if there is no error in his -account, it is, as 



' Natural History of the Slug-worm, 12 ./. 12, 13. 

 VOL. IV. M 



