INTERNAL ANATOMY OF INSECTS. 155 
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 or- 
der that the fissure or kerf may be made sufficiently 
wide for the blade to move easily. To answer this pur- 
pose 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 eaeh 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 (to- 
wards the end)*.” Valisnieri, Reaumur, and De Geer de- 
scribe the groove as being in the back ; but in Mr. Péck’s 
insect, if there is no error in his account, it is, as in the 
Cicade, in the’ saw itself®. In the genus Cimbez, be- 
longing to the same tribe, the saw differs in shape, being 
* Natural History of the Slug-worm, 12—. f. 12, 13. 
> Valisn. Esperienz. &c. Musca dé Rosai.  Reaum. v. 100—. 
De Geer ii. 916—. The last writer thought he saw in the back of 
the saw itself a longitudinal cavity (918), which adi to the gr oove 
would form an open canal. 
