INTERNAL ANATOMY OF INSECTS. 161 
which Is fixed : the saw of the Tenthredo is 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 saisc, their lateral effect is that of a rasp. 
In the artificial saw the teeth all point outward (towards 
the e?id) 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 
