SAW-FLIES. 



135 



lines parallel to, and at a small distance from, each otlier. 

 This is effected by slightly bending the points of the 

 alternate teeth right and left, so that one-half of the whole 

 teeth stand a little to the right, and the other half a little 

 to the left. The distance of the two parallel lines thus 

 formed is called the course of the saw, and it is only the 

 portion of wood which lies in the coiTi'se that is cut into 

 saw-dust by the action of the instrument. It will follow, 

 that in proportion to the thinness of a saw there will be 

 the less destruction of wood which may be sawed. When 

 cabinet-makers have to divide valuable wood into very 

 thin leaves, they accordingly employ saws with a narrow 

 course ; while sawyers who cut planks, use one with a 

 broad course. The ovipositor-saw being extremely fine, 

 does not require the teeth to diverge much ; but from the 

 manner in which they operate, it is requisite that they 

 should not stand, like those of our saws, in a straight line. 



Ovipositor-saw of Saw-fly, witli rasps shown in the cross lines. 



The greater portion of the edge of th^ instrument, on the 

 contrary, is towards the point somewhat concave, similar 

 to a scythe, while towards the base it becomes a little 

 convex, the whole edge being nearly the shape of an 

 Italic/. 



The ovipositor-saw of the fly is put in motion in the 

 same way as a carpenter's hand-saw, supposing the tendons 



