170 Insect Architecture. 



small distance- from, each other. 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 course 

 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 



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



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. The greater 

 portion of the edge of the 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 ol 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 attached 



