154 EVENINGS AT THE MICROSCOPE 



curved plates which form the sides of the slit are the ter- 

 mination of the sheath, in which the instrument lies con- 

 cealed till it is wanted by the insect. 



"The instrument thus brought into view is a very finely 

 contrived saw, made of horn, and adapted for penetrating 

 branches and other parts of plants where the eggs are to 

 be deposited. The ovipositor- saw of the insect is much 

 more complicated than any of those employed by our car- 

 penters. The teeth of our saws are formed in a line, but 

 in such a manner as to cut in two lines parallel to and at 

 a 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 sawdust 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 accord- 

 ingly employ saws with a narrow course; while sawyers 

 who cut planks use one with a broad course. The oviposi- 

 tor-saw, being extremely fine, does not require the teeth to 

 diverge much, but from the manner from which they oper- 

 ate, 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 toward the 

 point somewhat concave, similar to a scythe, while toward 

 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 



