130 EVENINGS AT THE MICROSCOPE. 



then show .you a specimen dissected out by myself, and 

 point out some agreements and some discrepancies between 

 it and them. 



" In order to see the ovipositor, a female saw-fly must 

 be taken, and her belly gently pressed, when a narrow 

 slit will be observed to open at some distance from the 

 anus, and a short, pointed, and somewhat curved body, 

 of a brown colour and horny substance, will be protruded. 

 The' curved plates which form the sides of the slit are 

 the termination of the sheath, in which the instrument 

 lies concealed 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 carpenters. 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 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 



