﻿20 NINTH ANNUAL REPOKT 



may be, possess these saws, though in one genus {Xijela) the saws 

 instead of being hard and horny throughout, are said to be soft and 

 membranous above and below*;* and in certain other Saw-flies, though 

 they are as hard and horny as usual, they are degraded and — to use the 

 technical term — ' defunctionated.' This will be seen at once from an 

 inspection of the following drawing (Fig. 8), copied by ourselves from 

 nature and very highly magnified. Here a represents the two saws of 

 the female of the Willow-apple Saw-fly ( Nematus salicispomum 

 Walsh), which belongs to the very same genus as our Currant-worm 

 Fly. Now, we know that the female of the Willow-apple Saw-fly depos- 

 its a single egg inside the leaf of the 

 Heart-shaped Willow {Salix cordata) about 

 the end of April, probably accompanying 

 the egg by a drop of some peculiar poison- 

 ous fluid. Shortly afterwards there grad- 

 ~l "^^^ * ually develops from the wound a round 



OviPOSiTOiiS or Saw-fliks :— a, per- ^, ni i.iir -i-t a 



feet; b, imperfect. fleshy gall, about half an inch m diameter, 



and with a cheek as smooth and rosy as that of a miniature apple; in- 

 side which the larva hatches out and upon the flesh of which it feeds. 

 In this particular case, therefore, as the female fly requires a complete 

 saw with which to cut into the willow leaf, nature has supplied her 

 with such saws, as is seen at once from Figure S, a. Now look at 

 Figure 8, h^ which is an accurate representation under the microscope 

 of the two saws of our Currant-worm Fly. It will be noticed at the 

 very first glance that, although the blade of the saw is there, the teeth 

 of the saw are almost entirely absent. 



What, then, are we to make of these and many other such facts? 

 Manifestly the teeth of the saw are in this last species degraded or 

 reduced to almost nothing, because the female fly, laying her eggs 

 upon the surface of the leaf, and not cutting into the substance of the 

 leaf, as does the female of the Willow-apple Saw-fly, has no occasion 

 to perform any sawing process. But why, it will be asked, is the blade 

 of the saw there in its normal size, and with the exception of the 

 degradation of the saw-teeth, as completely developed as in the other 

 species, when such a tool cannot be necessary for the simple process 

 of glueing an egg on to the surface of a leaf? The modern school of 

 philosophers will reply, that this is so, because the primordial Saw-flj^ 

 in the dim far-away vista of by-gone geological ages, had a complete 

 pair of saws, and our insect is the lineal descendant of that species, 

 slowly and gradually modified through a long series of years, so as to 



*'fiG».'Wesi\\ooA's\Introduclion, 11, p. 95. 



