SAW-FLIES. 157 



succession till she has deposited all her eggs, some- 

 times to the number of twenty-four. The grooves 

 are usually placed in a line, at a small distance from 

 one another, on the same branch ; but sometimes the 

 mother-fly shifts to another, or to a different part of 

 the branch, when she is either scared or finds it un- 

 suitable. She commonly, also, takes more than one 

 day to the work, notwithstanding the superiority of 

 her tools, Reaumur has seen a saw-fly make six 

 grooves in succession, which occupied her about ten 

 hours and a half 



The grooves, when finished, have externally little 

 elevation above the level of the bark, appearing like 

 the puncture of a lancet in the human skin ; but in 

 the course of a day or two the part becomes first 

 brown and then black, while it also becomes more 

 and more elevated. This increased elevation is not 

 owing to the growth of the bark, the fibres of which, 

 indeed, have been destroyed by the ovipositor saw, 

 but to the actual growth of the egg; for, when a new- 

 laid egg of the saw-fly is compared with one which 

 has been several days enclosed in the groove, the 

 latter will be found to be very considerably the larger. 

 This growth of the egg is contrary to the analogy 

 observable in the eggs of birds, and even of most 

 other insects; but it has its advantages. As it 

 continues to increase, it raises the bark more and 

 more, and consequently widens, at the same time, 

 the slit at the entrance; so that, when the grub 

 is hatched, it finds a passage ready for its exit. The 

 mother-fly seems to be aware of this growth of her 

 eggs, for she takes care to deposit them at such 

 distances as may prevent their disturbing one another 

 by their developement. 



Another species of saw-fly, with a yellow body and 

 deep violet-coloured wings, which also selects the 

 rose-tree, deposits her eggs in a different manner. 

 Instead of making a groove for each egg, like the 



VOL. IV 14 



