138 INSECT ARCHITECTURE. 



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 gi'owth 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 conse- 

 quently 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 ihem at 

 such distances as may prevent their disturbing one another 

 by their development. 



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 diff'erent manner. Instead of 

 making a groove for each egg, like the preceding, she 

 forms a large single groove, sufficient for about two dozen 

 eggs. These eggs are all arranged in pairs, forming two 

 straight lines parallel to the sides of the branch. The 

 eggs, however, though thus deposited in a common groove, 

 are carefully kept each in its place ; for a ridge of the 





Nest of eggs of Saw-fly, in rose tree. 



wood is left to prevent those on the right from touching 

 those on the left— and not only so, but between each egg 



