148 EVENINGS AT THE MICROSCOPE. 



introduces her egg ' into a puncture naade by her cu- 

 rious spiral sting, and in a few hours it becomes sur- 

 rounded with a fleshy chamber. M. Yirey says, the 

 gall-tubercle is produced by irritation, in the same way 

 as an inflamed tumour in an animal body, by the swell- 

 ing of the cellular tissue, and the flow of liquid matter, 

 which changes the organization, and alters the natural 

 external form." * 



Perhaps a still more charming example of animal 

 mechanics is that furnished to us by the Saw-flies {Tev^ 

 tJiredinidoB). These are very common four-winged 

 insects of rather small size, many species of which are 

 found in gardens and along hedges, in summer, pro- 

 duced from grubs which are often mistaken for true 

 caterpillars, as they strip our gooseberry and rose 

 bushes of their leaves ; but may be distinguished from 

 them by the number of their pro-legs, and by their sin- 

 gular postures ; for they possess from eight to four- 

 teen pairs of the former organs, and have the habit 

 of coiling up the hinder part of their body in a spiral 

 ring, while they hang on to the leaf by their six true 

 feet. 



These saw-fly caterpillars are produced from eggs 

 which are deposited in grooves, made by the parent-fly 

 in the bark of the tree or shrub whose future leaves 

 are destined to constitute their food ; and it is for the 

 construction of these grooves and the deposition of the 

 eggs in them, that the curious mechanism is contrived 

 which I am now bringing under your notice. 



Almost all our acquaintance with this instrument 

 and the manner of its employment, we owe to the em- 

 inent French naturalist Eeaumur, and to his Italian 



* Ins. Arch. 371. 



