INSECTS .* STINGS AND OVIPOSITORS. 129 



the oak, are not the proper fruit of the tree, but diseased 

 developments produced by a tiny insect, for the protection 

 and support of her young. Kirby and Spence tell us that 

 the parent fly introduces her egg into a puncture made by 

 her curious spiral sting, and in a few hours it becomes 

 surrounded with a fleshy chamber. Mr. Virey says the 

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

 an inflamed tumour in an animal body, by the swelling of 

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

 changes the organisation, and alters the natural external 

 form. 



Perhaps a still more charming example of animal 

 mechanics is that furnished to us by the Saw-flies (Ten- 

 thredinida). These are very common four- winged insects 

 of rather small size, many species of which are found in 

 gardens and along hedges in summer, produced from 

 grubs which are often mistaken for true caterpillars, as 

 they strip our gooseberry and rose bushes of their leaves ; 

 but which may be distinguished from them by the number 

 of their pro-legs, and by their singular postures ; for they 

 possess from eight to fourteen 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 eminent 

 French naturalist, Reaumur, and to his Italian con- 

 temporary, Valisnieri. Their details I shall first cite, as 

 they have been put into an English dress by Rennie, and 



I 



