GALL-FLIES. 



373 



QalUfly, and mechanism of ovipoator, greatly magnified. 



With this instrument the mother gall-fly pierces 

 the part of a plant which she selects, and, according 

 to our older naturalists, " ejects into the cavity a drop 

 of her corroding liquor, and immediately lays an egg 

 or more there ; the circulation of the sap being thus 

 interrupted, and thrown, by the poison, into a fer- 

 mentation that burns the contiguous parts and 

 changes the natural colour. The sap, turned from 

 its proper channel, extravasates and flows round the 

 eggs, while its surface is dried by the external air, and 

 hardens into a vaulted form."* 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 cham- 

 ber." f M. Virey says, the gall tubercle is produced 

 by irritation, in the same way as an inflamed tumour 



* Spectacle ile la Nature, i. 119. 

 t Introd., ii. 419. 



