GALL-FLIES. 375 



gall-fly to be globular, and covered, or coated with a 

 pellicle of gluten of uniform thickness, and conse- 

 quently opposing uniform resistance, or rather uni- 

 form expansibility, to the sap pressing from within. 

 It will also account for the remarkable uniformity in 

 the size of the gall apples; for the punctures and the 

 eggs being unitbrm in size, and the gluten by sup- 

 position, uniform in quantity, no more than the 

 same quantity of sap can escape in such circum- 

 stances. 



But though this explanation appears to be plau- 

 sible, it is confessedly altogether conjectural; for 

 nobody since Swammerdam, so far as we at pre- 

 sent know, has ever detected a gall-fly in the act 

 of depositing her eggs, and he did not attend to this 

 circumstance. The indefatigable R^'aumur, on one 

 occasion, thought he would make sure of tracing the 

 steps of the process in the case of the gall-fly, which 

 produces the bedeguar* on the wild-rose tree, and to 

 which we shall presently advert. His plan was to 

 enclose in a box, in which a brood of flies httd just 

 been produced from a bedeguar, a living branch from 

 a wild rose-tree; but, to his great disappointment, 

 no eggs were laid, and no bedeguar formed. Upon 

 further investigation, he discovered that the brood 

 of flies produced from the bedeguar were not the 

 genuine bedeguar insects at all, but one of the 

 parasite ichneumons {CaUimo7ie Bedegiiaris, Ste- 

 phens,) which had surreptitiously deposited their 

 eggs there, in order to supply their young with the 

 bedeguar grubs, all of which they appeared to have 

 devoured. It may prove interesting to look into the 

 remarkable structure of the bedeguar itself, which 

 is very different from the globular galls above de- 

 scribed. 



* The gall of the rose is so called botanically. 



