GAIL-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 uniform 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 conjectural; for though 

 Swammerdam detected a gall-fly in the act of de- 

 positing her eggs, he did not attend to this circum- 

 stance ; and in the instances which we have observed, 

 some unlucky accident always prevented us from 

 following up our observations. The indefatigable 

 Reaumur, 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 substance called 

 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 had 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 investiga- 

 tion, 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 

 (Callimone Bedeguaris, Stephens), 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 described. 



