GALL-FLIES. 379 



leafy structure, is caused by a superinduced disease, 

 as the Frencli think, or by the tbrm of the pores in 

 the pelhcle of gluten surrounding the eggs, or rather 

 by the tendency of the exuding sap of the oak to 

 form leaves, has not been ascertained; but that it is 

 intended, as in the case of the bedeguar, to afford 

 •an efficient protection against the weather to the in- 

 cluded eggs or grubs, there can be no doubt. 



From the very nature of the process of forming 

 ■willow-galls, bedeguar, and the artichoke of the 

 oak, whatever theory be adopted, it will be obvious 

 that their growth must be rapid; for the thicken- 

 ing of the exuded sap, which is quickly effected by 

 evaporation, will soon obstruct and finally close the 

 orifice of the puncture made by the parent insect. 

 It is accordingly asserted by Reaumur and other ob- 

 servers, that all the species of galls soon reach their 

 full growth. 



A very minute reddish-coloured grub feeds upon 

 dyer's broom [Genista), producing a sort of gall, fre- 

 quently globular, but always studded with bristles, 

 arising from the amorphus leaves. The stem of 

 the shrub passes through this ball, which is com- 

 posed of a great number of leaves, shorter and 

 broader than natural, and each rolled into the form 

 of a horn, the point of which ends in a bristle. In 

 the interior we find a thick fleshy substance, serving 

 to sustain the leaves, and also for the nourishment 

 of the grubs, some of which are within and some 

 between the leaves. They are in prodigious num- 

 bers, — hundreds being assembled in the same gall, 

 and so minute as scarcely to be perceived without 

 the aid of a magnifying glass. The bud of the plant 

 attacked by those grubs, instead of forming a shoot, 

 pushes out nothing but leaves, and these are all 

 rolled and turned round the stem. Some shrubs 



