. 



448 HABITATIONS OF INSECTS. 



at first he took for a tufted lichen a . From the same 

 cause the twigs of the common wild rose often shoot out 

 into a beautiful tuft of numerous reddish moss-like fibres 

 wholly dissimilar from the leaves of the plant, deemed 

 by the old naturalists a very valuable medical substance, 

 to which they erroneously gave the name of Bedeguar. 

 Noae of these variations is accidental or common to se- 

 veral of the tribe, but each peculiar to the galls formed 

 by a single and distinct species of Cynips. 



How the mere insertion of an egg into the substance 

 of a leaf or twig, even if accompanied, as some imagine, 

 by a peculiar fluid, should cause the growth of such sin- 

 gular protuberances around it, philosophers are as little 

 able to explain, as why the insertion of a particle of va- 

 riolous matter into a child's arm should cover it with 

 pustules of small pox. In both cases the effects seem to 

 proceed from some action of the foreign substance upon 

 the secreting vessels of the animal or vegetable : but of 

 the nature of this action we know nothing. Thus much 

 is ascertained by the observations of Reaumur and Mal- 

 pighi that the production of the gall, which however 

 large attains its full size in a day or two b , is caused by 

 the egg or some accompanying fluid : not by the larva, 

 which does not appear until the gall is fully formed c ; 

 that the galls which spring from leaves almost constantly 

 take their origin from nerves d ; and that the egg, at the 

 same time that it causes the growth of the gall, itself de- 

 rives nourishment from the substance that surrounds it, 

 becoming considerably larger before it is hatched than 

 it w r as when first deposited 6 . When chemically ana- 



a Introd. to Botany, 349. b Reaum. iii. 474. 



e Jbid. 479. d Ibid. 501. e Ibid. 479. 



