382 



HABITATIONS OF INSECTS. 



by Mr. Elliott growing on various species of dwarf oaks be- 

 yond the Jordan and in the Troad, to the twigs of which 

 Mr. Westwood remarks they are attached in a curious man- 

 ner, unlike what he has seen in any other galls, the narrow 

 end " rising upwards on each side and bending inwards, so as 

 to clasp the extremity of the twig somewhat like a pair of 

 wide and curved nippers." 



How the mere insertion of an egg into the substance of a 

 leaf or twig, even if accompanied, as some imagine, by a pe- 

 culiar fluid, should cause the growth of such singular protu- 

 berances around it, philosophers are as little able to explain, 

 as why the insertion of a particle of variolous 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 Malpighi — that the production of the gall, which, how- 

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

 the egg or some accompanying fluid ; not by the larva, which 

 does not appear until the gall is fully formed^ : that the galls 

 which spring from leaves almost constantly take their origin 

 from nerves^; and that the egg, at the same time that it 

 causes the growth of the gall, itself derives nourishment from 

 the substance that surrounds it, becoming considerably larger 

 before it is hatched than it was when first deposited.* When 

 chemically analysed, galls are found to contain only the same 

 principles as the plant from which they spring, but in a more 

 concentrated state. 



No productions of nature seem to have puzzled the ancient 

 philosophers more than galls. The commentator on Diosco- 

 rides, Mathiolus, who agreeably to the doctrine of those days 

 ascribed their origin to spontaneous generation, gravely in- 

 forms us that weighty prognostications as to the events of &e 

 ensuing year may be deduced from ascertaining whether they 

 contain spiders, worms, or flies. Other philosophers, who 

 knew that, except by rare accident, no other animals are to be 



1 Reaum. iii. 474. 

 3 Ibid. 501. 



2 Ibid. 479. 

 4 Ibid. 479. 



