19 



is found attached to the leaves of the Oak. These vary 

 very much in size, some being as large as a marble ; and 



each contains a single insect, which, when it arrives at 

 its perfect state, eats its way out through a great portion 

 of the solid substance of the gall. 



LEAF-GALLS. 



The habitation of all the parasitic insects hitherto men- 

 tioned is formed out of the pulpy substance of the tree : 

 one, however, which is not uncommon, and is called the 

 Artichoke-gall, is an irregular development of the bud, and 

 consists of a number of leafy scales overlapping each other. 



