( H.1 ) 



may pierce a large vefTel in the leaf, and afterwards fliift itfelf to 

 another place, and out of the large vefl'cl fo wounded, a gall-nut 

 may be produced without any maggot in it. The gall-nuts 

 which are formed without any infeifts in them, are generall}' the 

 heavieft, by reafon that they have not any cavity made in their 

 infides. 



With regard to my preceding obfervations on this fpecies of 

 fly, I was the more confirmed in my opinion, upon recoile6liou 

 of what I have obferved of the like kind in divers trees, and par- 

 ticularly the willow ; on the leaves of which I have feen certain 

 green tubercles or fwellings, on opening which I have found 

 animalcules within them, fome of which were alive, and others 

 dead ; all which I doubted not were produced by maggots hatched 

 from the eggs of the flies of the fame fpecies as thofe which 

 I had found in fuch tumours or fwellings. For it is. with me an 

 eftabliOied principle, that no living animal, whether worm, fly, 

 gnat, or mite, can be produced from the mere juice or leaf of any 

 tree or plant, nor from corrupted or decayed fubftances. 



I have caufed a drawing to be made of thefe galls, as they grow 

 or are produced on the leaves ; and in Plate V. fg. 17, A B C D, 

 is reprefented an oak leaf, with that fide upwards, which, when on 

 the tree is undermofi: ; for I never faw any of thefe galls on the 

 upper fide of the leaf ; the reafon of \\ hich I take to be, that if 

 the maggot was placed on the upper fide, and expofed to the fun's 

 heat but for a fhort time, it would, by reafon of its fmallnefs, be 

 dried up and perilh. On this leaf are feen two large galls, and 

 two fmaller ones, at the letters E, F, G, H. 



Fig. ]8, 18, exhibit a gall-nut cut in half, wherein, at N, N, 

 appears the cavity, in which the maggot lay ; fome of thefe 

 maggots, of different fizes, are fliewnat I, K, L M. 



Fig. 19, fliews one half of a gall-nut, with the fly in it, which has 

 opened to itfelf a pafi'age from the cavity as far as the furface of 

 the nut, and at fg. co, is the other half, exiiibiting the cavity in 



