OF NATURAL HISTORY. 297 



till the animal is hatched, and has pafled through its different tranf- 

 formations. Thefe tubercles are known by the name of galls, and 

 are very different in their form, texture, colour, and fize. Galls of 

 every kind, hovfever, derive their origin from the flings of infeds, 

 which generally belong to the clafs of flies. The female fly, by 

 means of h^r fling, makes incifions in the leaves or branches of a 

 tree, and in each incifion fhe lays an egg. This egg is at firft ex- 

 tremely minute ; but it foon acquires a confiderable bulk, and the 

 gall has arrived at its full fize before the worm is hatched. This 

 gall feems to be analogous to the membranes which invefl a foetus, 

 and expand in all diredions in proportion to its growth. That the 

 eggs of oviparous animals grow while in the ovarium is univerfally 

 known; but it is Angular that the eggs of gall-flies (hould grow af- 

 ter being feparated from the body of the mother. Thefe eggs muft 

 undoubtedly be furnifhed with external veflels, or a kind of roots, 

 by which they extradl juices from the internal cavity of the gall. 

 Malpighius afcribes the origin of galls to a corrofive liquor introdu- 

 ced by the fly into the wound. But Rtaumur, to account for the 

 growth of a gall, thinks it unncceflary to have recourfe to any fup- 

 pofed poifonous fluids, and attributes it to the fuperabundant nutri- 

 tious juices derived to that particular part by the continual adion 

 of the abforbent veflels of the egg, joined to its heat, which may be 

 compared to a little fire placed in the center of the tumour. 



Whether thefe caufes are fufficient to explain the growth of gaily, 

 we fliall fubmit to the judgment of the reader. But, that the eggs 

 depofited by the flies augment in fize ; that worms proceed from 

 them ; that thefe worms are nouriflied, and live a certain time im- 

 prifoned in the galls ; that they are there transformed into nymphs 

 or chryfalids; and, laftly, that they are metamorphofed into winged 

 infeds, which, by gnawing an aperture through the gall, take their 

 flight in the air ; are known and inconteftible fads, of the truth 

 2 t P P of 



