200 A BOOK OF INSECTS 



the gooseberry saw-fly. In the former case the damage 

 done to the willow tree is slight, even when the insect is 

 present in force. Each larva is confined throughout life 

 to one small part of the leaf — namely the gall ; and 

 although the formation and upkeep of this structure im- 

 poses a tax upon the tree's resources, the bulk of the 

 foliage remains to make good this loss. But the larvae 

 of the gooseberry saw-fly (A T . ribesli), not being confined 

 to galls, consumes one leaf after another with startling 

 voracity. A few hundreds of these caterpillars will 

 quickly strip the foliage from a plantation of gooseberry 

 and currant bushes, seriously affecting their vitality and 

 growth. Nevertheless, while the raison d'etre of galls is 

 fairly obvious, in that Nature delights to foster any scheme 

 for mutual economy, the origin of the individual gall is 

 by no means easy to explain. Pliny thought that galls 

 were fungi, in which insects bred by chance. A later 

 author opined that the parent insect laid its eggs in the 

 soil, whence they were drawn up with the sap to the 

 leaves where the galls were found. Redi actually assumed 

 that the plant had a vegetable soul, this vegetable soul 

 presiding at the origin of galls, with the eggs, larvae and 

 imagines, while it again gave issue to fruits — whatever 

 this jargon may signify. Yet he had himself successfully 

 refuted the theory of spontaneous generation which was 

 current in his day ! A more modern notion, and one 

 which until recently evoked universal credence, is that 

 the parent insect infects the plant tissue during oviposition 

 by injecting a small drop of poisonous fluid, and that this 

 gives rise to a morbid enlargement and subdivision of the 

 vegetable cells. 



It is a fact that gall-insects often inject a drop of fluid 

 into the wounds which they make, but they do this 

 either to provide a lubricant for the working of their 



