COLOUK-PKODUCING INSECTS. 61 



nourish themselves on the tissue of the excrescences, 

 thereby causing the sap to flow again to these parts. 

 As the little ball or wart grows in size, its interior 

 is excavated more and more by the increasing appe- 

 tite of the larvae, until the sides of the excrescence 

 have become tolerably thin. The larva thus becomes 

 a chrysalis, and when its metamorphosis is com- 

 pleted, the perfect insect without much difficulty 

 bores through the gall-nut and makes its exit. 



There are galls of all sorts and sizes, many of 

 which possess very curious forms ; but each diffe- 

 rent variety is produced by a distinct species of 

 Cynips. 



Reaumur and Malpighi, to whom we owe our 

 knowledge of the formation of gall-nuts, assure us 

 that one of these, however large, attains its full 

 size in a day or two, and that those which spring 

 from leaves constantly take their origin from the 

 nerves or veins of the leaf. 



The galls produced by Cynips gallce tinctorice, 

 fetch a high price in the markets. They were 

 formerly analysed by Sir Humphry Davy, who 

 found in them 63 parts of cellulose or vegetable 

 fibre, 26 of tannic acid, 6'2 of impure gallic acid, 

 2*4 of mucilage, and 2'4 of ash or mineral matter. 

 To the tannic acid they owe their highly astringent 

 property, on account of which they are employed 

 in medicine their gallic acid is indispensable for 



