GALL FLY. 



159 



whether the larva or caterpillars are possessed of 

 22 feet or 20 feet, or (for practical purposes) of no feet 

 at all, there is one method of prevention applicable to 

 them all, — to turn out the cocoons and destroy them. 



The " Giant," and the " Steel-blue " or " Common" 

 Sirex, may be taken as examples of the family of the 

 Uroceridce, which sometimes do great harm by means 

 of the galleries gnawed by their large grubs in solid 

 timber. 



The Sirex gigas (or ''Giant Sirex") is a splendid 

 insect, as large as, or larger than, the Sirex juvencns 

 (Fig. 122), and of a rich yellow colour banded with 

 black. 



Fig. 123. — Marble Galls. Gall Fly, grub, and pupa; magnified. 



The family of the Gall Flies (Cynipidce) is also one 

 which is not of much importance agriculturally, but 

 it may perhaps be convenient just to mention. 



The female Gall-fly has a kind of pointed tube, 

 or arrangement of bristles, by means of which she is 

 able to insert her egg or eggs into whatever part of 

 the plant she chooses for the purpose. In consequence 

 of this, various growths form, such as the Oak Marble 

 Gall ; the Oak-apple ; or the Spangles of the Oak-leaf ; 

 or the bright green and red moss-like bunch, which we 

 know as the Kobin's Pincushion of the Eose ; — and 

 within these the legless maggots hatch, feed, change 

 io chrysalids, and in due time from them they emerge 



