378 



INSECT ARCHITECTURE. 



through the influence of the warmth of the succeed- 

 ing summer, they undergo their final change into the 

 winged state ;* preparatory to which they eat their 

 way with their sharp mandibles through the walls of 

 their little cells, which are now so hard as to be cut 

 with difficulty by a knife.f 



Another structure, similar in principle, though dif- 

 ferent in appearance, is very common upon oak-trees, 

 the termination of a branch being selected as best 

 suited for the purpose. This structure is rather 

 larger than a filbert, and is composed of concentric 

 leaves diverging from the base, and expanding up- 

 wards, somewhat like an artichoke. Whether this 



Artichoke Gall of the Oak-bud, with Gall-fly (Cynipn querela 

 gemma?), natural size, and its ovipositor (a) magnified. 



* See Inacct Transformations, chaps, iv and vii 

 t J. R. 



