374 INSECT ARCHITECTURE. 



in an animal body, by the swelling of the cellular 

 tissue, and the flow of liquid matter, which changes 

 the organization, and alters the natural external 

 form.* This seems to be the received doctrine at 

 present in France.f 



Sprengel, speaking of the rose-willow, says, the 

 insect in spring deposits its eggs in the leaf-buds. 

 " The new stimulus attracts the sap, — the type of the 

 part becomes changed, and from the prevailing 

 acidity of the animal juice, it happens, that in the rose 

 and stock-shaped leaves which are pushed out, a red 

 instead of a green colour is evolved.'' J 



Without pretending positively to state facts which 

 are, perhaps, beyond human penetration, we may 

 view the process in a rather different light.§ Follow- 

 ing the analogy of what is known to occur in the case 

 of the saw-flies (see page 156), after the gall-fly has 

 made a puncture and pushed her egg into the hole, 

 we may suppose that she covers it over with some 

 adhesive gluten or gum, or the egg itself, as is usual 

 among moths, &c, may be coated over with such a 

 gluten. In either of these two cases, the gluten will 

 prevent the sap that flows through the puncture from 

 being scattered over the leaf and wasted; and the 

 sap, being thus confined to the space occupied by the 

 eggs, will expand and force outwards the pellicle of 

 gluten that confines it, till becoming thickened by 

 evaporation and exposure to the air, it at length shuts 

 up the puncture, stops the further escape of the sap, 

 and the process is completed. This explanation will 

 completely account for the globular form of the 

 galls alluded to; that is, supposing the egg of the 



* Hist. <les Mceurs et <le VInstinct, vol. ii. 



f Entomologie, ]>ar R. A. E. p. 242. Paris, 1826. 



% Elements of the Philosophy of Plants. Eng. Trans, p. 285. 



\ J. li. 



