HYMENOPTERA. TEREBRANTIA. 175 



ing wherewith to clothe all the excrescences which may 

 be formed upon it ; while an egg laid in one leaf-bud 

 of an oak-tree results in a formation resembling an arti- 

 choke, and another egg laid in another bud produces a 

 perfectly smooth, hard, round ball. Is then the cause of 

 the difference in the fly and not in the tree ? So far as 

 has yet been observed each species of insect has its own 

 form of gall, but this in no degree lessens the diffi- 

 culty ; for even supposing a chemical difference in the 

 poisonous secretions of the various species, it is altogether 

 inconceivable how so minute a drop as that to be 

 deposited by an insect under the T V of an inch in 

 length, and in so minute a wound, should occasion any 

 serious disturbance at all of the circulation ; but it is 

 still more so that some difference in its composition 

 should so regulate the whole process of change in the 

 natural action of the tree, as to produce growths totally 

 differing in appearance and in character. Conjectures have 

 been formed to explain the whole of this process, but, 

 like conjectures upon some other subjects, they are quite 

 as puzzling as the original problem.. 



It is not only in abstruse matters of physiology that 

 these little productions manage to baffle the naturalist. 

 It is often no easy matter to discover or determine the 

 owner of a gall from which a tenant has emerged. An 

 amusing instance of this occurred in the case of 

 Reaumur. Wishing to witness the growth of the mossy 

 rose-gall, he carefully tended a number of flies lately 

 hatched from one, supplying them with a branch from a 

 rose-tree, in order that they might lay their eggs therein 

 and prepare the way for future galls. After waiting for 

 some time, and finding that the flies showed no dis- 

 position to attack the rose-branch, he discovered his 



