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THE BRITISH NATURALIST. 



[April 



always equally obvious, and each must form his own opinion upon 

 its merits, until some new student comes forward to knock it on the 

 head. 



Galls sorely puzzled the ancients, who regarded them as very 

 mysterious objects. They were unable to account for the presence of 

 a grub or a fly in a substance where they could find no way in and no 

 way out. In those days spontaneous generation was the dominant 

 belief, and the only explanation that Pliny and his contemporaries 

 could apply to gall grubs. Redi successfuly combated this theory, but 

 himself fell into as great an error, in the case of gall grubs, in 

 supposing that the grubs were generated from the gall substance. 

 After this had been proved untenable it was supposed that the eggs 

 were laid in the earth by insects, and these eggs were drawn up along 

 with the sap of the plant, some lodging in the stems, others in the 

 leaves, others in the flowers, and otheis causing obstruction which 

 produced the galls. It is now well known that they are all produced 

 in the way I described — by eggs deposited by parent insects in the 

 substance upon which the gall grows. It is worthy of remark, how- 

 ever, that the gall begins to grow, and in many cases has attained 

 considerable size, before the grub hatches from the egg, so the grub 

 can have little, if anying, to do with the forming of the gall. It is 

 curious that the puncture by a given species should always produce 

 exactly the same gall. Supposing a liquid is secreted, is the secretion 

 different in ever} 7 species ? or how does it act to produce a different 

 gall ? It is also singular that very dissimilar species should produce 

 almost exactly similar galls, as the nail gall on lime, produced in 

 Britain by a mite and in America by a midge. 



The subject of parasitism is one of very great interest, but one 

 which scarcely falls within the scope of the present paper. Many 

 gall makers are very subject to parasites, and from the same galls the 

 student may often breed three or four or more sets of very different 

 flies. Some of these parasites prey on the gall-making grubs, some 

 feed in the substance of the gall ; some are hyper-parasites preying 

 on other intruders, while some only go into the galls for lodgment, or 

 protection, or undergo their transformations. 



Another very interesting phase in the study of gall insects is the 

 apparently unaccountable predomination of one sex over the other. 

 It was at one time thought that some species had no male, for 

 thousands of individuals have been reared everyone of which proved 

 to be a female and the male was not known. But the theory of 

 alternation of generation first announced by Mr. Walsh (U.S.) in 1870, 

 has thrown some light on this point, that is, that the same species 

 produces two different kinds of galls, the two kinds alternating, 

 generation after generation, and that males are produced in one 



