The Scottish Naturalist. 281 



THE GALL-MAKING DIPTERA OF SCOTLAND. 



BY PROF. J. W. H. TRAIL, A.M., M.D., F.L.S. 



(Head before the E.S. Union of Naturalists' Societies, in June, 1S87.) 



IN a paper read before the Perthshire Society of N.S. two years 

 ago, I gave an account of some of our Scottish galls and 

 gall-makers, restricting myself to the Cynipidae of the Oak. This 

 group of galls is perhaps the most interesting of all, because of 

 the strange relations that exist in the life-cycle of certain of the 

 species between successive generations of the insects and their 

 galls, constituting the phenomenon known as dimorphism. In 

 this two forms of gall-insects, formerly supposed to be entirely 

 distinct, are now known to be only stages in the life history of one 

 and the same species ; in which each generation produces a form 

 of gall peculiar to itself, the one being restricted to early summer, 

 and the other to autumn. Dimorphism has not been proved to 

 exist in any other group of gall-makers than the Cynipidae. I 

 now propose to lay before the East of Scotland Union a short 

 account of another and larger section of gall-makers, belonging to 

 the Diptera or two-winged flies, and of their galls. My personal 

 information has been, for by far the most part, gathered in the 

 counties of Aberdeen, Kincardine, Forfar and Perth ; and I may 

 thus hope to be able to throw some light in this paper upon the 

 fauna and flora of the East of Scotland. Gall-making Diptera 

 belong to only a few families ; and even these families include 

 very many species that are not gall-7nakers ; though they may live 

 as guests (inquilines) in the galls made by other species, or may 

 feed in some part (often in the flower) of a plant without deform- 

 ing it in any way. Many also live, as larvae, in decaying substances 

 or in mushrooms, &c. By far the larger number of true gall- 

 makers among the Diptera fall under the head of Gall-midges 

 or CecidomyidaB, though a few of the Scotch gall-makers be- 

 long to the Trypetidae, and a few others belong to other 

 families, differing in habits from their nearest allies. The life- 

 history of most of these isolated forms is not yet fully or reliably 

 known. The families of gall-making Diptera differ so much from 



