176 Notes on Natural Jlistori/. 



been known that the different galls on the oak— of which the " oak apple" is the 

 best known and most conspicuous form— are due to the action of the larvae of 

 different species of flies, but until within the last few years no one suspected that 

 the life history of these flies forms one of the most marvellous stories in Nature 

 — so marvellous, indeed, that if the facts had not been verified beyond the shadow 

 of a doubt by the patient researches of English and German naturalists, more 

 especially by those of Dr. Hermann Adler, of Schleswig, they would seem to be 

 incredible, so contrary are they to what are by most people regarded as the 

 universal laws of Natui-e. It is not so much because these facts constitute a 

 singularly fascinating chapter in natural history for the entomologist, as on 

 account of the important bearing which they have upon the theories of Weismann 

 as to heredity, and on other biological questions of the highest interest, that Mr. 

 C. E. Straton, F.R.C.S., and P.E.S., of Wilton, has translated Dr. Adler's 

 monograph on the gall flies of the oak, and has added to it a learned introduction 

 of forty pages of his own. 



To explain this let us see what Dr, Adler tells us of the life history of the 

 beautiful gall often seen on the leaf of the oak, of the size of a marble, yellow, 

 streaked and mottled with red. This gall {Dryoplianta scutellaris) is always 

 attached to the veins on the under side of the leaf. It appears in July, matures 

 in October, and the fly emerges from the gall in December, January, or February, 

 according to the weather. The flies only live a few days, and the difficulty 

 hitherto has beeu to bridge over the gap between January — when the perfect 

 insect appears — and July — when the gall containing the infant larva just hatched 

 from the egg first begins to show on the vein of the oak leaf. How is the egg 

 laid on the leaf ? Dr. Adler has solved the mystery by hatching out flies from 

 the galls and keeping them carefully under control and examination. "I had 

 kept a large quantity of galls out of doors through the winter, and in January 

 the flies began to take flight. I put them on a little oak tree indoors, and 

 observed that after a few days they began to oviposit, choosing the little adven- 

 titious buds that were on the stem. The buds were pricked in the following 

 manner. The fly reared itself, directing its ovipositor to the point of the bud, 

 and boring down into it perpendicularly. The fly is armed for this purpose with 

 a tolerably straight and strong ovipositor. Some time is required to complete 

 the act of ovipositing and the fly usually stands half an hour in the pricking 

 posture. In each bud only one egg is laid. If a pricked bud is examined it will 

 be seen that the egg lies at the base of the bud axis ... . therefore it may 

 be predicted with certainty that a bud gall will be the result. In my experiments 

 thirty-four buds were piicked between January 20th and January 26th, hut it 

 was not until the end of April that I was able to observe the beginning of gall 

 formation in any of the buds. The points of the buds became dark blue, and 

 soon the dainty velvety galls of Spathegaster Taschenhergi became evident ; 

 by the beginning of May eleven galls developed on the tree." 



Now this gall bears no resemblance whatever to the apple-like gall from which 

 the fly sprung from whose egg the larva causing it was hatched, and the flies 

 themselves which emerge from these galls at the end of May or beginning of 

 June are quite different from the parent flies, so much so as to have been always 

 regarded as actually belonging to distinct genera, yet Dr. Adler has proved that 

 the flies emerging from the apple gall on the leaf in January lay the eggs from 



