126 



KNOWLEDGE 



[July 1, 1893. 



wandered along countr)- lanes — perfectly spherical bodies 

 growing on young oak trees (Fig. 1), at first green and 

 soft, but becoming, later in the season, brown and very 



Fu,. ].— Marble Gall of Oak. 



hard, whence their popular name. They are one of the 

 commonest of the very numerous galls to which the oalv 

 tree is liable, and are often extremely abundant. While 

 the leaves are on the trees and the galls themselves are 

 green, they are, of course, not so noticeable ; but as they 

 remain on the twigs after the fall of the leaves, and pass 

 the winter in that position, they are at such times extremely 

 conspicuous, and the stunted oak bushes that in many 

 places form one of the chief constituents of the hedgerows 

 are often crowded with the dark brown marble-like balls. 

 So abundant are they, indeed, that they are often collected 

 in thousands to be strung on wires and worked up into 

 rustic baskets to hold ferns. 



The originator of these vegetable marbles is a little 

 brown-bodied fly (Fig. 2, a), with four clear transparent 

 wings, a thoroughly typical gall-insect. It has a globose 

 and hump-backed thorax, but the abdomen is the most 

 remarkable part of the insect. This is short, but broad 

 and deep, and attached to the thorax by a very short and 

 slender stalk. It is of a rusty brown colour, darker at the 

 base. The various segments of which it is composed are 

 very disproportionately distributed, the second being by 

 far the largest, occupying indeed about half the abdomen ; 

 while the others lap over one another like scales. Under- 

 neath, a sharp ridge runs longitudinally along the abdomen ; 

 this covers the oupositor, which rises from the head of the 

 abdomen instead of farther down, as is usually the case, 

 but part of its length is coiled up inside. The short, deep, 

 compressed, shining body is quite typical of a gall-fly, and 

 is one of the structural points by which the insects are 

 most easily recognized. The neuration of the wings, 

 again, is peculiar and is an aid to identification. 



With its onpositor the gall-fly pricks a minute hole in 

 a young leaf-bud of the oak, and deposits therein an egg. 

 It is probable that some irritant fluid is instilled at the 

 same time, since many of the Hymenoptera, as is well- 

 known, secrete fluids of this kind, and it is difficult to 

 account for the effects of the puncture except on such a 

 liypothesis. Either then by this means, or in consequence 

 of the mere presence of the egg, an irritation is produced 

 in the vegetable tissue, which results in the speedy growth 

 around the egg of a cellular mass of globular form, in a 



cavity in the centre of which reposes the egg. The 

 mother fly by its puncture taps, as it were, the current of 

 sap along the twig, and diverts its course so as to lead to 

 a rapid multiplication of vegetable cells around the 

 puncture ; in an incredibly short time this appears as a 

 green spherical mass of fleshy tissue, which has no 

 functions with regard to the tree from which it grows, but 

 is reserved exclusively for the insect's own use, or rather 

 for that of its offspring, and apparently interferes scarcely 

 at all with the general well-being of the tree. If, however, 

 the attack be very severe, thousands of galls appearing on 

 the same tree as the results of punctures by swarms of 

 flies, the vitality of the tree is naturally lowered through 

 the shutting oft' or shunting of so much of its energy, and 

 the effect of this would be felt first of all in its reproductive 

 processes, and the crop of acorns would in consequence be 

 diminished. 



During the early growth of the gall, the egg remains 

 unhatched, so that the grub is not called into independent 

 life till the materials for its support have been duly 

 elaborated. But, strange to say, the egg does not develop 

 solely at the expense of its own contents, as is usually the 

 case, but actually ip-ows through the absorption of nutrient 

 fluid from the gall ; so that, though in comparison with its 

 parent, it is proportionately large to begin with, it is 

 considerably larger before it is hatched. From it there 

 issues a fat, fleshy, footless grub, which has to spend its 

 whole life in the more or less spherical chamber, just of its 

 own size, that occupies the centre of the gall, from the rest 

 of which it is shut off by walls of a harder consistency. 

 It subsists at the expense of the gall itself, and the food it 

 absorbs appears to be wholly digested, so that the grub can 

 afford to dispense with any terminal outlet to its digestive 

 tract, and consequently no excrement accumulates, the 

 cell remaining always beautifully clean. 



In course of time the grub turns into a pupa, which 

 looks like a mummy of the perfect insect, with wings, legs, 

 and antennas folded close to its sides. This occurs towards 

 autumn, but meanwhile a great change has been taking 

 place in the gall itself, which is now hard and woody, and 

 brown outside, all traces of green having disappeared as 

 it became drier and harder. When cut open at this stage, 

 it is seen to be composed of a bi'own and more or less 

 spongy tissue, bounded within and without by a hard 

 woody layer, the spongy intermediate mass being arranged 

 in a radiating way from the centre (Fig. 3, a). 



A ^ 



l''ni. U. - (A) (tail Fly of Mai'bU\ CJall (Ci/ni/js Kutlari.) ( ii) Kgg 

 jiarasitt' (Cul/iiuome rct/iitA-). A is culargocl two diaiuetcrti j 3i, 

 tlirce and a half. 



The pupa changes to the perfect gall-fly while still lying 

 in the central chamber, absolutely imprisoned and without 

 patliway of escape. Of course it has been in darkness all 

 this time, and, therefore, needless to say, it has remained 

 of a creamy white colour ; all the air necessary for its 

 respiration has apparently been obtained by transfusion 

 through the mass of the gall, for no direct communication 

 with the outside is perceptible. The fly, when ready to 

 escape, eats a cylindrical burrow for itself along a radius 

 of the sphere, and thus reaches the outside by the most 



