ANIMAL GALLS,[GH] PRODUCED BY BREEZE-FLIES AND SNAIL-BEETLES.
The structures which we have hitherto noticed have all been formed of inanimate materials, or at the most of growing vegetables; but those to which we shall now advert are actually composed of the flesh of living animals, and seem to be somewhat akin to the galls already described as formed upon the shoots and leaves of plants. These were first investigated by the accurate Vallisnieri, and subsequently by Réaumur, De Geer, and Linnæus; but the best account which has hitherto been given of them is by our countryman Mr. Bracey Clark, who differs essentially from his predecessors as to the mode in which the eggs are deposited. As, in consequence of the extreme difficulty, if not the impossibility, of personal observation, it is no easy matter to decide between the conflicting opinions, we shall give such of the statements as appear most plausible.
The mother breeze-fly (Oestrus bovis, Clark;—Hypoderma bovis, Latr.), which produces the tumours in cattle called wurbles or wormuls (quasi, worm-holes), is a two-winged insect, smaller, but similar in appearance and colour to the carder-bee ([p. 75]), with two black bands, one crossing the shoulders and the other the abdomen, the rest being covered with yellow hair. This fly appears to have been first discovered by Vallisnieri, who has given a curious and interesting history of his observations upon its economy. “After having read this account,” says Réaumur, "with sincere pleasure, I became exceedingly desirous of seeing with my own eyes what the Italian naturalist had reported in so erudite and pleasing a manner. I did not then imagine that it would ever be my lot to speak upon a subject which had been treated with so much care and elegance; but since I have enjoyed more favourable opportunities than M. Vallisnieri, it was not difficult for me to investigate some of the circumstances better, and to consider them under a different point of view. It is not, indeed, very wonderful to discover something new in an object, though it has been already carefully inspected with very good eyes, when we sit down to examine it more narrowly, and in a more favourable position; while it sometimes happens, also, that most indifferent observers have detected what had been previously unnoticed by the most skilful interpreters of nature."[GI]
From the observations made by Réaumur, he concluded that the mother-fly, above described, deposits her eggs in the flesh of the larger animals, for which purpose she is furnished with an ovipositor of singular mechanism. We have seen that the ovipositors in the gall-flies (Cynips) are rolled up within the body of the insect somewhat like the spring of a watch, so that they can be thrust out to more than double their apparent length. To effect the same purpose, the ovipositor of the ox-fly lengthens, by a series of sliding tubes, precisely like an opera-glass. There are four of these tubes, as may be seen by pressing the belly of the fly till they come into view. Like other ovipositors of this sort, they are composed of a horny substance; but the terminal piece is very different indeed from the same part in the gall-flies, the tree-hoppers (Cicadæ), and the ichneumons, being composed of five points, three of which are longer than the other two, and at first sight not unlike a fleur-de-lis, though, upon narrower inspection, they may be discovered to terminate in curved points, somewhat like the claw of a cat. The two shorter pieces are also pointed, but not curved; and by the union of the five, a tube is composed for the passage of the eggs.
It would be necessary, Réaumur confesses, to see the fly employ this instrument to understand in what manner it acts, though he is disposed to consider it fit for boring through the hides of cattle. “Whenever I have succeeded,” he adds, “in seeing these insects at work, they have usually shown that they proceeded quite differently from what I had imagined; but unfortunately I have never been able to see one of them pierce the hide of a cow under my eyes.”[GJ]
Ovipositor of the Breeze-fly, greatly magnified, with a claw and part of the tube, distinct.
Mr. Bracey Clark, taking another view of the matter, is decidedly of opinion that the fly does not pierce the skin of cattle with its ovipositor at all, but merely glues its eggs to the hairs, while the grubs, when hatched, eat their way under the skin. If this be the fact, as is not improbable, the three curved pieces of the ovipositor, instead of acting, as Réaumur imagined, like a centre-bit, will only serve to prevent the eggs from falling till they are firmly glued to the hair, the opening formed by the two shorter points permitting this to be effected. This account of the matter is rendered more plausible, from Réaumur’s statement that the deposition of the egg is not attended by much pain, unless, as he adds, some very sensible nervous fibres have been wounded. According to this view, we must not estimate the pain produced by the thickness of the instrument; for the sting of a wasp, or a bee, although very considerably smaller than the ovipositor of the ox-fly, causes a very pungent pain. It is, in the latter case, the poison infused by the sting, rather than the wound, which occasions the pain; and Vallisnieri is of opinion that the ox-fly emits some acrid matter along with her eggs, but there is no proof of this beyond conjecture.