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]