The circumstance, however, of most importance to our purpose, is the agitation and terror produced both by this fly and by another horse breeze-fly (Gasterophilus hæmorrhoidalis, Leach), which deposits its eggs upon the lips of the horse as the sheep breeze-fly (Œstrus ovis) does on that of the sheep. The first of these is described by Mr. Clark as “very distressing to the animal, from the excessive titillation it occasions; for he immediately after rubs his mouth against the ground, his fore-feet, or sometimes against a tree, with great emotion; till, finding this mode of defence insufficient, he quits the spot in a rage, and endeavours to avoid it by galloping away to a distant part of the field, and if the fly still continues to follow and teaze him, his last resource is in the water, where the insect is never observed to pursue him. These flies appear sometimes to hide themselves in the grass, and as the horse stoops to graze they dart upon the mouth or lips, and are always observed to poise themselves during a few seconds in the air, while the egg is prepared on the extended point of the abdomen.”[GR]
The moment the second fly just mentioned touches the nose of a sheep, the animal shakes its head and strikes the ground violently with its fore-feet, and at the same time holding its nose to the earth, it runs away, looking about on every side to see if the flies pursue. A sheep will also smell the grass as it goes, lest a fly should be lying in wait, and if one be detected, it runs off in terror. As it will not, like a horse or an ox, take refuge in the water, it has recourse to a rut or dry dusty road, holding its nose close to the ground, thus rendering it difficult for the fly to get at the nostril.
a, The belly of the grub. b, Its back. c, The tail of the grub, greatly magnified. d, The bump, or gall, having its external aperture filled with the tail of the grub.
When the egg of the ox breeze-fly (Hypoderma bovis, Latr.) is hatched, it immediately (if Mr. Bracey Clark be correct) burrows into the skin; while, according to Réaumur, it is hatched there. At all events, the grub is found in a bump on the animal’s back, resembling a gall on a tree,—“a place,” says Réaumur, “where food is found in abundance, where it is protected from the weather, where it enjoys at all times an equal degree of warmth, and where it finally attains maturity.”[GS] When in an advanced stage, the bumps appear much like the swellings produced upon the forehead by a smart blow. These, with the grubs, are represented in the foregoing figure, and also at page 434.
Every bump, according to Réaumur, has in its inside a cavity, which is a lodging proportionate to the size of the insect. The bump and cavity also increase in proportion to the growth of the grub. It is not until about the middle of May that these bumps can be seen full grown. Owing to particular circumstances, they do not all attain an equal size. The largest of them are sixteen or seventeen lines in diameter at their base, and about an inch high; but they are scarcely perceptible before the beginning or during the course of the winter.
Fly, maggot, and grub of the Ox breeze-fly, with a microscopic view of the maggot.
It is commonly upon young cattle, such, namely, as are two or three years old, that the greatest number of bumps is found; it being rare to observe them upon very old animals. The fly seems to be well aware that such skins will not oppose too much resistance, and seems to know, also, that tender flesh is the most proper for supplying good nourishment to its progeny. “And why,” asks Réaumur, “should not the instinct which conducts it to confide its eggs to the flesh of certain species only, lead it to prefer the flesh of animals of the same species which is most preferable?” The number of bumps which are found upon a beast is equal to the number of eggs which have been deposited in its flesh; or, to speak more correctly, to the number of eggs which have succeeded, for apparently all are not fertile; but this number is very different upon different cattle. Upon one cow only three or four bumps may be observed, while upon another there will appear from thirty to forty. They are not always placed on the same parts, nor arranged in the same manner: commonly, they are near the spine, but sometimes upon or near the thighs and shoulders. Sometimes they are at remote distances from each other; at other times they are so near that their circumferences meet. In certain places, three or four tumors may be seen touching each other; and more than a dozen sometimes occur arranged as closely together as possible.
It is very essential to the grub that the hole of the tumor should remain constantly open; for by this aperture a communication with the air necessary for respiration is preserved; and the grub is thence placed in the most favourable position for receiving air. Its spiracles for respiration, like those of many other grubs, are situated immediately upon the posterior extremity of the body. Now, being almost always placed in such a situation as to have this part above, or upon a level with the external aperture, it is enabled to respire freely.[GT]