More Minor Horrors

by Sir A. E. Shipley

Anything that seriously destroys the continuity of the integument of our oxen, which interferes with the ‘wholeness’ of the hide which is the basis of leather, clearly affects—and affects detrimentally—an important munition of war. The bot- or warble-fly does this. But it does more: its attacks materially lessen the value of the beef which potentially lies beneath the hide, and thus in a double sense the warble-fly is the enemy of man whether he be soldier or sailor. Further, its attacks seriously lessen the milk-supply of the country.

Amongst the numerous families into which the true flies (Diptera) are divided, none are more harmful to human enterprise than that of the Oestridae, or bot-flies, inasmuch as every single species and every single member of this family passes its larval stage within the tissues of some vertebrate host, and frequently in those of domesticated cattle; sometimes even in man himself. One of the commonest genera of this family of flies is Hypoderma, which is represented in our islands, and in many other parts of the world where domesticated cattle are reared, by two species—H. bovis and H. lineatum, both commonly known as bot- or warble-flies.

The harm caused by these larvae, living as they do in the tissues of the body, beneath the skin, by piercing holes through the integument or skin, whereby they make their exit from the ‘warble’ or subcutaneous tumour in which they have passed their latest larval stage, is almost incalculable.

Fig. 8.—a, Hypoderma bovis; b, maggot of H. bovis; c, egg of H. bovis; d, puparium of H. bovis; e, egg of H. lineatum; f, maggot of H. lineatum; g, Hypoderma lineatum. All the figures are magnified. (From F. V. Theobald’s Second Report on Economic Zoology, British Museum, 1904.)

Miss Ormerod, who for so many years kept alight the lamp of economic entomology in England, published some statistics on this subject towards the end of the last century. In 1888, out of slightly over 100,000 hides dealt with in the Newcastle cattle and skin market, 60,000 were ‘warbled,’ and the loss to the trade amounted to £15,000. The same year at Nottingham 8500 out of 35,000 hides were largely spoiled; at Manchester 83,500 out of 250,000 suffered from the same cause: the losses in these towns being estimated for the year in question at about £2000 and £17,000 respectively. Taking the average from all sources in England, Miss Ormerod estimated the fall in value at from 5s. to 6s. on every warbled hide. The most riddled hides—that is, those with the most punctures—come to the sale-room during April and May, but the trouble extends from February to September.

There is also the loss caused by the warble to the butcher—and through the butcher to the Army Service Corps. The presence of the fly-larva, which is quite a large creature, induces chronic inflammation in the tissues, and a state of things known to the trade as ‘licked beef,’ and unless the meat-salesman cuts away the affected parts the meat is unsaleable in the market, or greatly depreciated in value. The average loss to the butcher on a warbled carcass is estimated at 6s. 8d.

Finally there is a loss to the stock-raiser and dairy farmer. We shall have occasion later to refer to the curious psychological effect the warble-fly has upon the cattle, causing them to ‘gad’ or stampede in wild gallops, which interferes with fattening, deteriorates the milk-supply, and is especially injurious to cows with calf. Mr. Imms, in his most useful summary of the warble-fly, tells us that the loss due to H. lineatum in America is calculated at 28 per cent. of their total value of all the cattle in the States. Some authorities place the total loss to the agricultural community in England at £2,000,000, others at £7,000,000, a year, whilst others estimate that the loss amounts to about £1 sterling on every head of horned cattle.

Curiously enough, the fly itself is rarely seen, and still more rarely taken. Mr. Imms records only two specimens of H. bovis in the collections of the British Museum, and but fifteen of H. lineatum. A similar scarcity of imagos in public collections obtains on the other side of the Atlantic, where for many years the last-named species was alone recognised. Two years ago, however, Dr. Hadwen, working in Canada, established the widespread existence of H. bovis in the Dominion; almost certainly it also occurs in the States; but Dr. Hadwen had to send to Dublin for specimens with which to confirm his find. None existed in the collections in Ottawa, and a ‘request for a specimen ... from the Bureau of Entomology at Washington, D.C., could not be granted owing to a scarcity of specimen’! These statements are interesting, since at present the tanneries of Canada are working night and day to help our shortage in leather at home.

H. bovis measures ⅝ in. in length, H. lineatum, somewhat less robust, ½ in.; the hairy covering of the last named is of a foxy red at the tail end, while that of H. bovis is yellow, both at the tail end and towards the front of the body. The flies are most abundant during July and August, though they are believed to occur throughout the summer. At Athenry (co. Galway) H. lineatum is common by the middle of May. They fly very rapidly, and are difficult to follow with the eye. They rejoice in warm, sunny weather, and remain in retirement during cold or cloudy days. Hadwen describes the egg-laying by the female ‘as a sort of frenzied process, the fly striking’ with its ovipositor twenty or thirty times rapidly, then leaving the animal for fifteen minutes or so, when the process was repeated. The eggs are attached one at a time to the hairs of the cattle and very close to the base of each hair, not near the tip, where the horse bot-fly deposits its ova. The eggs of H. bovis are scattered and isolated; those of H. lineatum are arranged in rows of some seven or more half-way up the hair and are contiguous. The favourite region for placing the eggs is on the hock and on the back of the knee, or on the thighs and flanks, and hence the American cowboys call the insect the ‘heel-fly.’ Undoubtedly by standing with their legs in water the herd is delivered from the pest—at any rate, for the time.