Fig. 9.—Eggs of H. lineatum, attached to hair of cow. Five of the eggs are hatched and six unhatched. Magnified 15 times. (From Carpenter, Hewitt, and Reddin, Journ. Dept. Agric. Ireland, xv., 1914.)
The eggs are large, 1·25 mm. in length, and enclosed in a whitish shell, which is prolonged behind into a brownish foot, and this foot, which exudes some sticky excretion, adheres to the ruminant’s hairs. The foot of the egg-shell, in fact, consists of two lobes or valves, which clasp the hair between their sticky inner surfaces.
Fig. 10.—Eggs of H. bovis attached to hairs. Note attachment near base. Slightly enlarged. (From Hadwen.)
Within the egg the youngest of the four larval stages is maturing. When hatched it is less than 1 mm. long, but it is ‘a terror for its size,’ being armed with a formidable spine and two hooks in the mouth, and with rows of strong spines on all the body-segments. Later, we find a second stage, very much smoother and less spiny than the first and this lies within the tissues of the host, embedded in its muscles and membranes, notably in the submucous coat of the gullet; and now the question confronts us, which once confronted George III apropos of the apple in the apple dumpling, ‘How the devil did it get in?’ There seems to be with Hypoderma but two possible modes of entrance into the body of its host—that is, domesticated cattle: (1) The eggs, or the newly hatched larvae, are licked up by the tongue, as are the eggs of the horse bot-fly—and this might be held to explain the not infrequent occurrence of the second larval stage in the walls of the oesophagus; or (2) the larvae bore their way directly through the skin. From experiments carried on for several years which show that cattle unable to lick themselves are not protected from warbles, Professor G. H. Carpenter of the Royal College of Science, Dublin, concluded that the larvae do not enter by the mouth. During the summer of 1914, he and his able assistant, the late Mr. T. R. Hewitt, definitely proved that ‘the newly hatched maggot does bore through the skin of cattle’; probably after an ecdysis they find their way to the submucous coat and muscles of the gullet, and here for a while they rest. I quote from the account of Carpenter and Hewitt some of their most crucial experiments carried out at the Athenry and Ballyhaise Stations of the Irish Department of Agriculture:—
In July 1914, twenty-four maggots were hatched in the incubator, and some of these were used for observations as to behaviour when placed on a calf’s body. Glaser, in 1913, had tried to carry out observations of this kind by placing maggots on a shaved portion of a calf’s skin; he found that they made no effort to bore through. Instead of being shaved, a small patch of the shoulder of one of the Ballyhaise calves was clipped, so as to have the conditions as normal as possible, when newly hatched maggots of H. bovis were placed on it. Immediately they started crawling down the clipped hairs to the skin, and, as soon as they reached the surface, they began to burrow. On account of their small size it is hard to discern them, but by carefully watching through a lens it was seen that they enter perpendicularly to the surface, evidently cutting into the epidermis with their mouth-hooks and occasionally bending their bodies. Mr. R. G. Whelan, A.R.C.Sc.I., Superintendent of the Ballyhaise Agricultural Station, kindly helped in the observations and confirmed them. Six hours after being placed on the calf, the maggots disappeared completely. Next morning the spots where they had entered were marked by little pimples, like those of the Athenry animals, easily to be seen with the naked eye. These increased slightly in size, but soon healed up, and in less than a week not a trace of the maggots’ entrances could be found. The boring-in of the maggots seemed at first to cause the calf a little pain, but the symptoms of discomfort soon passed away.
We have still to find out what happens to the first-stage larva after it has bored into the skin and how far it travels before it undergoes its first moult. Gläser found that some eggs of H. lineatum laid on his trousers hatched, and that a maggot bored right through into his own skin. From symptoms of swelling and pain in various regions he concluded that this maggot travelled to his gullet, and he finally extracted it (in the second stage) from his mouth.[4]
Fig. 11.—Entrance hole of H. lineatum maggot into the skin of a cow. The hairs around the hole have been clipped short. The white incrustation is due to a discharge from the hole, which has hardened. Magnified 12 times. (From Carpenter, Hewitt, and Reddin, Journ. Dept. Agric., Ireland, xv., 1914.)