The family of Œstrida has been fitly divided into three sections, namely, the Gastrophilinæ (the larvæ living in the gullet, the stomach, or the intestines), the Hypoderminæ (worble-flies), and the Œstrinæ (nasal or nostril flies); all the species are hairy or furry, and the gravid females fly slowly with loud buzzing, in a characteristic attitude peculiar by the bending downwards of abdomen and tail, with a much extruded ovipositor.
The sheep's nostril-fly, Œstrus ovis, has a chequered abdomen and is less hairy than others; it is the type of the section to which the generic term Cephalomyia is given in some books; species of this section attack deer and other animals.
The section termed Hypoderminæ comprises the "worble" flies or "marble" flies. One may imagine that the latter name indicates in the mind of the cowherd the appearance of the round pustulent boils on the hide of the suffering animal, and that the former name is a corruption of "worm-hole," originating with the tanner, observant of the deterioration of injured hides. A mixing of the terms worm-hole and marble probably originated the name "warble." The maggots live under the skin on the back of oxen, and breathe externally through openings in the boil-like excrescences. The discoloured flesh of infected oxen is called "flecked." Two species of worble-flies are prevalent, one or the other, in many parts of England.
The third section, to which the sub-family termed Gastrophilina is sometimes applied, comprises the "bot-fly," which commonly infects the horse; it is the imperfect knowledge of this latter which has led to erroneous surmises explanatory of the horribly disgusting fact of human intestinal myiasis.
All the species of all the three sections are single-brooded. Although the flies themselves can inflict no immediate pain, at their mere sight all the animals out at grass on the farm are seized with an instinctive terror, conspicuously greater than when attacked and copiously bled by any "blinden" breeze flies, which, however, fly more silently and settle on their victims very furtively. One can understand the violent efforts of the horse to free himself from the exceedingly painful bites of a newly attached forest-fly, but one can only wonder at the frantic galloping of oxen and horses to and fro when a non-biting œstrid fly buzzes about like a harmless fat bumble bee and slowly approaches.
The females of all the worble-flies, the nostril-flies, and the bot-fly are short-lived, appearing on the wing in August, possibly seen a few days earlier. In the act of ovipositing they make themselves very conspicuous; they lay their eggs whilst hovering in the air, their extruded ovipositors attaching glutinous eggs to their victims. The hatching of the eggs of the bot-fly is assisted by the habit of animals to lick themselves and each other, when certainly their warm, moist tongues will convey into their mouths the newly emerged bot-fly's maggots, which many months later are to be found attached to the internal lining of the unwilling host's stomach. When fully grown in June, these maggots loosen their hold, are discharged with the dung, and pupate in the soil.
No satisfactory account has yet been given as to the early stages of the maggots of the worble-flies. The eggs, having been attached to hairs on the host's hide in August, the prominent round pustulent swellings, called worbles, wherein the maggots dwell, do not become conspicuous until the following months of April and May. It is a reasonable surmise that the obscure and long first-period of the maggot's existence may more or less conform to that of some of those flies which are also single-brooded but are predaceous or parasitic on insects. The newly hatched maggot perhaps can crawl, but does not feed until after several moults; at each moulting the strange creature becomes smaller and smaller, but probably at the same time is provided with a new head well suited for the purpose of that period; firstly, with a burrowing or grappling head, and in due time with a feeding suctorial mouth, and then only does practical growth begin. No dipterid flies, at all events, known to be native to Great Britain, possess skin-piercing ovipositors.
I have been astonished to read in current literature much about œstrid flies which is not in agreement with my long course of personal observation; for instance, one high authority (F. R. S.) writes that œstrid "flies" appear from May until October, and hints that their egg-laying aggressions upon their victims are not conspicuously observable. I feel confident that the facts are quite otherwise.
That the bot-flies normally (and a few others abnormally, but for short periods only) pass a very long larval stage in the stomach and alimentary canal of herbivorous animals is one of the greatest marvels of insect life. All other growing creatures, which normally breathe in free air, require a certain large amount of breathable oxygen; and they would be stupefied or killed by a much smaller percentage of carbonic dioxide and other fermentive gases of digestion than undoubtedly exist in the strange abode wherein the bot-fly maggots dwell during the entire period of their feeding career. It has been stated that fly maggots artificially ingested into the human system have emerged alive in a normal condition, but the repulsive and objectionable experiment is not stated to have procured well nourished and full grown normally pupating larvæ. Some of the maggots of human intestinal myiasis are not perhaps amenable to artificial culture up to the stage of final metamorphosis; and they do not appear to have developed a breed or new species with a distinct habit of life. All the credible accounts of human intestinal myiasis point towards some fly which is plural-brooded, and of which the larvæ develop rapidly and promptly quit the body all at once; otherwise more than one infection must have occurred. The tales of prolonged continuous breeding, with slow and prodigiously copious emergings at intervals, should be altogether discredited.
It is an amply warranted criticism to say that recently published records by authorities, in an endeavour to comprise every reported instance of myiasic infection, seem to countenance mere coarse Gargantuan jokes. On the other hand, it is painful to read such a "cock-and-bull" story as that of the doctor about his elderly lady patient, up whose nostril a gravid female blue-bottle flew and successfully performed the prolonged and delicate operation of laying therein a large batch of eggs, in spite of all attempts to expel the invader by violent sneezing. Day by day the said doctor observed the terrible injury, and the symptoms accompanying the growth of the feeding maggots, whilst the injection of a spoonful of paraffin would have effected an instantaneous cure.