The family of the ŒstridÆ is the most curious and horrific of all the different tribes of flies; it is very limited in species, of which five or six are prevalent throughout Great Britain. The worst of these could be almost exterminated with ease, but unfortunately mistaken ideas have prevailed, and graziers commonly believe that though the sheep's nostril fly is conspicuously harmful and dangerous, the horse's bot-fly and its congeners are negligible as regards the practical health of the host. The bot-fly and the worble-flies are all of a largish size, only the sheep's nostril fly and Œstrus hÆmorrhoidalis, which latter infests the throat and rectum of the horse, are of a medium size.

It has been known from very ancient times that man himself was not exempt from some fly, which was imagined to resemble the horse's bot-fly, and it has been wrongly surmised that many different creatures and all ruminant animals were more or less subject to the attacks, each one of its own kind, of oestrid fly. It is undeniable that man is sometimes internally afflicted with dipterid larvÆ, but it is most certain that the fly to be incriminated is not a congener of the horse's bot-fly.

An old illustrated French encyclopÆdic work gives coloured pictures of the flies and larvÆ of Œstrus bovis (the worble-fly of the ox) and of Œstrus equi (the bot-fly of the horse), but only the larvÆ of a so-called Œstrus hominis is figured. Recently, however, new attempts have been made to identify the species causing intestinal myiasis, of which the larvÆ are observable from time to time in the course of post-mortem examinations and during anatomical study. Of recent years it has been suggested that the lesser house-fly is addicted to such a manner of breeding; then later that another species of the same genus has been found to be the real culprit. However, the peculiar larvÆ of these last-mentioned flies do not in the least resemble the fat round larvÆ of the true bot-fly or of the worble-fly, which are correctly represented in the above-mentioned French work, nor the round and rather smooth maggots which were observed in Westminster Hospital nearly fifty years ago, and at other places from time to time both before and since, giving rise to much wonder and discussion, and also to very incredible tales.

Another more credible surmise attributes the offence of human intestinal myiasis to Muscina stabulans; if this be correct, the infliction would be probably due to the subject having eaten damaged and egg-laden plums or similar fruit, for M. stabulans is credited with being normally, though not exclusively, fruitarian or vegetarian.

If any one of the above suppositions be true, it does not exclude any other one, amongst many explanatory surmises, from being possible. Judging from the remarkable attractiveness of the odour of humanity to the common house-fly, and from the fact of the maggots possessing well developed tenter-hooks on their heads (somewhat like those which the bot-fly maggots use for internal attachment), it is just as likely, nay more likely, that this species (as the writer stated for the information of the authorities of Westminster Hospital nearly fifty years ago) is more than any other capable of adopting such a life-cycle existence; these maggots would mature after five or six days feeding and then emerge. If there were a veritable "Œstrus hominis," however rare, the hairy and peculiar female would be conspicuously observable, a persistent hoverer about the person of her victim until she had attached eggs to his body, from which the maggots would not emerge until after nine months. Most of the tropical flies, which are said to similarly attack humanity, may be rather compared to the green-bottle flies which infest sheep, but the latest medical records and reports profess to identify ten or twelve species of very different genera as having myiasic capabilities.

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 oestrid 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 oestrid flies which is not in agreement with my long course of personal observation; for instance, one high authority (F. R. S.) writes that oestrid "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.