Very closely allied to the true breeze-flies in habit of life are the species of the genus Chrysops, of which two only are often met with in England, namely Ch. cœcutiens and Ch. relicta; these flies are very keen blood suckers; they are smaller, slightly more slender and brighter coloured than the commoner Tabanidæ; it is characteristic of the genus Chrysops that the antennæ are quite twice the length of the remarkably short horns of the majority of common full-bodied flies; all the species possess beautiful golden glittering eyes (whence the name Chrysops), and their wings are spotted and tinted.
One of the most horribly disgusting but serious facts connected with flies is observable most conspicuously amongst the wondrous family of the Œstrida. These pass the larval stage of life, not on, but inside the bodies of living animals; and the perfect insect, strange to say, is absolutely destitute of a mouth opening. Much misrepresentation has been prevalent, based entirely upon surmise, connecting "myiasis" in mankind, which is various but very rare, with the common infliction of horses and horned cattle with Œstrid maggots. Myiasis is the medical term given to all the various forms of animal infliction by internal parasitic maggots, and this subject is reserved for discussion in the next chapter.
The characteristics and natures of the very numerous tribes and families of other kinds of flies will be found summarised in the Appendix of this booklet.
CHAPTER IV
MYIASIS AND THE ŒSTRIDÆ
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 œstrid 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.