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.