The larvæ live parasitically in various parts of the body of mammals, such as the stomach (horse bot-fly), the subcutaneous connective tissue (warble-fly of cattle), or the nasal passage (sheep bot-fly or head maggot). There are on record many cases of the occurrence of the larvæ of Oestridæ as occasional parasites of man. A number of these have been collected and reviewed in a thesis by Mme. Pètrovskaia (1910). The majority of them relate to the following species. _Gastrophilus hæmorrhoidalis_, the red tailed bot-fly, is one of the species whose larvæ are most commonly found in the stomach of the horse. Schoch (1877) cites the case of a woman who suffered from a severe case of chronic catarrh of the stomach, and who vomited, and also passed from the anus, larvæ which apparently belonged to this species. Such cases are exceedingly rare but instances of subcutaneous infestation are fairly numerous. In the latter type these larvæ are sometimes the cause of the peculiar "creeping myasis." This is characterized at its beginning by a very painful swelling which gradually migrates, producing a narrow raised line four to twenty-five millimeters broad. When the larva is mature, sometimes after several months, it becomes stationary and a tumor is formed which opens and discharges the larva along with pus and serum. _Gastrophilus equi_ is the most widespread and common of the horse bot-flies. Portschinsky reports it as commonly causing subcutaneous myasis of man in Russia. _Hypoderma bovis_ (= _Oestrus bovis_), and _Hypoderma lineata_ are the so-called warble-flies of cattle. The latter species is the more common in North America but Dr. C. G. Hewitt has recently shown that _H. bovis_ also occurs. Though warbles are very common in cattle in this country, the adult flies are very rarely seen. They are about half an inch in length, very hairy, dark, and closely resemble common honey-bees. They deposit their eggs on the hairs of cattle and the animals in licking themselves take in the young larvæ. These pass out through the walls of the œsophagus and migrate through the tissues of the animal, to finally settle down in the subcutaneous tissue of the back. The possibility of their entering directly through the skin, especially in case of infestation of man, is not absolutely precluded, although it is doubtful.