_Chrysomyia macellaria_, the screw worm fly (fig. 107), is the fly which is responsible for the most serious cases of human myasis in the United States. It is widely distributed in the United States but is especially abundant in the south. While the larvæ breed in decaying matter in general, they so commonly breed in the living flesh of animals that they merit rank as true parasites. The females are attracted to open wounds of all kinds on cattle and other animals and quickly deposit large numbers of eggs. Animals which have been recently castrated, dehorned, or branded, injured by barbed wire, or even by the attacks of ticks are promptly attacked in the regions where the fly abounds. Even the navel of young calves or discharges from the vulva of cows may attract the insect. [Illustration: 104. Calliphora erythrocephala, (×6). After Graham-Smith.] Not infrequently the fly attacks man, being attracted by an offensive breath, a chronic catarrh, or a purulent discharge from the ears. Most common are the cases where the eggs are deposited in the nostrils. The larvæ, which are hatched in a day or two, are provided with strong spines and proceed to bore into the tissues of the nose, even down into or through the bone, into the frontal sinus, the pharynx, larynx, and neighboring parts. Osborn (1896) quotes a number of detailed accounts of the attacks of the _Chrysomyia_ on man. A vivid picture of the symptomology of rhinal myasis caused by the larvæ of this fly is given by Castellani and Chalmers: "Some couple of days after a person suffering from a chronic catarrh, foul breath, or ozæna, has slept in the open or has been attacked by a fly when riding or driving,--_i.e._, when the hands are engaged--signs of severe catarrh appear, accompanied with inordinate sneezing and severe pain over the root of the nose or the frontal bone. Quickly the nose becomes swollen, and later the face also may swell, while examination of the nose may show the presence of the larvæ. Left untreated, the patient rapidly becomes worse, and pus and blood are discharged from the nose, from which an offensive odor issues. Cough appears as well as fever, and often some delirium. If the patient lives long enough, the septum of the nose may fall in, the soft and hard palates may be pierced, the wall of the pharynx may be destroyed. By this time, however, the course of the disease will have become quite evident by the larvæ dropping out of the nose, and if the patient continues to live all the larvæ may come away naturally." For treatment of rhinal myasis these writers recommend douching the nose with chloroform water or a solution of chloroform in sweet milk (10-20 per cent), followed by douches of mild antiseptics. Surgical treatment may be necessary. [Illustration: 105. Larva of a flesh fly (Sarcophaga). Caudal aspect. Anterior stigmata. Pharyngeal skeleton.]