This fly attacks the host from June to September, depositing its viviparous larvae in the nostrils. The Reindeer attempts to fend off the fly, striking at it with its hoofs and keeping its nostrils closed as far as possible. Once deposited, the lively larvae crawl into the inner nasal passages and as far as the larynx, where they fasten themselves and live on the mucus. A Reindeer may harbor as many as 130 of these parasites. They range from 6 to 26 mm. in length. Their particular growth begins at the end of March, and they are ready for pupation up to May. The host assists their exit by continual sneezing and snuffling. In the last stages they are a great affliction for the host, and they sometimes cause its death. Pupation takes place in or on the ground, under some sort of cover, and it lasts for 15-19 days. The flies have been found emerging from July 12 to 31. The corresponding parasite of the Barren Ground Caribou is a similar or perhaps identical species, with a parallel life history. Its chief activity as an adult doubtless occurs in July and August. A number of the mature larvae were found in the throat of the buck of June 3; two of them that were preserved measure approximately 7 mm. in diameter and 27 and 30 mm. in length. A large mass of such sizable parasites in the throat might easily become a serious obstacle to comfortable living or even to survival on the part of the host. Presumably the larvae drop to the ground at about the same period as those of _Oedemagena_. Fred Schweder, Jr., remarked concerning the buck of August 17 that these larvae are never found at that season, and Charles Schweder made the same remark concerning a doe specimen of September 21. It would appear either that they remain so small as to escape detection at this time or that they do not reach the throat on their short journey from the nostrils until some later period of the year. Johansen (1921: 24) records larvae only 2-3 mm. long in the nasal passage at the end of May. Since the bulk of the Caribou population has passed well to the northward of the Nueltin Lake region by the time the larvae of _Oedemagena_ and _Cephenemyia_ drop out of the bodies of their hosts to pupate briefly on or in the ground (say in the latter part of June), one is tempted to speculate on the possibility that the adult flies found here in August may have followed their prospective victims for many miles in their southward migration. However, Porsild remarks (1943: 386) that they “apparently do not travel very far.” Certain kinds of behavior exhibited by the Caribou in attempting to fend off the parasitic flies are discussed in the section on _Shaking off moisture and insects_.