As far as I am able to judge from my own experience, mosquitoes are more or less equally numerous and ferocious in the Canadian, the Hudsonian, and the Arctic Zones of the Northwest. Naturally the season begins earlier in the more southerly localities. In two seasons (1914, 1920) at the western end of Lake Athabaska they began to be troublesome about the middle of June, whereas at Nueltin Lake this stage was reached about the first of July. In the Athabaska and Great Slave lakes region (Canadian and Hudsonian Zones) I have never had occasion to regard black flies as serious in respect to either numbers or ferocity; but there is universal agreement that conditions are vastly different and worse on the Barren Grounds. At Nueltin Lake the _Simulium_ hordes become troublesome at approximately the same time as the mosquitoes. Toward the end of August there is a merciful diminution in the numbers of both mosquitoes and black flies on the Barrens, and after the first of September they may be practically disregarded, except on an occasional day of unseasonable warmth. It may be remarked in passing that one of the insect terrors of the Athabaska region, the so-called “bulldog” (a species of Tabanidae), did not come to my attention as a pest at Nueltin Lake though I collected two species of _Tabanus_. Malloch (1919), in reporting on the Diptera of the Canadian Arctic Expedition of 1913-18, does not include a single species of Tabanidae. On the other hand, Twinn (1950: 18) states that 17 species have been found at Churchill; he refers to tabanids as “very abundant in forested regions of the North.” The “bulldog” may be presumed to contribute to the summer misery of the Woodland Caribou and the Moose as well as of man. The season during which the adult warble flies and nostril flies harass the Caribou probably lasts only a few weeks in July and August. While the adults evidently cause no pain, they no doubt arouse an instinctive dread in the prospective hosts of their larvae. While I have no information as to whether they follow the hosts into the wooded country, it would appear quite likely that they do so if we are to credit statements by Franklin (1823: 242), Richardson (1829: 251), and B. R. Ross (1861: 438) that these pests infest the Woodland Caribou as well as the Barren Ground species. Furthermore, _Cephenemyia_ has been reported in Pennsylvania as a parasite of the White-tailed Deer (_Odocoileus_) (Stewart, 1930?). The scarcity of available study material may be judged from the fact that the Canadian Arctic Expedition of 1913-18 secured only three adults of _Oedemagena_ and none of _Cephenemyia_ (Malloch, 1919: 55-56).