In material collected from Costello creek in Algonquin Park, Ontario, 1,549 males, 598 females and 2 pupae of a new species of blackfly were encountered. This species was present in marerial which was collected daily during the spring and summer of the years 1938-1943 using a cubic yard screen cage placed over the stream.

This species began to emerge between May 7 and 23 in most years, but the total number of flies was smaller when emergence began by May 10 than when it began on May 17 or later. The only exception to this occurred in 1938 when emergence began earlier and continued over a much longer period than usual. The main portion of the emergence ended between May 10 and 29 although occasional flies emerged as late as June 18. None of the new species was collected in 1942. Association of males and females of the new species was possible as both sexes emerged during the same period and as other species, emerging at the same time, were readily identified in both sexes.