It starts here in the western United States in mid-June in most years and its appearance is greatly anticipated by fly anglers. They come from the far corners of the globe to Montana, Wyoming and Idaho to experience it and only rarely does it disappoint. The yellow haze is caused by the appearance of what has been described as the most important fly hatches in the western states; the insects that comprise the flies commonly referred to as PMD's ? Ephemerella infrequens and Ephemerella inermis. [New names ? if you care ? Ephemerella dorothea infrequens and Ephemerella excrucians] There are also other insects that are a similar yellowish color that hatch during this time of the year, in the east, Midwest and west, but the insects that are called PMD's by anglers are what cause the greatest amount of attention.