A. I'd be willing to bet it is a coincidence. But, it is a coincidence based on the natural thermal history the stream goes through each year whereby the necessary number of degree days is accumulated at the time the stream reaches 48 degrees. Look at it this way. An insect requires a certain development time as it goes through its instars and this rate is governed by temperature. If your population of E. subvaria was, let's say, 75% through its development time and something happened that warmed the water quickly to 48 degrees, the insects would not be developmentally able to emerge. Their development would be accelerated and they would probably emerge earlier than normal (by acquiring the necessary number of degree days faster), but they still need to pass through the necessary developmental stages to reach maturity.

I'd also be willing to bet that your stream has a fairly constant temperature regime during the year - one that is not interrupted by abnormal fluctuations. This would explain why your population appears to obtain their necessary number of degree days at a fairly constant time, in terms of water temperature, each year.

I hope this helps. If you want to pursue it, contact me directly at cecushing@aol.com.