The Mayfly

Consider the mayfly (order Ephemeroptera). This winged aquatic insect is considered to be one of the ancients, a member of a primitive group that includes a dopplegänger of sorts, the dragonfly (order Odonata).

During the fall and spring, the mayfly can be found in abundance along the rivers and streams of Vermont, but that wasn’t the case last October. While surveying the Middlebury and New Haven Rivers last autumn, Vickie Backus and her Bio 140 class made a surprising discovery: the Mayfly population had plummeted.

Normally the dominant insect counted in biannual surveys of the two local rivers, mayfly numbers had dropped precipitously during the fall 2011 count. According to Backus, past semesters have shown fall counts anywhere between 9.4 and 17 mayflies per sample; last October the mayfly count was 2.3 per sample. In November it had dropped further, to 1.5.

Backus believes that the number of mayflies found in streams is dependent on the stream flow rate, and while mayflies are not resistant to change they are affected by major events. Events like Hurricane Irene, which slogged through Vermont six weeks before the biology class’s first sample.

This spring, counts have begun to rebound, though not yet to normal levels. (Counts in March and April revealed 8.2 and 5.5 mayflies per sample, respectively. Past spring counts have ranged between 10.9 and 14.8.) Come October, Backus and Bio 140 will be back in the Middlebury and New Haven Rivers collecting mayflies. Time will tell, if they once again become the dominant insect in our streams

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