Green Drake Mayfly

Fooled by a smooth black surface

Light reflected off water is horizontally polarized, which means its waves line up in one direction. Ephemera danica detects water with that signal. The more strongly a surface reflects horizontally polarized light, the more attractive it is to mayflies like this one. In a field test, smooth black plastic was the most attractive surface on offer. A cover textured like a rose petal was practically unattractive to them.

On each test day, between 18:30 and 19:15, mainly males flew around the surfaces at first. Then, in the second phase of swarming, most of the landing mayflies were females, often with visible egg batches. Sometimes they laid those egg batches on the test surfaces.

A burrower in the sand

This is a large mayfly, 1.5 to 3.0 cm long, known both as the common mayfly and the green drake mayfly. Its nymphs, the young aquatic stage, live in the depositional zones of streams and rivers with a sandy bottom. There they construct a tubular burrow in the sediment and feed by filtering or collecting fine particulate organic detritus, which is tiny bits of dead material. The species is widespread throughout Europe, and emerging adults have been collected in the littoral zone of Lake Constance in Germany. The littoral zone is the shallow water near a lake’s shore.

Nymphs collected from a Devon river for lab tests weighed between 15 and 128 mg fresh. In two Polish rivers, small mayflies (optimal size 11.63 mm) were mainly settled by ciliated protozoans, and larger ones (optimal size 28.77 mm) by midge larvae.

Passengers aboard On a single mayfly, researchers in two Polish rivers found between 0 and 18 midge larvae and from 0 to 46 ciliate colonies.

A two-year cycle

Across 2007 to 2013, E. danica at a cool, groundwater-fed reach of England’s River Dove kept a 2-year emergence cycle. The adults then fly: the main flight period is towards the end of May, though adults are often present between April and November.

As adults they do not feed, but they do use energy. In a study of the nymphs of this mayfly and another one, triglycerides, a type of fat, were the major energy reserve throughout development, at more than 84% of total energy storage in both. In male nymphs the triglyceride concentrations remained high until emergence, presumably because of the high energy demands of their swarming flights as adults.