by Piter Kehoma Boll
Mayflies make up the order Ephemeroptera, one of the oldest ones among insects. Closely related to dragonflies and damselflies (order Odonata), mayflies have an aquatic nymph and a terrestrial imago (i.e., adult). One considerably well-known species is Heptagenia sulphurea, commonly known as the yellow mayfly or yellow may dun.
Native from Europe, the yellow mayfly lives most of its life as a nymph. It prefers running and clean waters, where it lives under stones and feeds on decaying plant matter and associated bacterial biofilms. The nymph has a flattenned body of a dark color with several yellowish marks. The legs are short and white and have a series of alternating yellow and black sinuous transversal stripes. Like in all mayfly nymphs, the abdomen has visible gills on both sides and three longe cerci (tails) at the tip. During its final stage as a nymph, the yellow mayfly is about 1 cm long.
Most mayflies are very sensitive to pollution and the yellow mayfly is one of the most sensitive of all, at least in Europe. Whenever the water of a streams starts to get polluted, the yellow mayfly is the first mayfly species to disappear. Thus, its presence indicates water of very good quality.
Different from all other insects, mayflies have an intermediate stage between the nymph and the imago stages, the so-called subimago. This stage is already terrestrial like the imago and already has wings, although they are often less developed, making them poor fliers. This subimago stage is commonly known as dun and, in the yellow mayfly, it has a typical yellow color, hence the common name yellow may dun. Females have black and poorly developed eyes, while in males the eyes are larger and vary from dark gray to whitish. Nymphs molt into subimagos beginning in May, when the peak occurs, but may appear as late as July.
When the subimago molts into the adult, usually after only a few days, the body becomes light brown and the eyes whitish in both sexes, but the eyes are still smaller in females than in males. Adults have the sole purpose of reproducing and so they do. After mating, the male dies in a few hours, and so does the female after laying her eggs in a stream.
The yellow mayfly is often used as a fishing bait. Once a common species across Europe, its populations have decreased considerably in the last century due to the increase of water pollution. Some recent efforts to despolute streams may, fortunately, help this and other mayfly species to find again more room to thrive.
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References:
Beketov MA (2004) Different sensitivity of mayflies (Insecta, Ephemeroptera) to ammonia, nitrite and nitrate between experimental and observational data. Hydrobiologia 528:209–216.
Macan TT (1958) Descriptions of the nymphs of the British species of Heptagenia and Rhithrogena (Ephem.). Entomologist’s Gazette 9:83–92.
Madsen BL (1968) A comparative ecological investigation of two related mayfly nymphs. Hydrobiologia 31:3–4.
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