The ranks for this species group were reviewed based on one paper about the species found in the Mackenzie Valley collected in 1970s (Giberson & Burian, 2017).

The Ephemeroptera are a small order of insects with aquatic immature stages and terrestrial adults. They are the oldest order of winged insects still living today, dating from Carboniferous and Permian times.

As with other insects, they go through a number of larval or nymphal stages, but whereas most insects have a single adult winged stage for dispersal and mating (though note that some, such as fleas and lice, are wingless, even as adults) mayflies are unique among insects in that they have two winged stages. The first is known as the subimago (or in fly-fishing terminology, the dull-coloured “dun”), and the second stage is called the imago (the clear-winged and brighter coloured “spinner”).

Mayfly life cycles can range from a few weeks to several years, depending on the species and the location. Most of the life is spent as a nymph in the water, and growth is strongly dependent on temperature and food. The number of nymphal growth stages (instars) can range from 10 to 50 and is not fixed even within particular species’ life cycles. Once they reach a certain size and maturity as nymphs, emergence in most is triggered by specific temperature or light. The adults do not feed, so adults rarely live longer than a few hours to a few days. Mayflies in temperate and arctic areas are usually highly seasonal, with hatching from the egg and emergence to the adult controlled by a combination of temperature and light patterns.

Most mayflies deposit their eggs by flying to the water surface and dipping their abdomens into the water, releasing a few eggs at a time to fall to the substrate below. Some species in the genus Baetis can enter the water directly and swim to the bottom, laying the eggs directly onto suitable substrates. The eggs have attachment structures that stick them to the substrate materials so they are not dislodged in water currents.

Egg development can range from as little as a week to more than a year, depending on species-specific water temperature requirements. A few species have eggs that enter a resting stage called diapause if temperatures are too low, delaying hatching until conditions are suitable for the nymphs to survive. For example, the widespread northern small minnow mayfly, Baetis bundyae, can live in small streams and ponds that freeze to the bottom because it spends most of the year as a resistant egg, and only hatches once the water reaches a specific temperature in summer. This species completes development from egg hatch to adult in 3 to 4 weeks, so is easily missed when collecting mayflies. Most species, however, require habitat that does not freeze solid in winter.

Nymphs emerge (‘hatch’) to the first winged stage by swimming to the surface of the water and using the last nymphal skin (exoskeleton) as a raft while the winged stage crawls out of a split that forms along their back. After emergence, these “cast skins” can often be seen floating on the water in bays or backwaters. The subimago (dun) is covered with fine, water-resistant hairs to help keep them afloat as they harden their wings enough to fly from the water surface. The subimago doesn’t fly well, and usually seeks out streamside vegetation to attach to, avoiding predators. Tundra species can also spend their subimago stage under loose rocks on shore if there is little or no streamside vegetation. Within a few hours, they moult again, this time to the imago or sexually mature adult.

In temperate regions, males often form impressive mating swarms at dawn or dusk, and females enter the swarm and mate on the wing. In the North, mating swarms can occur at any time of day, and may be dependent on air temperature. Some mayfly species are parthenogenetic (where eggs develop without needed to be fertilized), so males may be rare or even absent in these species.

The nymphs live in a variety of freshwater habitats and show a range of body shapes and feeding relationships. Some (for example, the “minnow mayflies”) have a streamlined body shape that allows them to swim easily or to live in moderate water currents. Others are flattened in shape, which lets them hug the substrates in fast moving water without being dislodged and carried downstream. All mayfly nymphs have gills on their abdomens that increase their body surface area for picking up oxygen from the water, and these gills range from simple plate-like structures to complex feathery structures that can be moved in unison to move water past the gills to increase oxygen uptake. One group (the burrowing mayflies in the family Ephemeridae) burrow into the bottom mud or sand of lakes and large rivers and use their gill movements to circulate water and food through their burrows.

Most mayfly nymphs feed on plant or plant-like material, either by scraping algae from rocks or collecting and eating dead and decaying plant material (detritus). Some filter the fine detritus from the water, whereas others scoop up detritus deposited on the bottom. Some (Isonychia, Siphlonurus, Stenonema, and Ephemera) are omnivorous, but only a few species are primarily predaceous.

The nymphs have strong species-specific microhabitat preferences, and are generally intolerant of water pollution (especially acidification or nutrient enrichment that leads to low oxygen in the water), so they have become important bioindicator species for scientists to assess the health of streams or ponds where they are found. Interestingly, one group (the Baetidae) which is relatively pollution-tolerant (compared to other mayfly families) is also the most dominant family found in the North!

The nymphs are very important ecologically, as members of aquatic food webs and in cycling nutrients. They are eaten by other invertebrates and are particularly important food organisms for fish. Once in the terrestrial habitat, they are also important food for birds, bats, and shrews. Fly-fishers recognize their importance, and many important artificial flies for fishing attempt to mimic mayflies!

There are 85 species of mayfly confirmed present in the NWT. Two additional species are expected to be present.