Saskatchewan Mayflies (Ephemeroptera)
Ephemeroptera are found throughout the world except Antarctica. Currently 23 families are recognised in North America. Twenty-one are reported from Canada representing approximately 335 species, of which, over 110 species have been recorded from Saskatchewan.
Adult mayflies are relatively uniform in appearance. They range from 5 mm to 3 cm long, with one or two pairs of triangular wings (the front wing is the largest) and two or three long tails or caudal filaments. The adults usually have clear wings with distinct venation.
Larval mayflies are about one cm to 3 cm in length or more. They vary greatly in appearance from stream-lined swimmers to frilly-gilled burrowers and flattened rock-huggers. Their appearance can be quite striking with contrasting color patterns of pale yellow and dark brown to tinges of orange. The unifying feature is the series of abdominal gills variously modified as plates or feathery gills. There can be two or three tail filaments.
The greatest diversity of mayflies is found in cool rivers and streams. A number of species inhabit lakes particularly those with wave washed, rocky shorelines and cold water. Generally mayflies require unpolluted, well-oxygenated, cool waters to survive. This makes many mayfly communities useful indicators of ecosystem health. In many habitats they are important fish food as "fly fishers" will attest.
The life cycle of mayflies is unique among hemimetabolous insects in that larvae emerge into a short-lived, usually sexually immature, subimago or sub-adult stage which looks like the adult but has cloudy wings. This stage is followed by another moult into the sexually mature adult. The mayfly adult typically survives for only a few hours, just long enough to mate and lay eggs. Males have a pair of claspers on the end of their abdomen to grasp the female's abdomen for copulation and often have large eyes relative to the females. The adults have no functional gut and in one species, Lachlania saskatchewanensis, females lack functional legs.
If conditions are favourable large numbers of larvae can synchronously reach maturity resulting in an impressive mass emergence of hundreds of thousands of subimagoes and swarms of adults. Along certain areas of the Saskatchewan River system Ephoron album can emerge in huge numbers to create the illusion of a blizzard at dusk around bridge lights and ferries during August. In the morning all that remains are banks of dead adults under light standards. Similar mass emergences of other species can occur at lakes and ponds around the province.
Females lay their eggs by flying low over the water surface and dipping an extruded egg sac into the water. In some species, that inhabit swift flowing water, the female will crawl under the water surface to lay her eggs directly on submerged rocks. The duration of the egg stage can range from a few days to a number of months. Larvae undergo a number of instars depending on species and conditions. In Saskatchewan the length of the life cycle varies from one generation per year (univoltine) to two or more years to complete a single generation (semivoltine).
Larval mayflies exhibit many interesting body forms. Ephemeridae have large frontal tusks and burrow into the soft substrate of lakes and rivers. In rivers and streams some species have streamlined bodies to swim through the current. Others, members of the Heptageniidae, are flattened and cling to rocks to avoid swept away by the current (See above larval image). This has reached the ultimate level in the genus Rithrogena that have gills modified to act as a suction cup to hold the larvae on the rocks.
Larvae are the only feeding stage of the life cycle. In general, larvae feed by collecting or scraping bits of detritus and algae from the bottom, rocks, wood or plants. A few, Acanthametropus, Analetris, Anepeorus, Raptoheptagenia, have adapted to feeding on other aquatic insects. Others, Ametropus, Isonychia and Arthroplea have specialized mouthparts to filter food particles from the water current.
While some mayflies seem to be flourishing the populations of other species appear to be in decline and some are extremely rare or possibly extirpated in Saskatchewan. In a taxonomic survey of Saskatchewan mayflies, Webb (2002), recorded five species; A. eximia, Ac. pubescens, An. rusticus, M. nipawinia, and L. saskatchewanensis, which he considered truly rare in the sense that they are known from only a few localities and although these locations have been regularly sampled the numbers collected are extremely low suggesting the populations are small or now extirpated. The preferred habitats of these species are large, western North American, prairie rivers (Webb 2002). These large prairie rivers are some of the most utilized, and therefore impacted, aquatic habitats in North America. All have been impounded for agricultural irrigation, urban water supplies and hydroelectric purposes. Not only does this alter the flow regime and silt loads of the river, but it also changes thermal patterns down stream from the dams resulting in biologically significant temperature changes (Lehmkuhl 1972). These alterations coupled with large influxes of agricultural, industrial and urban contaminants have resulted in a significant decline in the quantity and quality of the water flowing through these rivers (Edmunds et al 1976). Since most mayflies, especially the species in question, are intolerant of such impacts many populations have become extirpated thus fragmenting the distributions of these species.
Below is a larval key to the families of mayflies found in Saskatchewan. It has been modified from Webb (2002).
1a: Top part of mesothorax (mesonotum) is in the form of a large shield that covers the metathorax and first few abdominal segments. (Arrow indicates shield-like mesonotum.)--Baetiscidae
1b: Thoracic segments not extensively modified.--2
2b: Head without projecting tusks. Gills not feathery.--4
3a: Tusks curved upwards.--Ephemeridae
3b: Tusks curved downwards with pointed tooth-like bumps (denticles) on the top surface.--Polymitarcyidae
4b: Second gill similar to the other gills on the abdomen or not present.--6
5a: Second gill square-shaped almost meeting at the center line. First gill a thin filament.--Caenidae
5b: Second gill triangle-shaped held more on the side of the abdomen. First gill absent.--Leptohyphidae
6b: Second abdominal gill present.--7
7b: Maxillary palps not as above.--8
8a: Foreleg with claws forked in two.--Metretopodidae
10b: Claws on legs very much shorter than tarsi.--Heptageniidae
11a: Forelegs similar to other legs in appearance.--13
12b: Three tail filaments present.--Isonychiidae
13b: All legs have similarly shaped claws.--14
14a: Gills on abdomen all forked. Both forks similar in appearance.--Leptophlebiidae
14b: Gills different from above.--15
15a: Gills with a thickened brown band along the side margin.--Ameletidae
15b: Gills without brown band. Uniform in color and appearance.--16
16a: Labrum, the upper lip, usually with a notch in the middle. If notch is absent antennae longer than head.--Baetidae
16b: Labrum without a notch and antennae shorter than head.--17
17a: Hind claws only slightly shorter than hind tarsi. Found only in the Saskatchewan River system in SK.--Acanthametropodidae (This family is rarely collected due it being found in large river sandy habitats and its sensitivity to habitat changes. Likely should be considered endangered or at risk in SK (Webb 2002).
17b: Hind claws much shorter than hind tarsi. Found throughout SK.--Siphlonuridae
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