Saskatchewan Mayflies (Ephemeroptera)
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. The subadults, or subimagoes, have cloudy wings.
Larval mayflies range in size from about one cm to 3 cm 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 orange tinges. The unifying feature is the abdominal gills variously modified as plates or feathery gills. There can be two or three tail filaments.
Ephemeroptera are found throughout the world except Antarctica. Currently twenty families are reported from Canada represented by 321 species. The Saskatchewan fauna is made up of 107 species belonging to 18 families.
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. In warm prairie sloughs and potholes only three species have been recorded. Generally mayflies require unpolluted, well-oxygenated, cool water to survive. This makes many mayfly communities useful indicators of ecosystem health. In many habitats they are important fish food, as "fly fishermen" 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 prior to moulting into the sexually mature adult. The mayfly adult typically survives for only a few hours, just long enough to mate and lay eggs. Adults have no functional gut and in one species, Lachlania saskatchewanensis, females lack functional legs. The adults are often seen flying in mating swarms. 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.
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 (growth stages) 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 being swept away by the current. 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.
If conditions are favourable for large numbers of larvae to synchronously reach maturity an impressive mass emergence of hundreds of thousands of subimagoes and swarms of adults may occur. Along certain areas of the Saskatchewan River 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 the light standards. Similar mass emergence of other species can occur at lakes around the province.
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)
Arrow indicates shield-like mesonotum. is in the form of a large shield that covers the metathorax and first few abdominal segments.--Baetiscidae
1b: Thoracic segments not extensively modified. 2
2a: Mouth with forward projecting tusk-like 
Arrow indicates "tusk". structures. Gills 
~~~~ on abdomen in two parts (forked) and "feathery". 3
2b: Head without projecting tusks. Gills not feathery. 4
3a: Tusks curved upwards Arrow indicates "tusk". .--Ephemeridae
3b: Tusks curved downwards 
Arrow indicates "tusk".. With pointed tooth-like bumps (denticles) on the top surface.--Polymitarcyidae
4a: Second abdominal gill modified into a shield
Arrow indicates shield-like gill two. that covers all or almost all of the remaining gills of the abdomen. 5
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
6a: Second abdominal gill absent.--Ephemerellidae
6b:Second abdominal gill present. 7
7a: Maxillary palps extending beyond the sides of the head. They are covered in a fringe of long "hairs" or setae. Found in extreme northern SK.--Arthropleidae
7b: Maxillary palps not as above. 8
8a: Foreleg with claws forked in two.--Metretopodidae
8b: Foreleg claws single. 9
9a: Head wide and flat. 10
9b: Head more rounded, not flat. 11
10a: Claws on legs are all long. As long as tarsi.--Pseudironidae
10b: Claws on legs very much shorter than tarsi.--Heptageniidae
11a: Forelegs similar to other legs in appearance.13
11b: Forelegs (femora and tibiae) with long hairs (setae)
Arrow indicates setae. on under (ventral) surface.--12
12a: Two tail filaments at the end of the abdomen.--Oligoneuriidae
12b: Three tail filaments present.--Isonychiidae
13a: Foreleg claws much shorter than claws on the other two pairs of legs.--Ametropodidae
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 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 middle
Arrow indicates notch on labrum. in it. 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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Saskatchewan species list to mayflies (Ephemeroptera): The species list below is primarily from Webb 2002.
(Please note if you do not have a "wheeled" mouse and a list expands below the page bottom just left click on the first entry of the list in question and use the keyboard's down arrow button to scroll down the list.)
References
Waltz, R.D. and S.K. Burian. 2008. Chapter 11. Ephemeroptera. In. Merritt R.W. and K.W. Cummins. Ed. An Introduction to the Aquatic Insects of North America. 4th Edition. Kendall/Hunt Publishing Co. Dubuque, Iowa.
Lehmkuhl, D.M. 1970. Mayflies in the South Saskatchewan River: pollution indicators. Blue Jay 28:183-186.
McCafferty, W.P. and R.P.Randolph. 1998. Canada mayflies: A faunistic compendium. Proc. Ent. Soc. Ontario. 129:47-97.
Web, J.M. 2002. The mayflies of Saskatchewan. MSc. Thesis. University of Saskatchewan, Saskatoon, Saskatchewan.430 pp.
Webb JM, Sun LL, McCafferty WP, Ferris VR. 2007. A new species and new synonym in Heptagenia Walsh (Ephemeroptera: Heptageniidae: Heptageniinae) based on molecular and morphological evidence. 16pp. Journal of Insect Science 7:63, available online: insectscience.org/7.63
Webb, J., D.W. Parker, D.M.Lehmkuhl, and W.P. McCafferty. 2004. Additions and emendations to the mayfly (Ephemeroptera) fauna of Saskatchewan, Canada. Ent. News 115:213-218