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Introduction to the Mayflies (Ephemeroptera) of Gunnison County, Colorado

Updated 17 May 2020
TSN 100502
The Baetis or small minnow mayflies to the right are some of the most common mayflies in the Gunnison basin. They were caught and released from the Gunnison River. Common and abundant in Gunnison county and the entire US, this genus and many others are essential to the aquatic food web. As with many other mayflies in our area, Baetis lives crawling around the rocks of the streambed, grazing on the diatoms and algae that grow on the surface of the rocks.

This mayfly species also "drifts", or releases from the bottom to swim in the current. Of course foraging fish love this behavior, so Baetis and many other mayflies that drift are eagerly sought out as food by the variety of introduced and native fishes in our watersheds.


The scientific name is from the Greek Eøémepos which is the basis for the english word "Ephemeral". It means "enduring but a day" or short lived. Then add the suffix -optera which means winged or "has wings". Adult mayflies may only live for a few hours let alone a whole day!

Translations of terms used by Fishermen and Scientists

Subimago = Dun
Imago = Spinner
Exuvia = Shuck

You may click on a mayfly below to go to their webpage :-)

Epeorus longimanus, Spotted Epeorus or flat headed mayfly mymph. Another Epeorus longimanus, Spotted Epeorus or flat headed mayfly mymph. Young Epeorus mayfly nymph in a pan of water on the bank of West Brush Creek. Small Epeorus mayfly nymph in a pan of water. Epeorus nymphs are everywhere! Another youthful Epeorus mayfly nymph. Another young Epeorus mayfly nymph. Young Epeorus mayfly nymph in a pan of water. Young Cinygmula nymphs from a rocky stream. Cinygmula nymphs from a stream in the Southern Rocky Mountains Immature Cinygmula nymphs from a rocky stream. Big, tank=like Drunella nymph in a pan of water with a bunch of other mayflies

Good Links

On this website:
Key to Adult Mayfly families

Other Websites:


Brittain,JE 2008 Mayflies, biodiversity and climate change. International Advances in the Ecology, Zoogeography, and Systematics of Mayflies and Stoneflies, pp.1-14.
     Abstract: "Mayflies (Ephemeroptera) are an ancient order of insects that are globally distributed in both northern and southern hemispheres and have survived major environmental shifts. Despite the problems associated with selection processes operating in both terrestrial and aquatic environments, mayflies have successfully colonized a wide range of freshwater habitats from the tropics to the arctic, a somewhat greater range than other hemimetabolic aquatic insects such as the Plecoptera and Odonata. While many species of Ephemeroptera require specific environmental cues, others display considerable flexibility in life cycle length and timing in relation to environmental changes. This is particularly apparent in arctic and alpine species. Climate change scenarios predict rapid shifts across many environmental gradients, including temperature and the frequency and magnitude of floods and droughts. Changes in the mayfly fauna are hypothesized in the light of the environmental tolerances, life cycle plasticity and the dispersal mechanisms of present day mayflies. During periods of rapid environmental transition certain species traits will be beneficial. Generalists will do better; specialists with strict environmental limits and poor powers of dispersal may become extinct."

Clements,WH; Cherry,DS; Cairns,J 1988 Impact of heavy metals on insect communities in streams: a comparision of observational and experimental results. Canadian Journal of Fisheries and Aquatic Sciences 45 11, 2017-2025.
     Working in the Clinch River of Russell County, Virginia and outdoor experimental streams, they measured population responses of macroinvertebrates to natural conditions and 12 µg of copper and zinc in the artifical streams. They used 6 replicates of substrate-filled trays everywhere and counted all the animals (no subsampling). Both stream mesocosm experiments and Clinch river sites showed similar results. They found abundance or total numbers of aquatic insects declined at all high effluent sites associated with the Clinch River coal-fired power plant, recovering 3- 4 kilometers downstream. Low levels of copper and zinc reduced species richness (number of different taxa) and total numbers as well as caused a shift in the species composition of dominant taxa. Metal contamination caused macroinvertebrate populations to shift from control (clean) sites dominated by Mayflies and Tanytarsini Midges to polluted sites dominated by Hydropsychidae caddisflies and Orthocladiinae midges.

Edmunds Jr.,GF; Jensen,SL and Berner,L 1976 The Mayflies of North and Central America. University of Minnesota Press, Minneapolis, Minnesota. 330 pages pages.

Edmunds Jr,GF and McCafferty,WP 1988 The mayfly subimago. Annual review of entomology, 33(1)509-527. PDF
     Abstract: "Mayflies are unique among present-day insects because they molt after becoming fully winged; in other words, they have a winged preadult life stage. This is the subimago. Because mayfly metamorphosis includes this unique life stage and is thus different from other existing kinds of insect metamorphosis, entomologists have long pondered the exact nature and role of the subimago. Many of the proposed explanations have been based on little or no data. Is the subimago comparable to the adult of other insects or is it perhaps equivalent to the pupa or even the larva? Does it have a functional role or is it merely a relict of a primitive lifestyle? Why and how in certain groups of mayflies has the subimago evidently replaced the adult? Here we review the historical thinking, synthesize the pertinent aspects of available data with considerable unpublished data,and draw conclusions about the function and evolution of the subimago."

Edmunds,GF and Tennessen,KJ 1996 Ephemeroptera. In: An Introduction to the Aquatic Insects of North America. 3rd ed. Eds: Merritt,RW; Cummins,KW Kendall/Hunt Publishing Company, Dubuque, Iowa, 126-163.

Peckarsky,BL; McIntosh,AR; Àlvarez,M and Moslemi,JM 2015 Disturbance legacies and nutrient limitation influence interactions between grazers and algae in high elevation streams. Ecosphere, 6(11)1-15. PDF
     Abstract: " Debate about control of interaction strength among species is fueled by variation in environmental contexts affecting food webs. We used extensive surveys and two field experiments to test the individual and interactive influences of variation in the assemblages and associated traits of grazers as shaped by the legacy of disturbance, nutrient limitation and the presence of top predators on the accrual of basal resources. We quantified hydrologic variation and streambed movement to describe the legacy of disturbance and sampled biota of 20 streams over five years in a high-elevation catchment in Colorado, USA. Grazer assemblages switched from caddisfly-dominated to mayfly-dominated as disturbance increased. We manipulated the composition of grazer assemblages and the availability of nutrients (N and P) within flow-through mesocosms assembled adjacent to 10 streams, and also deployed larger in-stream channels manipulating the presence of top predators (brook trout) in five streams varying in disturbance regimes. In both experiments we compared the rate of accrual of benthic algae and the strength of grazer-algal interactions among treatments. We observed no indirect effects of top predators on grazer mobility, grazer consumption of algae, or accrual of algal biomass (no trophic cascades). However, in both experiments accrual rates of algae yielded a unimodal pattern and grazer impacts on algae decreased with increasing disturbance, but only at ambient (limiting) nutrient conditions. When nutrients were amended in the mesocosm experiment, algal accrual was uniformly high and grazer impacts on algae were consistently low. Reduced algae accrual at high disturbance levels may be explained by direct effects of environmental harshness on algae, and at low disturbance by indirect effects on grazer traits (behaviors) rather than on grazer density. In more benign streams per capita and per unit biomass grazer impacts on algae were high and drift dispersal was low, both behaviors that reduced accrual of algae. We conclude that nutrient limitation and indirect effects of disturbance on accrual of algae mediated by grazer traits can be stronger than indirect effects of predators on algae, providing a new contribution to the debate about the influence of environmental context on the strength of food web interactions."

Perry,JA and Schaeffer,DJ 1987 The longitudinal distribution of riverine benthos: A river dis-continuum?. Hydrobiologia, 148(3) 257-268.
      They studied Tomichi Creek in Gunnison County.

Schloss,AL and Haney,JF 2006 Clouds, shadows or twilight? Mayfly nymphs recognise the difference. Freshwater Biology, 51(6), 1079-1089. PDF
     Summary: "1. We examined the relative changes in light intensity that initiate night-time locomotor activity changes in nymphs of the mayfly, Stenonema modestum (Heptageniidae). Tests were carried out in a laboratory stream to examine the hypothesis that nymphs increase their locomotion in response to the large and sustained reductions in relative light intensity that take place during twilight but not to short-term daytime light fluctuations or a minimum light intensity threshold. Ambient light intensity was reduced over a range of values representative of evening twilight. Light was reduced over the same range of intensities either continuously or in discrete intervals while at the same time nymph activity on unglazed tile substrata was video recorded.
2. Nymphs increased their locomotor activity during darkness in response to large, sustained relative light decreases, but not in response to short-term, interrupted periods of light decrease. Nymphs did not recognise darkness unless an adequate light stimulus, such as large and sustained relative decrease in light intensity, had taken place.
3. We show that nymphs perceive light change over time and respond only after a lengthy period of accumulation of light stimulus. The response is much lengthier than reported for other aquatic organisms and is highly adaptive to heterogeneous stream environments. "

Waltz,RD and Burian,SK 2008 Ephemeroptera. In: An Introduction to the Aquatic Insects of North America. 4th ed. Eds: Merritt,RW; Cummins,KW; Berg,MB Kendall/Hunt Publishing Company, Dubuque, Iowa, 181-236.

Ward,JV, Kondratieff,BC and Zuellig,RE 2002 An Illustrated Guide to the Mountain Stream Insects of Colorado. 2nd ed. University Press of Colorado, Boulder, Colorado. 219 pages.

Wiggins,GB and Mackay,RJ 1978 Some relationships between systematics and trophic ecology in nearctic aquatic insects, with special reference to Trichoptera. Ecology 59 6, 1211-1220. PDF

Brown,WS 2005 Introduction to the Ephemeroptera or Mayflies of Gunnison County, Colorado

"When one tugs at a single thing in nature, they find it attached to the rest of the world." - John Muir