Among the Instars: Ephoron leukon
Hey there, bug folks! Here is my first focus on mayflies, and what better way to start than with the first species described from North America, back in 1802: Ephoron leukon (family: Polymitarcyidae), or the white fly. This critter is part of the “burrowing mayflies,” which are distinguished from other mayflies by the tusks protruding from their head. They tend to be overlooked in a lot of monitoring methods, since the larvae and adults tend to be nocturnal. The larvae travel deeper in their burrows during daytime and come up to the stream bed around sunset. When they are mature, they even emerge as adults after sunset, so some have referred to Ephoron as the only genus that never gets to see the sun!
Ephoron leukon is a species of burrowing mayfly in the family Polymitarcyidae.
Ephoron leukon Life Cycle
The large eyes on this Ephoron leukon confirm that it is a male specimen.
The life of an Ephoron leukon starts as an egg in the middle to late summer and stays in that form all through the winter. In spring, the eggs hatch and the larvae spend several weeks developing in streams before they’re ready to emerge as adults. In the photographs, the dark color of the wingpads indicates that these specimens were mature and would have emerged soon. Slightly after sunset, it begins with most individuals emerging all in the same night, with densities that can look like a cloud or blizzard rising from the river. The first form of adults are subimagos, which require another molt before mating. It’s estimated that this subimaginal form only lasts for a few minutes, and then the adults mate, drop the eggs back into the stream, and immediately die. The whole process lasts a very short amount of time, which is the basis for the name Ephoron, meaning “insect of an hour.” You won’t see them hanging out on trees or structures during the daytime. However, the adults are attracted to artificial lights so you might find them at night near a stream. The emergence period starts in the middle to end of July, so keep an eye out!
Ecology of Ephoron leukon
Habitat
Ephoron leukon is mostly found in the eastern half of North America, in stream riffle habitats. They build tube-shaped burrows on the bottom side of rocks, especially those partially buried in silt sediments. As mentioned earlier, the nymphs tend to be nocturnal, so they travel deeper into their burrows during the day and may be difficult to find. Try again in the evening when they come up to feed, and there could be a strong population present. The large eyes and black sensory ocelli probably help them sense their surroundings with low levels of light.
The shape of the tusks and presence of dorsal tubercles help identify this specimen as Ephoron leukon. The long hairs around the head are used in filtering food particles from the water column.
Food Preferences
The main food source for Ephoron leukon is detritus suspended in the water column. They also eat diatoms and sometimes small algae. This mayfly is in the category of filter feeders because they can filter particles from the water using the long hairs surrounding their face and mouth.
Tolerance Values
Mayflies typically indicate good water quality, and Ephoron leukon follows that pattern. In the Upper Midwest where my specimens are found, their tolerance value is 2.0 on a scale from 0 to 10. This suggests that they prefer fairly clean streams with high oxygen levels and without much pollution from organic inputs. The score is even lower in the Southeast region with a value of 1.5. Part of the difference in scores can relate to variability in habitat preferences depending on the location.
Ephoron leukon Taxonomy
Taxonomy for Ephoron leukon is moderately easy, since there are few species. The family Polymitarcyidae contains four genera in North America, and Ephoron has 13 species but only two are found in North America. Mayflies are distinguished at the order level from other insects by the combination of these features:
Developing wingpads
3 pairs of moveable legs, each with one claw
2 or 3 filaments at the end of the abdomen
Gills on sides of some abdominal segments
Within the mayfly order, an early couplet usually separates the burrowing mayflies from the rest, by the presence of mandibular tusks and the abdominal gills forked and with a fringed margin. Polymitarcyidae can be separated from the other families by front legs modified for burrowing and the tips of the tusks curving downward. The tubercles on the top surface of the tusks, as seen in the photo above, distinguish Ephoron from the other genera. The intricate network of dark coloration in the gills separates E. leukon from the other North American species, E. album.
Ventral view of Ephoron leukon, showing the color pattern and the dark colored tracheation in the gills.
Special thanks to the Aquatic Biomonitoring Laboratory at UW-Stevens Point for allowing me the use of their microscope camera. Check out these photos in the gallery and Ephoron print products in the shop!
Sources
Hilsenhoff, W.L., 1995. Aquatic Insects of Wisconsin. Keys to Wisconsin genera and notes on biology, habitat, distribution, and species. Publication of the Natural History Council, University of Wisconsin-Madison 3:11-17.
Ide, F.P., 1935. Life history notes on Ephoron, Potamanthus, Leptophlebia and Blasturus with descriptions (Ephemeroptera). The Canadian Entomologist 67:113-125. https://doi.org/10.4039/Ent67113-6
Lenat, D.R., 1993. A biotic index for the southeastern United States: derivation and list of tolerance values, with criteria for assigning water-quality ratings. Journal of the North American Benthological Society 12:279-290. https://doi.org/10.2307/1467463
McCafferty, W.P., 1975. The burrowing mayflies (Ephemeroptera: Ephemeroidea) of the United States. Transactions of the American Entomological Society 101:447-504. https://www.jstor.org/stable/25078178
Merritt, R.W., K.W. Cummins, and M.B. Berg, 2019. An introduction to the aquatic insects of North America, 5th edn. Kendall/Hunt Publishing Company, Dubuque, Iowa.
O'donnell, B.C., 2009. Early nymphal development in Ephoron leukon (Ephemeroptera: Polymitarcyidae) with particular emphasis on mouthparts and abdominal gills. Annals of the Entomological Society of America 102:128-136. https://doi.org/10.1603/008.102.0114
Shapas, T.J. and W.L. Hilsenhoff, 1976. Feeding habits of Wisconsin's predominant lotic Plecoptera, Ephemeroptera, and Trichoptera. The Great Lakes Entomologist 9:175-188. https://scholar.valpo.edu/tgle/vol9/iss4/3
Snyder, C.D., L.D. Willis, and A.C. Hendricks, 1991. Spatial and temporal variation in the growth and production of Ephoron leukon (Ephemeroptera: Polymitarcyidae). Journal of the North American Benthological Society 10:57-67. https://doi.org/10.2307/1467764
Williamson, D., 1802. On the Ephoron leukon, usually called the white fly of Passaick River. Transactions of the American Philosophical Society 5:71-73. https://doi.org/10.2307/1004977
