Adult giant mayflies can grow to 8.7 to 27.3 mm in length. Like all adult mayflies, they are soft-bodied, with large forewings and a smaller pair of hindwings, which are held together above their body when they are at rest. Two hairlike tails come out from the end of their abdomen and they have small antennae. Members of their family (Ephemeridae) can be identified by the 4 segments on their hind legs, as well as by the veins on their wings. Adults, also known as imagos, can be told apart from subimagos by their clear wings. Female imagos have lighter yellow bodies and smaller eyes than males, they are also up to twice the weight of males and several millimeters longer. Giant mayflies can be yellow or white or even shades of brown; they can also have a variety of patterns and sizes. Giant mayflies often live in the same area as Hexagenia rigida, another burrowing mayfly. Male imagos of these two species can be told apart by their genitalia. Giant mayflies also have a dark stripe along the outer edge of their hind wing that H. rigida does not have. (Bachteram, et al., 2005; Borror and White, 1970; Bustos and Corkum, 2013; Corkum, 2010; Corkum, et al., 1997; Green, et al., 2013; Hunt, 1951; Reynoldson and Hamilton, 1993)
Nymphs have long cylindrical bodies. They have 3 thread-like tail filaments extending from the end of their abdomen and gills along the outer edges of their abdomen. Nymphs also have unique mandibles (part of their mouths) that are long, hardened tusks used for burrowing. When they hatch nymphs are 1 mm long, males can grow to 23 mm long, while females grow to 30 mm long. Older nymphs have dark, prominent wing pads. Eggs are 0.3 by 0.2 mm and have an ellipsoid shape. They are white and transparent enough to see the embryo inside. Dark or black eggs will not hatch. (Bachteram, et al., 2005; Borror and White, 1970; Bustos and Corkum, 2013; Corkum, 2010; Corkum, et al., 1997; Green, et al., 2013; Hunt, 1951; Reynoldson and Hamilton, 1993)
Giant mayflies (Hexagenia limbata) are native to the Nearctic region. They are found across almost all of the United States and have been seen in every state except Arizona and Alaska. They are particularly common in the Great Lakes region. Populations near Lake Erie have been the focus of research and attention during the last few decades, as they have disappeared and later reappeared in the area. They are also found throughout Canada, their range extends to just south of Alaska and Nunavut. They are the most widespread burrowing mayfly species in North America. (Giberson and Rosenberg, 1994; Green, et al., 2013; McCafferty, et al., 2012; Shipley, et al., 2012)
Nymphs of giant mayflies live in the water and dig u-shaped burrows in the soil and sand at the bottom of lakes and streams in mild habitats. The best habitats for these nymphs have well-mixed, shallow water about 3 meters deep, so that enough air gets into the sediment and there is plenty of detritus, or litter, available for food. Adults and subimagos (mayflies in the developmental stage prior to sexual maturity) are typically found in riparian habitats, close to the bodies of water from which they emerged. Subimagos are often found resting in trees and bushes on shore before molting to adults, while adults can usually be found flying in swarms on shore. (Corkum, 2010; Giberson and Rosenberg, 1994; Green, et al., 2013; Shipley, et al., 2012)
Like all mayflies, giant mayflies go through incomplete metamorphosis. They develop from an egg into a nymph, also called a naiad, with several stages called instars. They molt into non-mating adults, called subimagos or duns, then molt into mating adults, called imagos or spinners. Females lay their eggs in the water just after mating, they either drop into the sediment or are moved by flowing water. Most eggs hatch after winter, as their eggs can survive in cold temperatures (8 degrees Celsius) for over a year. Development and hatching depend on the amount of oxygen in the water and water temperatures. Eggs hatch a couple of days or weeks after the water warms up. Some eggs hatch before winter, a few weeks after being laid. The nymphs that hatch early have little growth and few molts before winter. (Bustos and Corkum, 2013; Corkum, 2010; Corkum, et al., 2006; Corkum, et al., 1997; Giberson and Rosenberg, 1994; Green, et al., 2013; Shipley, et al., 2012)
Nymphs shed their skin (molt) and go through different stages, called instars, as many as 30 times, this can take anywhere from 14 to 22 months. Giant mayflies have a two year life cycle, the majority of which is spent as a nymph, however, in colder regions they may have a 3 or 4 year life cycle. The final nymphal instar swims to the surface and molts into a subimago. Subimagos look like adults but are not yet able to mate and reproduce. Nymphal development time is temperature dependent so development times vary, subimago emergence takes place from spring to early fall, not until water temperatures have reached 20 degrees Celsius. Subimagos move on shore where they rest for 24 to 48 hours. They molt more one time to become adult imagos. Imagos live for up to two days, in which time they must find a mate and breed, before dying. Most adults emerge from the water during June and July, though they can emerge anytime from late spring to early fall. (Bustos and Corkum, 2013; Corkum, 2010; Corkum, et al., 2006; Corkum, et al., 1997; Giberson and Rosenberg, 1994; Green, et al., 2013; Shipley, et al., 2012)
After 24 to 48 hours as subimagos, or non-mating adults, giant mayflies molt into imagos, or mating adults. As imagos, males and females have one or two days to find a mate. Males form giant mating swarms just before sunset. Females fly through the swarm and are grabbed by males out of the air. They mate in mid-air for a brief time, males hold on to females, before separating. Males likely die shortly after mating, while females move on to lay their eggs. Due to their short lifespan, these mayflies are monogamous and only mate once. Most mating takes place in June and July, though some adults may emerge before or after this time in much smaller numbers. (Bustos and Corkum, 2013; Corkum, 2010; Morgan, 1913)
After mating, female giant mayflies gather in big swarms for an egg-laying flight after sunset, this may help decrease predation. Females fly out over the lake or stream, flying back and forth about 10 to 20 feet above the surface for a few minutes, before dropping to the surface. Most hit the water hard and flutter around, then lift their abdomen to drop their eggs in the water. If they cannot get up from the water surfaces, and most are not able to, then they drown. Some females land gracefully, deposit a few eggs, then rise back into the air to do it again. Sometimes, females drop their eggs into the water from several feet above the surface. (Bustos and Corkum, 2013; Corkum, 2010; Corkum, et al., 2006; Green, et al., 2013; Hunt, 1951)
Larger females carry more eggs. Females carry 2,000 to 7,000 eggs, with an average of 4,000 eggs. Due to their short lifespan, they only lay one batch of eggs and die shortly after. Eggs may be moved to other locations by flowing water. When strong winds are present, these female swarms may be pushed ashore to lay their eggs on the cement under street lights, rather than in the water. This may be because they are attracted to light, or because the streetlights look like moonlight reflected on water. Eggs laid on cement will dry out and will not hatch. (Bustos and Corkum, 2013; Corkum, 2010; Corkum, et al., 2006; Green, et al., 2013; Hunt, 1951)
Female giant mayflies provide nutrients in their eggs and lay their eggs in a body of water that will provide a good habitat for the eggs and nymphs to grow and develop. Since adults live only for a day or two, they do not provide any more parental care. (Green, et al., 2013)
Giant mayflies typically have a 2 year life cycle, though it can be longer in colder regions and shorter in warmer regions. Development time of nymphs takes 14 to 22 months and their subimago stage usually lasts 24 to 48 hours. After molting into an imago, they have an incredibly short adult lifespan, living only 1 or 2 days. One laboratory was able to keep a female alive for 8 days. Stormy weather can cause the death of many emerging mayflies by drowning them if waves are rough or too high, or if wind speeds are too high. (Bustos and Corkum, 2013; Carey, 2002; Giberson and Rosenberg, 1994)
Nymphs can be buried up to 10 cm in the sand and sediment. They may share burrows and often group together in laboratories. While in their burrows, nymphs beat and wave their gills to create a water current. This current not only brings oxygen into the burrow for the nymphs to breathe, it also sorts sediment particles and draws food towards their mouths. Their gills are in motion about 75% of the time; they can beat their gills for hours at a time. Nymphs are almost constantly in motion and are active throughout the day and night. They push sediment around with their head and front legs, switch directions in the burrow, and rub their gills together, probably to clean them. Subimagos emerge during the day, usually the morning and afternoon and can typically be found resting on shore. Adults are active both day and night, throughout their short lifespan. Adults gather in large swarms, males gather in non-mating and mating swarms and females gather in egg-laying swarms. They are not strong fliers and usually depend on the wind to move them. (Bachteram, et al., 2005; Corkum, et al., 2006; Edwards, et al., 2009; Fincke and Tylczak, 2011; Gallon, et al., 2008; Giberson and Rosenberg, 1994; Green, et al., 2013)
Adults are poor fliers and often have to rely on winds to move them ashore after emerging from the water. They travel about 1.2 km on average from where they first emerged from the water. Nymphs mainly stay in their burrows. (Corkum, et al., 2006; Green, et al., 2013)
Adult mayflies have large eyes and view their environment visually. They are also attracted to light. However, nymphs are photophobic and stay away from light, which is not usually a problem in their underwater burrows. (Bustos and Corkum, 2013; Gallon, et al., 2008)
Nymphs of giant mayflies are detritivores. They filter feed bits of organic material and occasionally algae out of the water by creating a current in their burrows with their gills. By waving their gills, the water current causes the food particles to move to their mouthparts. Like all mayflies, adults do not feeding during their short lifespan because their mouthparts do not work. Instead, their digestive tract is actually filled with air, which helps with flight. (Morgan, 1913; Shipley, et al., 2012)
Many aquatic animals prey on giant mayfly nymphs, such as dragonfly larvae including swift river cruisers and many freshwater fish species including yellow perch, lake whitefish, and smallmouth bass. Though nymphs burrow deep into the sediment, the burrows do not seem to protect the mayfly nymphs from being eaten by dragonfly larvae. Many bird species, such as tree swallows and other aerial predators, such as adult dragonflies, can prey on the adult mayfly swarms. These swarms are one way the mayflies defend against predators; with so many mayflies for a predator to choose from, the chances of any one mayfly getting eaten decreases. (Clady and Hutchinson, 1976; Corkum, et al., 2006; Corkum, et al., 1997; Fincke and Tylczak, 2011; Giberson and Rosenberg, 1994; Papp, et al., 2007)
Populations of giant mayflies in Lake Erie have received considerable research attention as this species disappeared from the area for over 30 years due to water pollution. The pollution caused the presence of too many nutrients in the lake and changes in the amount of oxygen in the water, which significantly changed their habitat. The disappearance of mayflies and the terrible state of the lake caused people to change their habits of dumping phosphorous and other pollutants into the water. After the lake was restored, giant mayflies reappeared in the 1980s and are once again common in the region. As a source of prey for fish, birds, and insects, their disappearance could have a large impact on the other animals in their ecosystem. Likewise, these populations are dealing with the effects of invasive zebra mussels (Dreissena polymorpha) and quagga mussels (Dreissena bugensis) that have spread throughout and now dominate the Great Lakes. There are fewer giant mayfly nymphs in areas with a lot of mussels because their shells build up on the bottom of the lake, making it difficult for mayfly nymphs to build their burrows in the sediment. However, mussels produce feces that nymphs can eat, which could be helpful. (Freeman, et al., 2011; Green, et al., 2013)
Nymphs cause bioturbation in their habitat, which means they move around and change the sand and soil at the bottom of lakes and streams. The currents created by nymph's gills and their burrow building cause some chemicals, such as cadmium, to stay in the water, although it would normally sink into the sediment and be out of the aquatic environment. This may make it more difficult for area contaminated with chemicals to return to healthy conditions. Since nymphs need oxygen to live, they cause less oxygen in areas where the water meets the sand and soil. Giant mayflies often live in the same habitat as Hexagenia rigida, another burrowing mayfly, particularly in Lake Erie and their eastern range. The species with the larger population changes from season to season, the two species often co-dominate the habitat. Giant mayflies seem to be the dominant species in the last few years, which may be because of their larger body size, which allows them to produce more offspring. Parasitic trematodes, Crepidostomum cooperi, which infect sunfish from family Centrarchidae, may live inside giant mayfly nymphs during part of their life cycle. (Bachteram, et al., 2005; Corkum, 2010; Edwards, et al., 2009; Marcogliese, et al., 1990)
Swarms of giant mayflies and other Hexagenia species can be huge and are often annoying to communities. Swarms were a problem particularly in the 1950s in the Lake Erie region and as populations have returned in that region, they are again causing problems. Since mayflies are attracted to lights, large groups often collect at street lights, causing power outages when swarms collect at illuminated power transformers. Dead bodies of mayflies can also quickly pile up. (Corkum, 2010; Corkum, et al., 2006; Reynoldson and Hamilton, 1993)
Giant mayfly nymphs can be used by scientists to determine if lakes and streams are contaminated by chemicals. Nymphs have evidence of these chemicals in their body, which tells researchers if there are harmful chemicals in their habitat. They can also be bought and used to test water samples from other areas. Giant mayflies emerge from the water in large groups, molting from nymph to subimago, which can be an important time for fisherman. Fishing flies are often designed to look like mayflies, as many fish prey on this species. Fisherman have also learned to imitate the way subimagos sit on the water before taking off for shore, tempting fish to bite their line. Fishing during a "Hex hatch" can be very successful for fishermen. (Neuswanger, 2013; Nguyen, et al., 2012)
Giant mayflies are not an endangered species, but as shown by their populations in Lake Erie, they can die out due to human pollution and invasive species. Efforts to prevent pollution and keep ecosystems in their natural, healthy state are important to keeping this mayfly species alive. (Freeman, et al., 2011)
In July 1999, a huge swarm of giant mayflies and Hexagenia rigida on the shore of Lake Erie was so large that it was visible on Doppler radar, which is normally used to detect weather. The swarm was thought to be 3 to 6 km wide, 16 to 25 km long, and 125 to 250 m high. (Corkum, 2010)
Angela Miner (author), Animal Diversity Web Staff, Leila Siciliano Martina (editor), Animal Diversity Web Staff.
Borror, D., R. White. 1970. A Field Guide to Insects. New York: Houghton Mifflin Company.
Bustos, C., L. Corkum. 2013. Delayed egg hatching accounts for replacement of burrowing mayflies Hexagenia rigida by Hexagenia limbata after recolonization in western Lake Erie. Journal of Great Lakes Research, 39/1: 168-172.
Carey, J. 2002. Longevity minimalists: life table studies of two species of northern Michigan adult mayflies. Experimental Gerontology, 37/4: 567-570.
Corkum, L. 2010. Spatial-temporal patterns of recolonizing adult mayflies in Lake Erie after a major disturbance. Journal of Great Lakes Research, 36/2: 338-344.
Corkum, L., J. Ciborowski, R. Poulin. 1997. Effects of emergence date and maternal size on egg development and sizes of eggs and first-instar nymphs of a semelparous aquatic insect. Oecologia, 111/1: 69-75.
Edwards, W., F. Soster, G. Matisoff, D. Schloesser. 2009. The effect of mayfly (Hexagenia spp.) burrowing activity on sediment oxygen demand in western Lake Erie. Journal of Great Lakes Research, 35/4: 507-516.
Fincke, O., L. Tylczak. 2011. Effects of zebra mussel attachment on the foraging behaviour of a larval dragonfly, Macromia illinoiensis. Ecological Entomology, 36: 760-767.
Freeman, K., K. Krieger, D. Berg. 2011. The effects of dreissenid mussels on the survival and condition of burrowing mayflies (Hexagenia spp.) in western Lake Erie. Journal of Great Lakes Research, 37/3: 426-431.
Gallon, C., L. Hare, A. Tessier. 2008. Surviving in anoxic surroundings: how burrowing aquatic insects create an oxic microhabitat. Journal of the North American Benthological Society, 27/3: 570-580.
Giberson, D., D. Rosenberg. 1994. Life-histories of burrowing mayflies (Hexagenia limbata and H. rigida, Ephemeroptera, Ephemeridae) in a northern Canadian reservoir. Freshwater Biology, 32/3: 501-518.
Green, E., A. Grgicak-Mannion, J. Ciborowski, L. Corkum. 2013. Spatial and temporal variation in the distribution of burrowing mayfly nymphs (Ephemeroptera: Hexagenia limbata and H. rigida) in western Lake Erie. Journal of Great Lakes Research, 39/2: 280-286.
Hunt, B. 1951. Reproduction of the Burrowing Mayfly, Hexagenia limbata (Serville), in Michigan. The Florida Entomologist, 34/2: 59-70.
Marcogliese, D., T. Goater, G. Esch. 1990. Crepidostomum cooperi (Allocreadidae) in the Burrowing Mayfly, Hexagenia limbata (Ephemeroptera) Related to Trophic Status of a Lake. American Midland Naturalist, 124/2: 309-317.
McCafferty, W., R. Randolph, L. Jacobus. 2012. Mayflies of the Intermountain West. Gainesville, Florida: The American Entomological Institute.
Morgan, A. 1913. A contribution to the biology of may-flies. Annals of the Entomological Society of America, 6: 371-413.
Neuswanger, J. 2013. "Mayfly Species Hexagenia limbata (Hex)" (On-line). Troutnut.com. Accessed October 05, 2013 at http://www.troutnut.com/hatch/32/Mayfly-Hexagenia-limbata-Hex.
Nguyen, L., M. Vandegehuchte, H. van der Geest, C. Janssen. 2012. Evaluation of the mayfly Ephoron virgo for European sediment toxicity assessment. Journals of Soils and Sediments, 12/5: 749-757.
Papp, Z., G. Bortolotti, M. Sebastian, J. Smits. 2007. PCB congener profiles in nestling tree swallows and their insect prey. Archives of Environmental Contamination and Toxicology, 52/2: 257-263.
Reynoldson, T., A. Hamilton. 1993. Historic changes in populations of burrowing mayflies (Hexagenia limbata) from Lake Erie based on sediment tusk profiles. Journal of Great Lakes Research, 19/2: 250-257.
Shipley, M., K. Wellington, A. Rao, T. Ritchie, R. Vogtsberger. 2012. Fatty Acid Composition of a Burrowing Mayfly, Hexagenia limbata (Ephemeroptera: Ephemeridae), from a North Central Texas Lake. Journal of the Kansas Entomological Society, 85/3: 245-258.