From Swimmers to Swarmers: The Biology, Ecology, and Importance of Mayflies

Imagine this: you’re sitting on the edge of a stream. Conditions are calm and peaceful, and you’re enjoying the tranquility of your surroundings. You’re so relaxed in fact, that you barely notice as one specific species of insect begins to congregate over the water’s surface. At first, the numbers are low enough, and their disturbance light enough, that you pay them no mind. However, the number of these insects over the water continues to increase, up and up and up until you finally acknowledge them and wonder, slightly concerned, at what is building up in front of you. On and on it continues as you watch with a mix of fascination and concern, and eventually, this swarm not only grows in thickness, but in area, and concern morphs to horror as the edge of the swarming mass extends towards you. You’ve decided you’ve had enough, and hurriedly pack your things and retreat to your car to leave. However, you’re too late, as by the time you start your engine, the swarm has engulfed your vehicle, and you can’t see any distance in front of you. You resign yourself to your fate and, appreciating the shelter you have, hunker down for the long haul as you wait for the swarm to abate.

Mayflies covering a car. (Source: Mark Johnson/Sue Bixler 2019)

The event you’ve just witnessed is a mayfly mating swarm, and this comes towards the end of their life cycle. This species, part of the order Ephemeroptera within Paleoptera, begins its life under the water’s surface. They start out small, less than 1 mm long and even lacking gills. However, over subsequent molts, they grow in size, up to about 3 cm long at maximum, and develop various body features including gills and sex indicators, such as clasping organs on males and oviducts on females. Eventually they reach the phase to where they’re ready to move to the air, a stage called the subimago, at which point they empty their guts, fill them with air, and release whatever’s anchoring them in the water, allowing them to float to the surface, where they’ll sit for a time as they release their wings and build up the strength for flight. This is their most vulnerable stage, so as soon as they’re strong enough, they fly to any available shelter nearby and prepare to molt once again into their imago form. This molt includes a longer tail, increasing their stability in flight, and longer legs, for easier grasping during mating. This development takes only 24-48 hours to occur, and their final life stage only lasts for a few hours to a day, during which it participates in the massive mating swarm you witnessed on the creek. After mating and finally completing their lifes’ purpose, mayflies die (Salles, 2000).

Life cycle of a mayfly. (Source: Encyclopaedia Britannica 2008)

The number of mayflies involved in a swarm can be absolutely massive. Their emergence events, if coming from a large enough body of water, have been recorded to produce up to 87.9 billion individuals over a period of several hours, which is a release of 3,078.6 tons of biomass into the airspace. These large emergence events are so colossal that their flights have been detected by radar, and this has been used to track and study long-term changes in their annual biomass transport over large areas (Stepanian et al., 2020). Of course, it’s easy to see how such large numbers could become a nuisance at times, and there are records of this happening. For example, the insects flying everywhere means they can get in people’s way in all sorts of tasks, and when they mate and shortly after die, there can be so many that their bodies accumulate everywhere they could reach, from beaches to streets to cars to even window screens. There’s even the rare instance where the buildup of mayflies on roads is so great that driving over them becomes slippery, which would pose a distinct hazard to human health (Freedman, 2012).

However, those instances are few and far between, and the vast majority of the time, mayflies are unquestionably more beneficial than they are harmful. Due to their often large numbers, mayflies play important roles in their ecosystems, especially in food webs. As they are often herbivores or detritivores, their grazing action can be incredibly important in preventing the buildup of large amounts of algae or detritus, and in continuing the cycling of nutrients. Even if they’re predators, a similar logic can easily be applied, that they play an important role in maintaining the populations of their prey. The reverse is also true, with mayflies providing a source of food for many aquatic species, most notably sportfish that provide game and sustenance for fishers. There’s even a fishing technique derived from mayflies, using lures that resemble mayflies, so these insects have even contributed to people’s success at catching fish. Finally, due to their specificity in their choice of habitat and water conditions, mayflies often serve as indicators of water quality, with a particularly famous example being the drop in mayflies in Lake Erie during the 1950s and 1960s, which was reported in the popular press and contributed to public awareness of water cleanliness that has led to the lake becoming much cleaner than it once was. (Freedman, 2012) Therefore, while these insects have the potential to be a major nuisance, they tend to be the exact opposite, and are an important order of insects that deserve understanding and respect.

Examples of mayfly-resembling fishing lures. (Source: Aardvark McLeod)

Works Cited

Freedman, B. (2012). Mayflies - ecological and economic importance of mayflies. Lake, Mayfly, Species, and Press - JRank Articles. Retrieved May 19, 2022, from https://science.jrank.org/pages/4188/Mayflies-Ecological-economic-importance-mayflies.html#:~:text=Grazing%20by%20mayflies%20is%20important,for%20many%20species%20of%20predators.

Salles, A. (2000, August 4). Mayflies. Ephemeroptera. Retrieved May 19, 2022, from https://ucmp.berkeley.edu/arthropoda/uniramia/ephemeroptera.html

Stepanian, P. M., Entrekin, S. A., Wainwright, C. E., Mirkovic, D., Tank, J. L., & Kelly, J. F. (2020). Declines in an abundant aquatic insect, the burrowing mayfly, across major North American waterways. Proceedings of the National Academy of Sciences, 117(6), 2987–2992. https://doi.org/10.1073/pnas.1913598117