The mayfly is a fascinating benthic macroinvertebrate that has been around longer than the dinosaurs, serving as an indicator of clean and unpolluted water. But what happens after the mayfly nymph leaves the safety and familiarity of the water? And how does the mayfly larvae transform into its two-phase adult form? To explore these questions, we’ll start at the very beginning -- when mayfly eggs are first laid.

To commence the mating ritual, male mayflies form a swarm above the water where they will mate with female mayflies that fly into their path. Once mating has occurred, the female will lay her eggs on the surface of the water and lie motionless where fish will pick away at her wings and the male mayfly will go off to die on the nearby land. The eggs will descend to the depths of the water falling onto plants and stones. A particular family of mayflies, the Baetidae will actually pull themselves under the surface of the water to directly attach their eggs to the bed of their environment before being taken by the current.

The mayfly eggs will take anywhere from a few days to a number of weeks to hatch.  The exact science is dependent on water conditions and the particular species. Mayfly nymphs will then spend up to three years foraging on the bottom of the riverbed before emerging as adult mayflies. During this time, they may undergo up to as many as 20 instars, or transformation stages of development. As nymphs mayflies are omnivores, however once they become adults they no longer have a functioning mouth and cannot eat.

When the mayfly nymphs are ready to enter their adult phase, they will reach the water surface and emerge as a sub-imago or a dun. During this first phase of their adult form, the mayflies are dull and somewhat colourless with very short tails. It can take up to 30 seconds for this process of emergence from the nymphal skin to the sub-imago phase to occur. The mayfly is extremely vulnerable as it struggles to break through the water surface and dry their newly exposed wings without being preyed upon. During this phase the genitalia, eyes, and legs are not fully developed therefore they cannot fly properly and do not have attractive colouration to attract mates.

With its wings now dry, the mayfly heads towards the closest vegetation it can find to continue onto the second phase of its adult transformation. The process is dependent upon environmental conditions and can take anywhere from half an hour to several hours. It involves the sub-imago shedding its skin, giving way to the fully sexually mature, brightly coloured imago or spinner that is ready to mate and begin the cycle all over again.                                                                                         

Fun Fact: When insects are above water they do not eat or drink and usually live anywhere from 2 minutes to 2 days, so mating must occur quickly.

Fun Fact:  The shortest lifespan a mayfly is known to have is a survival time of just 5 minutes. This was seen for the mayfly species Dolania americana.

Interested in learning more about benthic macroinvertebrates and how they can be used to measure the health of rivers and streams? Be sure to follow EcoSpark’s social media to stay updated on our Changing Currents program and our other citizen science and environmental education programs

References

Dr. C Bennette. (2018). Ephemera Danica. Retrieved from:http://www.tomsutcliffe.co.za/fly-fishing/my-fly-fishing/item/1112-the-dry-flies-of-oliver-kite.html

Carina holds a Bachelor of Honors in Environmental Studies and a diploma in Ecosystem Technology, both of which developed her interest in using geographic information systems to communicate about complex issues, such as climate change. In her role as Lead Environmental Educator, Carina designs geospatial education tools for EcoSpark’s citizen science programs. When outdoors in the field, you will often find her streamside identifying aquatic critters, communicating her enthusiasm for the outdoors, and demonstrating her dedication to stewardship practices with youth and adults throughout the GTA.