Mayflies are common insects often observed in significant numbers, particularly near freshwater bodies. Their sudden appearance, sometimes in massive swarms, prompts curiosity about their origins. These delicate, soft-bodied insects are active during warmer months, not solely in May, and are found globally.
The Journey Begins in Water
Mayflies begin their lives in freshwater environments such as rivers, streams, and lakes, where females deposit their eggs. These small eggs, typically ranging from 0.5 to 1.5 millimeters, can be oval or elliptical and often possess a sticky coating that helps them adhere to submerged surfaces like rocks or aquatic plants. Depending on the species and environmental factors like water temperature, the eggs may hatch within a few days to several weeks.
Once hatched, tiny mayfly nymphs, also referred to as naiads, start their prolonged aquatic stage. This nymphal phase is the longest part of their life cycle, lasting from a few months to several years, though an annual cycle is common. Nymphs have elongated, cylindrical, or flattened bodies adapted to their underwater habitat, complete with gills for breathing and specialized mouthparts for feeding on algae, diatoms, or decaying plant matter; some larger species are predatory. They serve as a food source for fish and other aquatic creatures, and their presence often indicates clean, oxygenated water. Nymphs undergo multiple molts, shedding their exoskeletons as they increase in size.
From Water to Air
The transformation from an aquatic nymph to a winged insect is a distinct event for mayflies. When a nymph is fully developed, it rises to the water surface, often in a bubble of air. There, it undergoes a molt, shedding its nymphal skin and emerging as a winged form known as a subimago. This subimago stage is unique to mayflies, making them the only insects that molt again after developing functional wings.
The subimago is a brief, transitional stage, typically lasting from a few minutes to several days, though often overnight. During this time, the subimago usually rests in sheltered areas near the water, such as bankside vegetation. Its wings appear duller and more opaque, often tinted gray, blue, yellow, or olive, and its body is softer and less vibrant than the final adult form. Following this short period, the subimago undergoes one final molt, shedding its skin to reveal the fully mature adult, known as an imago. The imago possesses clear, transparent wings and a shinier exoskeleton, capable of more rapid flight.
The Short Adult Life
The adult mayfly has an extremely short lifespan, typically ranging from just a few hours to a few days, with some species living less than five minutes. This brief existence is entirely dedicated to reproduction, as adult mayflies do not possess functional mouthparts and do not feed, relying instead on energy reserves accumulated during their nymphal stage. Males often gather in large swarms over freshwater bodies, performing a mating flight.
Females fly into these swarms to mate, with copulation usually occurring in flight. After mating, females promptly return to the water to deposit their eggs. Depending on the species, a female may lay anywhere from fewer than 50 to over 10,000 eggs, which then sink to the bottom or attach to submerged objects. Once the eggs are laid, the adult mayfly’s purpose is fulfilled, and it dies, often falling onto the water surface, completing the life cycle.