Mayflies do molt, or shed their exoskeleton, but their process is highly unusual among insects. Molting, scientifically known as ecdysis, is necessary for growth and transformation in invertebrates. While most winged insects complete their final molt before their wings are functional, mayflies are the only known insect group to possess a fully winged, flying stage that still undergoes a complete molt. This unique two-part transformation into the adult form is a distinguishing characteristic of the entire Ephemeroptera order.
Frequent Molting in the Aquatic Phase
For the vast majority of their lives, mayflies exist as aquatic larvae, often called nymphs, submerged in freshwater environments. This extended period can last from a few months to several years, during which their primary function is growth. Because the exoskeleton is rigid, the nymph must periodically shed its outer skin to increase in size. This aquatic molting is a repeated process of expanding body mass and forming a new, larger exoskeleton.
The frequency of this growth-related molting is remarkably high compared to other insect larvae. Mayfly nymphs can shed their skin anywhere from 12 to 50 times before leaving the water, though 15 to 25 molts is common. Each successful molt, or instar, allows the organism to accommodate the rapid increase in body size required for the transition to the winged form. The cast-off exoskeletons remain in the water, marking the many developmental stages the aquatic form has undergone.
The Unique Subimago Stage and Winged Molt
When the aquatic nymph reaches maturity, it emerges from the water and transforms into the first winged stage, known as the subimago, or the “dun.” This subimago is a fully winged, flying insect, but it is an intermediate life phase, not the final adult form. Mayflies display their most extraordinary trait here: molting while already possessing functional wings. This final ecdysis, where the subimago sheds its skin to become the imago, is unique among winged insects.
The physical difference between the two winged stages is noticeable. The subimago’s wings possess a dull, opaque appearance, often described as dusty or cloudy. They are covered with minute hairs called microtrichia and a fine fringe, which helps the insect shed water quickly after emergence. The subimago then flies to a sheltered location, such as bankside vegetation, where it waits a period ranging from a few minutes to 48 hours before the final molt occurs.
During this final transformation, the entire outer skin of the subimago splits and is shed, including the skin covering the wings. The insect that emerges is the imago, or the “spinner.” The imago’s wings are immediately distinguishable, appearing clear, glossy, and transparent, having lost the dull microtrichia layer. This final shedding of the integument, including the wing surfaces, is the definitive characteristic separating mayflies from all other winged insects.
Biological Purpose of the Final Transformation
The last molt from the subimago to the imago is performed not for physical growth, but for reproductive and aerial optimization. The subimago stage serves as a crucial, short-term transition, allowing the insect to safely leave the water and find shelter before completing maturation. The final imago stage is the true reproductive adult, fully prepared for mating.
Transformation into the imago completes the sexual maturation process, with reproductive organs becoming fully developed and functional. Male imagos undergo significant physical refinement, particularly in their forelegs and eyes. The forelegs lengthen considerably, allowing the male to grasp the female during the mating flight in aerial swarms. The improved structure of the wings, now clear and less fuzzy, also enhances flight performance, making the imago a more agile flier. With no functional mouthparts, the imago stage is entirely dedicated to reproduction, and its lifespan is consequently very short, often lasting only a few hours or days.