Mayflies, ancient insects distinguished by a distinctive life stage not observed in other winged counterparts, undergo a remarkable transformation. Fully-grown nymphs transition into a winged subadult phase, known as the subimago, a stage incapable of reproduction. After a brief duration, this subimago sheds its skin, along with its wings, metamorphosing into the adult insect.
Led by Dr. Arnold Staniczek, an insect specialist at the Natural History Museum in Stuttgart, an international team harnessed modern technologies to unravel a pivotal mechanism in the molting process from subadult to adult. Contrary to prior assumptions about the function of bubble-shaped areas, termed “bullae,” in mayfly wing veins, the team's findings, detailed in the journal BMC Biology, unveil that these bubbles are not associated with flight but instead play a crucial role in the molting process leading to the final adult stage. Beyond shedding light on this intricate metamorphic process, the results offer insights into the evolutionary aspects of winged insects.
Modern technologies help analyze wing structures
For seven decades, the prevailing belief held that the bullae in mayfly wing veins played a crucial role in flight, facilitating passive downward bending during the upstroke to reduce air resistance and enhance lift. However, this long-standing theory has been overturned through modern research methods employed by the authors.
Dr. Staniczek, leading the investigation, explains, “When we took high-speed videos of mayflies in flight, we soon realized that the previous theory could not be upheld: Instead of the wings bending during the upstroke, they are simply held vertically to avoid air resistance, but the wing itself remains stable.”
In-depth exploration of wing bullae and their structural intricacies utilized advanced techniques such as scanning electron microscopy, synchrotron micro-CT, and fluorescence microscopy. The findings confirmed that the bullae serve as membranous weak points in the wings of both winged stages of these insects.
During the observation of the molting process from subadults to adults, researchers noted a distinctive behavior. “In order to pull out the new wings from the cast-off skin, the mayflies lift their wings, bending them at the defined weak points to more easily extract them undamaged,” says Staniczek. Drawing an analogy, he adds, “We do a similar thing when bending our arms to take off a jumper more easily.” This hypothesis received indirect validation through the observation that bullae are absent in mayflies that, over evolutionary processes, abandoned subimaginal molting. The study's revelations offer a fresh perspective on mayfly wing mechanics and the intricacies of their molting process.
Important clues to the evolution of insect flight
The bullae emerge as a distinctive indicator of subimaginal molting, serving as a reliable marker. Upon scrutinizing fossilized remains, the researchers made a remarkable discovery: bullae were discernible in 272 million-year-old mayfly fossils from the Permian period. This finding indicates that the molting mechanism involving bullae in wings is an ancient feature of mayflies, persisting through their evolutionary history.
Notably, bullae and the winged subadult life stage stand out as unique to mayflies among present-day insects. The preservation of this intermediate stage in mayflies may be attributed to selective advantages associated with the transition from aquatic larval habits to adult life on land.
Dr. Staniczek emphasizes the broader implications of their work, stating, “This work provides new insights and a basis for further research into the evolution of flying insects. A thorough search for bullae in the fossil record might reveal whether other winged insects also had a winged subadult and at what point in evolution it was lost.” The study opens avenues for deeper exploration into the evolutionary trajectories of flying insects, offering a glimpse into ancient features preserved in the fossilized records of these remarkable organisms.