In their latest research, staff from the ELKH Centre for Ecological Research (CER) and the Eötvös Loránd University (ELTE) Institute of Plant Biology have sought to find out what can be seen by mayflies that develop in the water as larvae before leaving the water, and how this influences their behavior. During their studies with electroretinography, the researchers showed that the visual system of the Danube mayfly, a protected species in Hungary, adapts to changes in light conditions during its development. An article presenting their results has been published in the prestigious journal Proceedings of the Royal Society B.
The larvae hatch from the eggs of the one-year-old Danube mayfly around April. They feed on organic debris and live coating and develop until the late summer swarming season. Swarming begins after dark, at which point the larvae float to the surface, where they transform into a winged insect. After the males transform once again, mating takes place, and then the fertilized females fly a few miles upstream of the river so that the eggs are laid in the water at roughly the same place where the females that laid them matured.
The swarming of the Danube mayfly takes place in a relatively short time in the twilight hours. At this time, it is already dark for the human eye, but the scattered light of the sun still dominates the sky. The ratio of blue and ultraviolet light to lighter wavelength components is highest in the light of the sky at this time. This period follows the short time window called the ‘blue hour’ by photographers.
In their most recent study, the researchers measured the spectral sensitivity of the complex eye of the Ephoron virgo mayfly in larval and adult individuals by electroretinography and found significant differences between these groups. “The larvae of the mayfly were more sensitive to the green spectral range, whereas the eyes of the adult mayfly were more sensitive to ultraviolet light,” said Ádám Egri, a staff member at the CER Institute of Aquatic Ecology and the primary author of the study.
The Ephoron virgo mayfly develops underwater, where in a sometimes cloudy, brownish environment the longer wavelength range of the spectrum dominates and the light intensity is also lower than on the surface. This means that the highest proportions of green, yellow and red wavelengths are present, so it is preferable for the larvae if its eyes are primarily sensitive to the green spectral range.
Using recordings from webcams in Budapest, the researchers determined that the Ephoron virgo mayfly typically collects en-masse between 14 and seven degrees below the horizon, when the ratio of ultraviolet photons to green components in the light of the sky is at its highest. This means that the primarily ultraviolet-sensitive eyes of adult mayflies suggest adaptation to the swarming time window.
“We have previously shown that the Ephoron virgo mayfly is most strongly attracted to ultraviolet and blue light, which is in line with the results of our current laboratory experiments,” said György Kriska, associate professor at ELTE, who began his visual ecological research on the destruction of mayflies in 2012.
A well-known phenomenon is the mass destruction of the Ephoron virgo mayfly observed in public lighting lamps, which can be reduced by the correct choice of the spectrum of outdoor lamps. Most short-wavelength light sources, such as bluish-cold, cold white LEDs, attract far more mayflies than those whose spectrum is predominantly comprised of longer wavelengths, such as the yellowish, warm-white LED.
This means that the example of the Ephoron virgo mayfly also confirms the theory that the longest wavelength, warm white lights of the artificial light sources, cause the least damage to the living world, which is why it makes sense to use them.