Everyone gains new perspectives as they grow up, but insects that go through metamorphosis see with a completely different set of eyes as adults. Mayflies spend most of their lives under water as larvae before emerging as adults for one final day of speed dating in a last-ditch effort to produce the next generation. In order to compare how mayflies navigate the two different visual landscapes in which they live, Ádám Egri with Ádám Mészáros and György Kriska at the Eötvös Loránd Research Network Centre for Ecological Research, Hungary, measured how mayflies’ eyes respond to different wavelengths of light and compared what they found to the light levels where the insects tend to congregate.
Adult mayflies don't have much time to travel so they spend most of their airborne life mingling around the local neighborhood. By reviewing footage from webcam videos, the researchers determined that Ephoronvirgo mayflies typically swarm in the evening when the sun elevation is low, about –15 to –7 deg below the horizon. They then measured the visual environment when the sun is at different elevations with a spectrophotometer, which measures the strength of light across a range of wavelengths. It turned out that light levels when the adults congregate at dusk are low and the majority of the light is made up of short wavelength colours, closer to ultraviolet. In contrast, the water where the larvae grow up is murky, so longer wavelengths like visible green light are better able to permeate the darkness. These differences suggest that the insects’ eyes may be adapted to distinct environments throughout their life cycle.
In order to measure what the insects can see, the team collected larval and adult wild mayflies to test their vision in the lab. Light is sensed when photons are absorbed by specialized cells in the eye that communicate with the brain via electrical signals. The researchers measured these electrical signals with a fine electrode probe while shining lights of different wavelengths onto the insects’ eyes. The adult mayfly's eyes were less sensitive to light that is visible to humans and instead picked up on short ultraviolet wavelengths that are invisible to us. Given that the adults emerge in the evening and die the following day, the reduced sensitivity to visual light suits the insects. However, the larvae living in muddy riverbeds sense light and darkness through simple eyes called ocelli, which absorb photons and send electrical signals to the brain, but can't resolve shapes. The researchers found that larval eyes were also sensitive to UV light, but they were most sensitive to light with longer wavelengths closer to green, which is fitting given the murky world they grow up in.
Many insects can see ultraviolet, but these experiments shed light on the green sensitivity of larval ocelli. Although they can't see as well as other animals, the simple eyes of larval mayflies are well matched for the muddy riverbed conditions where they spend most of their lives. As adults, mayflies emerge and face their first and last evening of life above water with a fresh set of eyes that help them find a mate in the dark.
