Eye Doctor’s Tool Offers New Look at Marvel of Moth Eyes
By Ed Ricciuti
If you are into puns, you might call it an eye-opening innovation.
An optometrist in the United Kingdom has adapted technology for diagnosing human eye disease to instead scan how the eye of a living nocturnal moth regulates light input. To date, this light-regulation process has been visualized only in still images from dead specimens, but the new technique records in real time the moth eye adapting to changing light as it unfolds, dynamically.
An article by optometrist Simon Berry, MCOptom, published in June in the journal Environmental Entomology, describes the first use of optical coherence tomography (OCT) to view anatomical detail in the compound eye, common to insects, crustaceans, and other arthropods. Like medical ultrasound, OCT technology images biological tissue but does so by using light instead of sound. It is widely used in ophthalmology to obtain cross-sectional information about structures within the eye, making it an important diagnostic tool in the evaluation of human eye diseases. You may have peered into one if you have been examined for macular disease or if you are elderly; it is used routinely in many patients over 70.
Adapting to seeing in the dark is one of the evolutionary problems that nocturnal animals have had to overcome. Conversely, they can be challenged by the bright light of day. “During the night the light levels are low, so their eyes need to be very sensitive; but, they also need a way of adapting to environmental light conditions, and protecting those sensitive organs, if a bright light is encountered,” says Berry. “Human eyes have a pupil that changes size to regulate light input to the eye. Moths use a light-absorbing pigment that moves position to limit the light within the eye.”
In the moth’s eye, photopigment granules are stored between crystalline cone-shaped structures, or Semper cells, beneath the cornea. Behind that layer, the compound eye of nocturnal insects—defined as a “superposition” eye—has a transparent region called the clear zone. To decrease the brightness of light, the dark pigment is extruded from the cones into the clear zone. Like clouds blocking the sun, the pigment restricts the amount of light reaching the rhabdoms, photoreceptive structures in a layer at the back of the eye. In darkness, the pigment migrates away from the zone back into the cone layer. In effect, the concentration of pigment granules lessens to permit more light and increases to reduce it.
The migration of pigment is difficult to record because it is a dynamic process, Berry says, and takes place only when a moth is alive. “By necessity, any microscopic examination of the eye requires dissection of a dead insect and will show a snap-shot of the adaptive state at that point in time,” Berry writes his paper. Thus, the fact that OCT is non-invasive is critical to the new method for observing this process.
Moths used in the study were trapped, scanned, and later released. During the experiment, the moths were adapted to darkness in a dark bag for at least an hour. The first scan was completed with the room in darkness to try and ensure the insect stayed dark adapted. A white LED light source was then turned, on and various scans were taken as the insect became light adapted.
Berry found that when a moth is in a dark-adapted state, the clear zone is optically transparent, and light emitted by the OCT passes through it to the rhabdom layer, which serves like the retina of the human eye, resolving wavelengths of light so it can be processed to images by the brain. In a light-adapted state, pigment that has migrated into the clear zone changes its composition so it filters out light.
OCT is well suited to observing the physiological adaptation process to light because the process is relatively slow—circa 30 minutes—says Berry, and during this period the insect’s perception is not optimized for the environmental light levels. For example, if a light source causes an insect to light adapt and then that light source is taken away, it will take a period of time for it to become dark adapted and see effectively in low light levels.
From the OCT scans, it appears that the beginning of the pigment migration is not instantaneous but rather the pigment migration becomes visible after a short delay. “This may be because it takes time for the pigment to migrate and show in the scan,” says Berry. However, there could possibly be a biological reason why this may occur. The lag before pigment migration means that if the insect encounters a brief flash of bright light, it may be able to recover quickly because the pigment migration has not started. It may not lose its fully dark-adapted state immediately, as humans do, and so its vision not impeded. Conversely, the time lag in transition from light to dark adaption may disadvantage moths with light-adapted eyes for a time period if they move away from a light source into the dark.
“Further research is needed to determine whether the state of light adaption affects moth behavior,” says Berry. “I really do think that OCT can be a useful tool in entomology and could possibly help explain some of moth behaviour around light sources. It opens up another way of examining the compound eye, and because it is non-invasive it can be used to look at dynamic processes like light adaption in ways not previously possible.”
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.