We may have to re-think what we know about how the little creatures see.

Arthropods such as this Calliphora vomitoria fly have compound eyes. Image credits: JJ Harrison.

Insects see the world much differently from us, that much is clear. For the longest time, researchers thought they are unable to see fine images due to the way their eyes are built. Most insects have compound eyes which consist of many (up to thousands) tiny lens-capped ‘eye-units’. Together, these work to create a low resolution, pixelated image.

Contrasting to that, our own eyes have a single lens, a “megapixel camera” that can actively change the lens shape according to different needs and can keep both nearby and far away objects in sharp focus, based on our different needs. The end result of our eyes is a densely-packed, high-resolution image. Very different from that of insects — or at least that’s what we thought.

Researchers from the University of Sheffield’s Department of Biomedical Science challenge that long-held view. Working with colleagues from Beijing, Cambridge, and Lisbon, they found that insect compound eyes can also generate surprisingly high-resolution images, due to the way the photoreceptor cells deal with image movement.

Unlike the human lens, the insect eyes cannot move to accommodate different images. But the University of Sheffield researchers found that they do something else to compensate for that: underneath the lenses, photoreceptor cells move rapidly in and out of focus as they sample the world around them. This twitch-like movement is so fast that we can’t see it with the naked eye, and has long escaped detection from biologists. In order to thoroughly study it, researchers had to improvise a special microscope.

Researchers conducted in vivo electrophysiological measurements to understand how insects see. Image credits: Mikko Juusola et al — University of Sheffield.

A photoreceptor cell is a specialized type of cell found in the retina that absorbs light (photons). By triggering a change in the cell’s membrane potential, they transform this sensorial input into electrical signals which are then passed on to the brain. Compound eyes are better at detecting edges and are capable of forming images, but were thought to fare worse in terms of overall image quality. They still fare worse than human eyes, it’s just not as bad as we thought.

“By using electrophysiological, optical and behavioural assays with mathematical modelling we have demonstrated that fruit flies (Drosophila) have much better vision than scientists have believed for the past 100 years.”

If these findings are confirmed, then insects combine these normal head/eye movements with super-fast twitching to resolve the world in much finer detail. So far, this improved vision has only been detected in fruit flies (Drosophila), but researchers will soon move on to other insects, as well as vertebrates, in the hope of identifying similar patterns.

Mikko Juusola, Professor of Systems Neuroscience at the University of Sheffield and lead author of the study, said:

“From humans to insects, all animals with good vision, irrespective of their eye shape or design, see the world through fast saccadic eye movements and gaze fixations.It has long been known that fast visual adaptation results in the world around us fading from perception unless we move our eyes to cancel this effect. On the other hand, fast eye movements should blur vision which is why it has remained an enigma how photoreceptors work with eye movements to see the world clearly.”

“Our results show that by adapting the way photoreceptor cells sample light information to saccadic eye movements and gaze fixations, evolution has optimised the visual perception of animals. ”

The findings have been published in the open-access journal eLife.

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