Beetles that see red break the rules of insect vision

Most insects, including many beetles, cannot see red. Their eyes detect ultraviolet, blue, and green light. Bees often visit red flowers, but not for their color. They are drawn to ultraviolet reflections, not the red hue itself.

But two Mediterranean beetles break that pattern. Pygopleurus chrysonotus and Pygopleurus syriacus can truly perceive red. They seek out red flowers, especially poppies, anemones, and buttercups, for pollen. Their eyes are uniquely tuned to do this.

Beetles with a four-color vision system

‘To our knowledge, we are the first to have experimentally demonstrated that beetles can actually perceive the color red,’ said Dr Johannes Spaethe.

The researchers confirmed this by recording electrical activity from photoreceptors in beetle eyes. They discovered four photoreceptor types. These respond to ultraviolet, blue, green, and red wavelengths.

This tetrachromatic system is rare among beetles. Most have just two or three receptor types. Blue sensitivity, previously missing in some beetle lineages, reappears here. It likely evolved through a gene duplication event and a spectral shift from ultraviolet.

Beetles choose red over grey

Photoreceptors are light-sensitive cells in the eye. Just having them doesn’t prove an animal can see colors. To show true color vision, an animal must tell colors apart even when they have the same brightness.

To test this, scientists used a trick from bee research by Von Frisch. They gave beetles a choice between red circles and grey ones that varied in brightness. If beetles picked red over all shades of grey, it meant they could see red as a distinct color – not just as a lighter or darker patch.

That’s exactly what happened. In every test, beetles chose red first. The researchers also ran a second test in the wild using traps in five colors. Almost all P. chrysonotus beetles ended up in red traps. This showed their natural color preference matched the lab findings.

Opponency and complex visual processing

Recordings also revealed opponency signals. These occur when one receptor type’s signal suppresses another. It is a marker of complex color processing.

For example, red receptors showed suppressed responses in green wavelengths. This confirmed interaction between channels.

This kind of opponency, previously seen in butterflies, suggests advanced neural processing. It likely helps the beetles make sharp color distinctions in complex natural environments.

Evolution of beetles and red flowers

Why do these beetles care so much about red? It may reflect a mutual evolutionary pathway. Red flowers in the Mediterranean are mostly pollinated by Glaphyridae beetles. These flowers form the “poppy guild.”

‘The prevailing opinion in science is that flower colours have adapted to the visual systems of pollinators over the course of evolution,’ says Spaethe.

Yet this case flips that assumption. Glaphyrid beetles may have evolved their vision to match flower color diversity.

Within their family, different genera prefer different flower colors such as yellow, white, violet, and red. This suggests their visual systems may be flexible and evolving alongside flowers.

Red vision helps beetles access flowers

Specializing in red gives beetles an advantage. Most insects cannot see red. That means red flowers are invisible or less noticeable to them.

But P. chrysonotus and P. syriacus can see red clearly. This gives them special access to food – like pollen – from red flowers that others ignore.

Because fewer insects compete for red flowers, the beetles face less crowding. They also become more reliable pollinators for those flowers.

Over time, flowers may evolve to become red on purpose. This helps attract only beetles and avoid unwanted visitors like bees that may steal pollen without pollinating. This is what scientists call a “private niche in color space.”

Rare window into insect vision

P. chrysonotus and P. syriacus are now among the few insects with confirmed red color vision. Their unique tetrachromatic eyes, behavioral responses, and flower preferences paint a new picture of insect-flower interaction.

This research does more than reveal a rare trait. It creates a valuable model for studying pollination, vision, and co-evolution, all through the eyes of beetles.

The study is published in the Journal of Experimental Biology.

Image Credit: Johannes Spaethe / University of Würzburg

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