Jewel beetles don't hide their light under a bushel. Their glimmering metallic wing cases are prized by jewellery designers and adorned the dresses of Victorian gentlewomen. One gown, in London's V&A Museum, is bedecked with more than 5000 shimmering emerald wing cases. Yet, no one was sure how members of the exotic family perceive their own gleaming exoskeletons. Gregor Belušič from the University of Ljubljana, Slovenia, explains that the insects seem to be sensitive to brilliant colours, as they are easily lured into brightly coloured traps. However, thin structures in the exoskeleton surface also polarize the light reflected by their iridescent wing cases and bodies, possibly enhancing the vivid colours in the eyes of their own species. Intrigued by the possibility that their cuticular colours may appear even brighter to the jewel beetles themselves, Belušič and Carmen Quero from the Institute for Advanced Chemistry of Catalonia, Spain, began investigating the appearance and the retina of Coraebus undatus beetles – captured using traps in nearby cork oak woods – to learn more about the image that they project and what they actually see.
Opening the black wing coverings to reveal the beetle's iridescent abdomen, Belušič and Andrej Meglič, also from the University of Ljubljana, recorded ultraviolet and blue wavelengths from the back, and green and yellow wavelengths from the underside. Interestingly, when they recorded the light reflected by traps that the beetles were caught in – which are used by foresters in North America to catch other jewel beetle pests – the spectrum of colours matched the blue back of the insect's abdomen reasonably well, but with a strong additional red peak that made the plastic appear purple to human eyes.
Belušič and Marko Ilić from The Graduate University for Advanced Studies, Japan, then measured the electrical activity from individual light receptors. They shone light ranging from ultraviolet to red on the insect's eye, while also adjusting the degree of light polarization, to find out which colours and directions of polarization the receptors in the eye were tuned to. Most of the light receptors were tuned to green wavelengths (540 nm), while a few picked up UV and blue light and two receptors were attuned to red light. In addition, most of the photoreceptors were mildly sensitive to light polarization. However, when Belušič, Meglič and Kentaro Arikawa, also from The Graduate University for Advanced Studies in Japan, examined the arrangement of the light receptors, they found that the photoreceptors were randomly rotated relative to each other, essentially cancelling out each other's response to light polarization. Even though the light reflected by the beetle's vividly coloured abdomen is slightly polarized, the insect is apparently incapable of seeing the effects.
Jewel beetle eyes seem to see colour well; they are immune to the effects of light polarization, which can brighten and dim specific shades at certain times of day. This allows them to always see the true colour of an object. Belušič says their insensitivity to polarized light also explains why jewel beetle traps work so well. ‘The plastic coloured traps are usually coated with glue, which is shiny and creates a polarized reflection’, he says. The insect's inability to see the effects of polarization means that the traps closely resemble the colour of their own abdomen at all times. ‘The structure and physiology of the retina explain why the sticky traps work at all’, concludes Belušič.