Australian imperial blue butterflies see polarised light, even on the move

Abstract

While our world appears enveloped in colour, there is one palette that evades mammalian eyes: the effects of light polarisation. Although it is not possible to say how polarised light vision might affect our vision, the ability to see polarisation light plays a major role in the lives of butterflies – helping them to select mates and navigate by the sun. Knowing that tropical jumping spiders pay attention to the polarisation of their shiny colours, Matthew Lim, now at the National University of Singapore, contacted Naomi Pierce (Harvard University, USA) to suggest that they investigate whether Australian imperial blue butterflies (Jalmenus evagoras) are also members of the polarisation vision club. Together with Richard Rabideau Childers, also from Harvard University, Pierce travelled to New South Wales, Australia, home to the shimmering butterflies, to find out whether the butterflies view the world through polarised eyes.‘The challenge was that we had very little idea which, if any, of the mock wings, wing-beating robots or lights we took would actually work to attract and test the responses of these butterflies’, says Rabideau Childers, who arrived in Australia with suitcases packed full of robot and toy butterflies, colourful printed wings and polarisers to try out on imperial blues in the wild. Setting up pairs of identically coloured (blue, red or yellow) robot butterflies – one with polarised wings and the other without light polarisation – in trees for the wild butterflies to visit, the duo realised that the insects were most attracted to the blue unpolarised wings compared with the polarised wings, while the degree of light polarisation had no effect on the attractiveness of the red and yellow wings. So, the butterflies see blue polarised light well. And when Benedict Hogan at Princeton University, USA, analysed how the butterflies perceive each other's wing colours, he discovered that female wings would appear bluer – into the UV range – to male viewers, while male wings would appear greener to the females. In addition, Cheng-Chia Tsai, Heqing Huang and Nanfang Yu (Columbia University, USA) took a close look at the light reflected by the males’ and females’ wings, revealing that the females’ bluer wings reflected more polarised light than the males’ greenish wings and the light reflected from the females’ wings was most polarised when viewed head on during the morning and afternoon.However, creatures that see polarised light often have problems discerning the outline of an object, so Gary Bernard from the University of Washington, USA, wondered how the butterflies have resolved this issue. Bernard took detailed photographs of the mosaic-like retina of the butterflies’ eyes, looking for changes in the polarised light reflected by the individual photoreceptors, known as eyeshine, to find out how the individual photoreceptors are arranged relative to one another. Sure enough, pairs of the light-sensitive structures within each photoreceptor were arranged to see polarised light, confirming butterflies have polarised vision. Then, he scrutinised how the photoreceptors were arranged relative to each other and discovered that pairs of neighbouring photoreceptors were aligned with each other, helping the butterflies to see the outlines of objects more clearly. However, a substantial fraction of neighbouring photoreceptors were not aligned. Bernard realised that this misalignment could allow the constantly moving insects to see polarised light continually, as neurons within the retina sum the input from all of the misaligned photoreceptors so that the insects’ polarisation vision never drops out even when they are on the move.The team suspects that the females’ heightened sensitivity to polarised light might help them to be more choosy, selecting the most attractive male, as they mate only once, while serially unsuccessful males can keep an eye out for the next passing female.