2 min readCaterpillars of Peppered Moth Perceive Colour Through Skin

Liverpool, UK — Cephalopods, chameleons and some fish camouflage themselves by adapting their colour to their surroundings. These animals have a system to perceive colour and light independently of the eyes. Some insects, such as caterpillars of the peppered moth (Biston betularia), also match their body colour to the twig color of their food plant; although this colour change is rather slow compared to other animals. Until now, scientists have not known how insect larvae can perceive the color of their environment and how the colour change occurs. Two theories dating back more than 130 years proposed that the colour change could be caused by the diet or by the animal seeing the colour. As some insects are known to be able to perceive light – but not colour – by the skin, researchers from Liverpool University and the Max Planck Institute for Chemical Ecology pursued three different approaches to finally solve the riddle of how caterpillars of the peppered moth match the colour of their surroundings. 

First, they tested if caterpillars of the peppered moth, whose eyes were painted over with black acrylic paint, could still adjust their colour to the background. The blindfolded caterpillars were raised on white, green, brown and black branches and their body colour observed. Even without being able to see, the caterpillars changed colour to resemble the background to the same extent as caterpillars whose eyes were not covered. “It was completely surprising to me that blindfolded caterpillars could still change their colour and match it to the background. I don’t think my supervisor, Ilik Saccheri, believed me until he saw it by himself”, says Amy Eacock, one of the lead authors of the new study and currently a postdoc at the Max Planck Institute for Chemical Ecology.

In behavioural experiments, blindfolded caterpillars had the choice to move to differently coloured twigs. Consistently the caterpillar rested on the twig most similar to their own colour. 

In a third approach, the researchers examined in which parts of the body genes related to vision were expressed. They found them not only in the head of the caterpillars, where the eyes are, but also in the skin of all body segments. One visual gene was expressed even more in the skin than in the heads of the caterpillars. “We assume that this gene is involved in the perception of background colour by the skin,” notes Hannah Rowland, second lead author and leader of the Max Planck Research Group, Predators and Toxic Prey.

“One of the major challenges animals face is how to avoid being eaten by predators. Numerous species have evolved camouflage to avoid being detected or recognised. A considerable problem, however, is how prey animals can match the range of visual backgrounds against which they are often seen. Colour change enables animals to match their surroundings and potentially reduce the risk of predation,” says Hannah Rowland, highlighting the study’s ecological context. Amy Eacock adds: “We constructed a computer model that can ‘see’ the same way birds do, so we are able to conclude that these adaptations – colour change, twig-mimicking, behavioural background-matching – likely evolved to avoid visual detection by predators.” Caterpillars with better colour sensing may have been eaten less by birds, while birds with improved vision may prey more upon these larvae, continuing the evolutionary predator-prey arms race.

The study expands our understanding of how lepidopteran larvae protect themselves from predation.

Article adapted from a Max Planck Institute for Chemical Ecology news release.

Publication: Adaptive colour change and background choice behaviour in peppered moth caterpillars is mediated by extraocular photoreception. Eacock, A et al. Nature Communications (August 02, 2019): Click here to view.

caterpillars, Colour change, genes, peppered moth

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