Caterpillars of the peppered moth perceive color through their skin
It is difficult to distinguish the caterpillars of the pepper moth from a twig. Caterpillars imitate not only the shape but also the colour of a branch. In a new study, researchers at the University of Liverpool in the United Kingdom and the Max Planck Institute for Chemical Ecology in Germany are demonstrating that caterpillars can feel the colour of the branch with their skin. The blindfolded caterpillars changed their body colour according to their background. When given the choice of background to lean on, the blindfolded caterpillars were always moving towards the background they looked like. The researchers also discovered that the genes needed for vision were expressed not only in the eyes of the caterpillars, but also in their skin. The study is published in Communications Biology.
Cephalopods, chameleons and some fish camouflage themselves by adapting their colour to their environment. These animals have a system to perceive colour and light independently of the eyes. Some insects, such as the caterpillars of the pepper borer (Biston betularia), also associate their body colour with the colour of the twigs of their edible plant, although this colour change is rather slow compared to other animals. Until now, scientists did not know how insect larvae can perceive the colour of their environment and how the colour change occurs. Two theories dating back more than 130 years have suggested that the change in colour could be caused by the diet or by the animal seeing the colour. Since some insects are known to be able to perceive the light - but not the colour - of the skin, researchers at the University of Liverpool and the Max Planck Institute for Chemical Ecology have adopted three different approaches to solving the enigma of how pepper butterfly caterpillars adapt to the colour of their environment.
First, they tested whether the caterpillars of the pepper moth, whose eyes were covered with black acrylic paint, could still adjust their colour in the background. Blind caterpillars were raised on white, green, brown and black branches and their body colour was observed. Even without being able to see, the tracks changed color to look like the background to the same extent as the tracks whose eyes were not covered. "I was completely surprised that blindfolded caterpillars could still change color and match it to the background. I don't think my supervisor, Ilik Saccheri, believed me until he saw it for himself," says Amy Eacock, one of the lead authors of the new study and currently a postdoctoral fellow at the Max Planck Institute for Chemical Ecology.
In behavioural experiments, blindfolded caterpillars had the option of switching to twigs of different colours. Constantly, the caterpillar rested on the branch that most closely resembled their own colour.
In a third approach, researchers examined in which parts of the body the genes related to vision were expressed. They found them not only in the heads of caterpillars, where the eyes are, but also in the skin of all body segments. A visual gene was expressed even more in the skin than in the head of the caterpillars. "We assume that this gene is involved in the perception of background color by the skin," notes Hannah Rowland, second lead author and head of the Max Planck Research Group, Predators and Toxic Prey.
"One of the main challenges facing animals is to avoid being eaten by predators. Many species have evolved into camouflage to avoid detection or recognition. A considerable problem, however, is how prey can correspond to the range of visual backgrounds on which they are often seen. The colour change allows animals to adapt to their environment and potentially reduce the risk of predation," explains Hannah Rowland, emphasizing the ecological context of the study. Amy Eacock adds: We have built a computer model that can "see" in the same way as birds, which allows us to conclude that these adaptations - colour change, twig imitation, background behavioural matching - have probably evolved to avoid visual detection by predators. Caterpillars with a better colour perception may have been eaten less by birds, while birds with better vision may attack these larvae more, thus continuing the race for evolving predator-prey weapons.
The study expands our understanding of how lepidopteran larvae protect themselves from predation.
