Phenotypic plasticity in color without molt in adult grasshoppers of the genus Sphingonotus (Acrididae: Oedipodinae)

Abstract

Homochromy (i.e. that individuals have a similar color as their environment) is frequent in grasshoppers, and probably functions to reduce detection by potential predators. Nymphs of several soil-perching grasshopper species are known to show color changes during development that increase homochromy, with color being determined with each molt. While this is well documented for young individuals, the only color change in response to the environment that has been recorded for adult grasshoppers of these species is an overall darkening of the individual when exposed to dark surfaces. Whether grasshoppers can also adaptively change color hue is relevant for our understanding of the evolution of locally adapted crypsis. We therefore exposed two groups of adult grasshoppers to a bluish-gray substrate or a reddish-brown substrate, and recorded their color over time. Quantitative digital image analysis showed that adult soil-perching grasshoppers remained capable of adapting to changes in the color of their surroundings through a plastic response. Compared to nymphs, the changes are not as strong and much slower. We suggest that color change in adults occurs through the ongoing deposition of melanins, with eumelanin making individuals more bluish-gray and pheomelanin making individuals more reddish-brown. The fact that color change is possible but slow supports that other mechanisms, such as habitat choice or selective predation, may also play a role in adapting local populations to substrate color. In addition, the ability of these grasshoppers to produce different melanins in response to the environment supports a previous suggestion that they might be useful in the future development of animal models to study melanin-related diseases like melanoma and Parkinson´s disease.