Damselflies that prefer dark habitats illustrate the importance of light as an ecological resource

Habitat associations provide clues to the resources that influence the life of animals. Food distribution, structural microhabitat or degree of insolation can determine species' strategies for energy acquisition, locomotor strategy or thermoregulation. A growing body of research suggests that insolation may be important not for heat, but rather for visual performance in communication, crypsis or prey capture. Odonates (damselflies and dragonflies) are famous for their heliothermic habitat associations. Megalagrion nigrohamatum nigrolineatum is a forest-dwelling damselfly endemic to the island of O'ahu and part of an ecologically diverse adaptive radiation with spectacular body coloration. Although many Megalagrion exploit full sun, nigrolineatum can be curiously found in deep shade raising the possibility that it is shade-seeking. Here, we show that nigrolineatum selects perches based on light, and not perch type or temperature. Surprisingly, they did not select the brightest locations available (as might be expected if they are extending their visual function in a challenging habitat), but chose darker perches in a fairly dark habitat. This strategy opens up niche space that is abundantly available in forests, yet little-occupied by other odonates. We discuss implications of shade-seeking for communication, evolutionary diversification and preserving future evolutionary potential.