Camouflage by counter-brightness: the blue wings of Morpho dragonflies Zenithoptera lanei (Anisoptera: Libellulidae) match the water background

Bright iridescent colours are widespread in several aquatic and terrestrial animal taxa and are usually involved in intraspecific communication and/or predator avoidance. Camouflage by iridescence may be one strategy to avoid predators when the animal exhibits bright colours that match the brightness of its surroundings. Hence, animal structural colouration may have a "brightness matching" or "counter-brightness" function when observed against bright or glossy backgrounds. Here, we addressed the role of such counter-brightness effect of the iridescent wings of the Morpho dragonfly Zenithoptera lanei for avoiding detection. We hypothesized that the bright reflectance of the dragonfly wings is cryptic against the bright water surface and the glossy vegetation where they naturally occur, protecting the dragonfly from visually oriented predators, deceiving prey and signalling to conspecifics when desired. We addressed whether (1) the iridescent colours of Z. lanei wings function as a visual strategy to reduce their wing detectability by brightness matching the background and (2) the detectability of wings against vegetation and water varies according to the observer. For this, we modelled how conspecifics, dipteran prey and predatory birds see the odonate wings against the vegetation of the Neotropical Savannah and against the water surface where the dragonflies perch. Our results suggest that Z. lanei dragonflies can avoid detection by predators, prey and conspecifics when perched on their natural habitats (i.e., ponds) against the bright background of the water surface. Here, we add evidence to the multifunctionality of structural colours in animals and the function of iridescence in camouflage. The bright iridescence and ultraviolet reflective nanostructures of Z. lanei wings when coupled with striking behavioural displays may provide a dynamic and safe intraspecific communication channel.