Should I stay or should I go? Perching damselfly use simple colour and size cues to trigger flight

How do flying insects correctly respond to visual stimuli in complex natural environments? The spectacular coloration of some orders suggests that colour cues are important. Size may contain useful information as well, but insects are limited in resolving fine spatial detail due to the structure of their compound eyes. Although there have been many studies of experimentally altered body colour and pattern, we know surprisingly little about simple, isolated cues that insects use to take off after objects. Specifically, whether it is colour, size or some combination that triggers pursuit. We presented artificial bead stimuli of varying colours and sizes to perching males of the Hawaiian orangeblack damselfly Megalagrion xanthomelas in their natural forested stream habitat. Damselflies were most vigorously responsive to conspecific colours - attacking red (males) and tracking brown (females). Other colours with lesser biological relevance inspired lower response rates. Interestingly, size strongly modulated responses, whereby attack responses towards the smallest bead sizes transitioned to tracking or avoidance at the larger bead sizes. Although small beads are inherently more difficult to see, they triggered responses from greatest distances, even beyond the calculated stereopsis range of 17.5 cm. Damselflies had an object detection threshold of 0.34° within a high-resolution frontally directed acute zone with an interommatidial angle of 0.82°. We found evidence that size and colour serve as discrimination filters to efficiently identify objects of interest while ignoring visual noise. Simple schemes for object discrimination may help to explain how insects can differentiate among prey and conspecifics, and why multiple species can exist in the same community despite being similar in colour. We discuss implications for visual capabilities, and how visual systems that can readily discriminate size and colour can contribute to extraordinary phenotypic diversity.