Nocturnal bees learn landmark colours in starlight.

Honeybees, like humans and most other vertebrates, are colour-blind in dim light. Bees are primarily day-active and have apposition compound eyes, the typical eye design of diurnal insects. Most bees are trichromats with photoreceptors sensitive in the UV, blue and green [1]. While their diurnal colour vision was established almost 100 years ago, honeybees are known to be colour-blind in moonlight [2]. Here, we present the first evidence that the only known obligately nocturnal bee, the Indian carpenter bee Xylocopa tranquebarica (Fabricius), which flies even on moonless nights [3], uses colour vision to discriminate artificial landmarks at the nest in starlight. Humans, in contrast, are colour-blind at half-moon illumination. This finding, obtained using natural nests under natural illumination, is remarkable because insensitive apposition eyes were thought unable to support nocturnal colour vision. Hitherto, nocturnal colour vision was known only in nocturnal hawkmoths [4] and geckos [5], animals with eyes well adapted to nocturnality.