Tsetse flies (Glossina spp.), the vector for African trypanosomiasis, are highly attracted by blue and black surfaces. This phototactic behaviour has long been exploited to trap tsetse flies as one measure in the control of African trypanosomiasis. However, why blue and black are so attractive for tsetse flies is still unknown. We propose that the combination of blue and black is attractive for many Glossina species because when searching for a shady resting place to pass the day, the flies are probably guided by the blueness and darkness of daytime shadows. In contrast to people's experience that daytime shadows are colourless, actually on a sunny day all shadows are tinted bluish by the scattered blue skylight.