The yellow specialist: dronefly Eristalis tenax prefers different yellow colours for landing and proboscis extension.

Droneflies, imagoes of the hoverfly Eristalis tenax, are known to possess a preference for yellow flowers, i.e. they prefer to visit yellow flowers and prefer to extend the proboscis to yellow colours. In this study we disentangle these colour preferences by investigating the landing reaction and proboscis reflex with particular reference to intensity, spectral purity and dominant wavelength of colour stimuli and their UV reflection properties. In multiple-choice tests, naïve and non-trained flies prefer to land on yellow colours independent of their UV reflection characteristics, but also accept blue, white and pink colours if they absorb UV and are of sufficient brightness. Flies trained to land on colours other than yellow still prefer yellow colours to some extent. Moreover, the flies prefer bright over dark yellow colours even if trained to dark yellow ones. The flies refuse to land on dark colours of all hues. Naïve flies exhibit the proboscis reflex only to pure yellow pollen. These experiments show for the first time that landing in droneflies is triggered by a yellow colour independent of its UV reflection properties, but proboscis extension is triggered by yellow colours strongly absorbing blue and UV. The ability to discriminate colours is better than predicted by the categorical colour vision model. The colour preferences in E. tenax represent a fine-tuned ability to visit yellow flowers displaying a UV bull's-eye pattern.