Abstract
A century ago, in his study of colour vision in the honeybee (Apis mellifera), Karl von Frisch showed that bees distinguish between a disc that is half yellow, half blue, and a mirror image of the same. Although his inference of colour vision in this example has been accepted, some discrepancies have prompted a new investigation of the detection of polarity in coloured patterns. In new experiments, bees restricted to their blue and green receptors by exclusion of ultraviolet could learn patterns of this type if they displayed a difference in green contrast between the two colours. Patterns with no green contrast required an additional vertical black line as a landmark. Tests of the trained bees revealed that they had learned two inputs; a measure and the retinotopic position of blue with large field tonic detectors, and the measure and position of a vertical edge or line with small-field phasic green detectors. The angle between these two was measured. This simple combination was detected wherever it occurred in many patterns, fitting the definition of an algorithm, which is defined as a method of processing data. As long as they excited blue receptors, colours could be any colour to human eyes, even white. The blue area cue could be separated from the green receptor modulation by as much as 50°. When some blue content was not available, the bees learned two measures of the modulation of the green receptors at widely separated vertical edges, and the angle between them. There was no evidence that the bees reconstructed the lay-out of the pattern or detected a tonic input to the green receptors.
Highlights
The study of bee colour vision has attracted many naturalists over the past centuries, but has not revealed the kind of mechanism involved
Polarity was not distinguished in some patterns lacking green contrast
No polarity was detected from the outer edges because polarity required a relative position of a blue input, and the contrasts at vertical edges of buff or black on a white background were all saturated and appeared equal to the bees (Table 1)
Summary
The study of bee colour vision has attracted many naturalists over the past centuries, but has not revealed the kind of mechanism involved. In the 19th century, some critical observers found that blue was the preferred colour, and that bees were attracted to landmarks, not to the colours of flowers [1,2,3]. Karl von Frisch showed that bees trained to go to a coloured target for a reward of sugar would distinguish that colour (except bluish-green or grey) from all shades of grey [4]. These results provided support for the popular belief that bees have colour vision, but this conclusion was seriously challenged. From extensive testing of patterns behind glass to eliminate effects of odours, Hess [5] found that bees trained to go to yellow could not PLOS ONE | DOI:10.1371/journal.pone.0116224 January 24, 2015
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