A black line enabled bees to distinguish patterns equiluminant for green receptors.

(A) The training patterns. (B) The trained bees discriminated the blue panels alone. (C) Moving the black line to the outer edge of the blue reversed the preference (p < 0.02). (D, E, F, G) Narrow blue bars, buff panels alone, a black line on a buff target, or on a buff panel, were not discriminated. (H) A black line and a blue bar, forming a skeleton pattern, were as good as the training pattern. (J) The training pattern was easily distinguished from the skeleton pattern. (K) Changing the blue contrast at the central midline to green contrast had little effect because the black line already displayed strong green contrast. For scores of 62 or more, or 38 or less, and n = 100, p < 0.02 (2%), which was acceptable.