Fig. 1.

(A) Illustrations of the cosexual artificial flowers used in the experiments. The flowers attract visits from bumble-bee workers by offering sucrose solution at a nectary that is continuously replenished by capillary action. ‘Sexual organs’ are housed in a plastic superstructure, formed by 3-D printing, that is positioned over the nectar cup by a piece of tape serving as a hinge. The flowers receive and transfer pollen analogue dye when a bee pushes under the hinged superstructure: the sticky tape stigma picks up dye and a slit in the dye chamber sifts dye on the back of the bee. (B) Spatial arrangement of artificial flowers designed to represent an idealized transition zone between two plant species. Blue and yellow flowers provide 1.4 m and 0.6 m sucrose, respectively.