Fig 1. Stimuli qualities and experimental setup.

A: Spectral sensitivity of the three photoreceptor types in Bombus terrestris (data obtained from [41]), overlaid by transmission of the four tested color filters (435 nm, 455 nm, 488 nm, and 528 nm). B: Intensities (photons per second and mm²) of the monochromatic light stimuli generated by means of different ND filters (13%, 51% and 100% transmission). C: Illustration of the set-up for visual PER conditioning. See text for description. fh, filter holder; s, movable sleigh. D: Y-maze set-up used for the transfer test after differential PER conditioning. The diffusor is omitted in the left arm to make the color filter (f) and the opening between the two chambers visible. See text for description. d, diffusor (parchment paper); fc, filter camber; f, color filter; tc, testing camber; ec, entrance chamber; dc, decision chamber.