Setup for the wavelength choice test.

(a) The experimental arena used for the wavelength choice test (modified from [22]). Two dodecagonal clear plastic plates (top and bottom) were separated by six semicircular black spacers. A piece of filter paper was placed on the bottom plate. A hole was made in the center, through which the fly could climb into the experimental arena via the plastic tube. Six LED lights of different colors were placed on the open sides of the arena. We used UV (365 nm), VL (405 nm), BL (450 nm), GR (525 nm), OR (590 nm), and RD (660 nm) LEDs. The light-emitting surface was made of an optical diffusion filter framed with an aluminum seal. Dashed lines indicate imaginary borders, separating each LED area from the central hexagonal area. (b) Irradiance spectra of the LEDs. The integral of the spectra was adjusted so that the photon fluxes were equivalent for all six LEDs. (c, d) Example trajectories of the flies. Several trials were merged in each Fig The color of a trajectory reflects the peak wavelength of the LED that was eventually chosen by the individual. (c) Example trajectories of the flies that chose UV or VL. (d) Example trajectories of the flies that chose LEDs other than UV or VL.