F, an image of the corneal surface illuminated from the inside of the eye, showing the filtering effect of the pigments. E, an unstained transverse section of the ommatidia showing red and yellow pigments. UV, ultraviolet B, blue DG, double‐peaked green R, red V, violet SG, single‐peaked green BB, broad‐band. B–D, spectral sensitivities of photoreceptors contained in type I ( B), type II ( C) and type III ( D) ommatidium. Type II has fluorescence pigment (Fp) in the distal portion of the rhabdom. The rhabdom (Rh) is surrounded by red (type I and II) or yellow (type III) perirhabdomal pigments (Pp). An ommatidium contains nine photoreceptor cells (1–9) that together form the rhabdom in the centre. Longitudinal view with transverse sections on the left. We have found that the development of butterfly eyes shares its molecular logic with that of Drosophila: the three‐way stochastic expression pattern of the transcription factor Spineless determines the fate of ommatidia, creating the random array in Papilio.Ī, schematic diagram of three types of ommatidia, I, II and III. To address the question of how the spectrally complex eyes of butterflies evolved, we studied their developmental process. The random array of three ommatidial types is a common feature in butterflies. Our simulation indicated that the Papilio vision is tetrachromatic, employing the ultraviolet, blue, green and red receptors. We then used mathematical modelling to infer which photoreceptors are involved in wavelength discrimination. We found that Papilio can detect differences as small as 1–2 nm in three wavelength regions, rivalling human performance. To determine whether Papilio use all of their receptors to see colours, we measured their ability to discriminate monochromatic lights of slightly different wavelengths. The Papilio eye is thus a random patchwork of three types of spectrally heterogeneous ommatidia. The eyes of the Japanese yellow swallowtail ( Papilio xuthus) contain ultraviolet, violet, blue, green, red and broad‐band receptors, with each ommatidium housing nine photoreceptor cells in one of three fixed combinations. Unlike the trichromatic retinas of humans (blue, green and red cones plus rods) and honeybees (ultraviolet, blue and green photoreceptors), butterfly retinas typically have six or more photoreceptor classes with distinct spectral sensitivities. Butterflies use colour vision when searching for flowers.
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