The adaptation to coloured backgrounds of green and permanentiy brown specimens of Rana esculenta, and the anatomical and physical bases for their difference in colour were studied by means of reflectance microspectrophotometry and electron microscopy.
Adaptation to the lightness of the background took place in the green type, but not in the brown, whereas adaptation to hue and purity in both types seems negligible.
The iridophores of this species probably represent a λ/4-system. The colour difference between the two types seems mainly to be caused by a difference in the selective reflectivity of the iridophores because of a difference in thickness of the platelets and the cytoplasmic interspaces. Furthermore there is a difference in the amount and, probably, composition of the xanthophore pigment.