Stiles-Crawford effect in garter snake

The optical geometry of the eye of the American garter snake suggests a highly pronounced Stiles-Crawford effect occurs at the level of its cone photoreceptors. However, it is demonstrated here that inner segments of the cones in this species have a high density of microdroplets, up to 0.1 microns in diameter, which increases the refractive index of the ellipsoid. Theoretical considerations involving geometrical optics show that these droplets, like conventional oil droplets, may be a specific adaptation of the garter snake to increase the sensitivity of its cones to oblique rays, thus reducing the potentially large Stiles-Crawford effect predicted for this vertebrate.