The optical geometry of euphausiid eyes

Summary1.Euphausiid eyes have all the features of classical refracting superposition eyes. The crystalline cones, which resemble those of moths very closely, are hard, circular in cross section and hexagonally packed. They are thus unlike the optical elements of macruran decapods with reflecting superposition optics, which are soft and square in section.2.The clear zone always has a thickness of about half the local radius of curvature of the eye, when the centre of curvature is defined as the point of intersection of the axes of the cones. This zone appears to contain only the clear cytoplasm of the receptor cells, and there are no structures that could be considered to be light-guides.3.It is shown that the crystalline cones have the property of bending incident light across their axes, so that it emerges into the clear zone at an angle equal and opposite to the angle of incidence at the eye surface. This is a necessary condition for superposition image formation.4.Many deep-water euphausiids have double eyes, usually with a region of enlarged facets pointing upwards and covering a narrow angle, and a downward pointing region covering a wider angle. The upper eye often has a peculiar geometry, with the eye surface centred on a point in the receptor layer, but the long axes of the cones centred on a point twice as deep in the eye. It is shown that this condition produces a superposition image with no axial spherical aberration.5.A consequence of this arrangement is that the focal plane is flat, not curved as in spherical-eyed forms, and this explains why the retina has a flattened appearance in the upper but usually not the lower eyes.6.Unlike the eyes of both moths and macruran decapods, euphausiid eyes do not show eye-shine. The rhabdoms are relatively short and wide, and are not shielded by reflecting pigment.