Photomechanical adaptation in the eyes of Squilla mantis

In the eye of Squilla mantis adapted to the dark the layer of the crystalline cones shortens and the retina becomes longer; the opposite occurs in light adaptation. With dark adaptation the acceptance angle becomes more than twice as large as in light adaptation. With this also the visual field of the ommatidium and the gaussian sensitivity function increase. Myofibrils between the crystalline cones are immersed in veils which are attached to the cornea. At the widest part of the cone a reinforced cone membrane folds inside; the fold contains a ring to which the veils are attached. This unique structure serves possibly as a harness to transfer the mechanical traction of the contractile structures to the cone. Around the distal, tapering part of the cone, the corneagenous cells could act as hydraulic shock-absorbing cushions. Six accessory pigment cells stretch between the distal retina and the basement membrane and contain myofibrils. The shortening of the cones during dark adaptation is presumably due to the contraction of the myofibrils in the veils and the simultaneous relaxation of those in the accessory pigment cells. In light adaptation the latter contract and the fibers in the veils relax.