The fine structure of the lateral eyes ofNeocarus texanus Chamberlin and Mulaik, 1942 (Opilioacarida,Acari, Arachnida,Chelicerata)

Summary Neocarus texanus, a primitive mite, bears two pairs of eyes, which are principally similar in ultrastructure. Each eye is covered externally by a cuticular cornea. It is underlain by flat sheath cells which send extensive processes into the retina. The retina is composed of distal and proximal cells. The 20 distal cells of the anterior eye are inversely orientated and form 10 disc-like rhabdoms. They represent typical retinula cells. Each rhabdom encloses the dendritic process of a neuron, the perikaryon of which is located outside the retina (proximal cells). The significance of this cell is not known. The retina is underlain by a crystalline tapetum. In the posterior eye 14 retinula cells form 7 rhabdoms in an arrangement similar to the anterior eye. The eyes of one side of the body are located within a capsule of pigment cells. Together the axons of the distal and proximal cells form the two optic nerves, one on each side of the body. The optic nerves leave the eyes anteriorly and terminate in two optic neuropils located in the brain.From structural evidence it is concluded, that the resolution of the eyes must be rather low.The peculiar proximal cells have not been observed previously in Acari. They probably resemble at best the eccentric cells and arhabdomeric cells of xiphosurans, scorpions, whip-scorpions and opilionids. Also, inverse retinae and tapeta of the present type have not been found in Acari until now, but are present in other Arachnida. Thus the eyes ofNeocarus texanus evidently represent a unique type within the Acari.