Structure of the visual system of the two-spotted spider-mite, Tetranychus urticae

The pair of eyes on each side of Tetranychus urticae consists of fifteen retinular cells, six pigment cells, six corneal cells, and one vitreous cell. Five rhabdomeres lie beneath the anterior lens, twelve beneath the posterior lens. The pigment and corneal cells appear to determine the positions at which rhabdomeres occur. The volumes of all rhabdomeres have been measured. A pair of fibrous masses are associated with tendons at the dorsal apodemes; the nerves of these ‘apodemal organs’ join the optic nerves en route to the brain. There are a pair of optic neuropiles, each surmounted by a large secretory cell.