Like most spiders, the nocturnal hunting spider Cupiennius salei is able to move the retinae of its antero-median (AM) eyes. In the present study, the morphological and physiological properties of the eye muscles and the mechanism and behavioural relevance of retinal movements are investigated. The retinal movements are brought about by two forces: (i) contractions of the dorsal and ventral eye muscles, and (ii) the passive elastic restoring force of the eye tube and eye muscles (the presumed counteracting force). The dorsal eye muscle consists of 15­18 striated fibres and is 600 µm long. The ventral eye muscle is longer (650 µm) and consists of 20­22 striated fibres. The direction of the gaze of the retinae brought about by the eye muscles (active retinal movements) depends on the contraction states of the two eye muscles. The medially directed action of both eye muscles does not allow active movements of the eye tube in any lateral direction. Thus, the direction of gaze cannot actively be shifted medially. After active displacement of the retina, the elasticity of the eye tube and eye muscles passively moves the eye tube back to its resting position. There are two types of retinal movements. (i) Spontaneous microsaccades (duration 80 ms; excursion 3 °) are caused by the spontaneous contraction of only the dorsal eye muscle. They are ideally suited for preventing the adaptation of the sensory cells since their mean excursion (3 °) perfectly fits the inter-receptor angle (2.9 °) in the AM eye of Cupiennius salei. (ii) Induced movements (duration 100­500 ms; excursions 4­15 °) are caused by the contraction of both eye muscles and occur only after mechanical stimulation. Induced movements were elicited by stimulating the mechanosensory organs (trichobothria and slit sense organs) of the spider's legs. A stimulus on one side of the spider induces movements of the ipsilateral retina only. We therefore suggest that induced retinal movements are saccades shifting the gaze of the spider laterally towards the site of mechanical stimulation. According to behavioural experiments, the ability of a spider to locate an immobile target is highly impaired after blinding its AM eyes. We suggest that the motility of the AM eyes is required to locate stationary objects.

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