Vision
Jumping spiders have very good vision centered in their anterior median eyes (AME). These eyes are able to create a focused image on the retina, which has up to four layers of receptor cells in it (Harland & Jackson, 2000). Physiological experiments have shown that they may have up to four different kinds of receptor cells, with different absorption spectra, giving them the possibility of up to tetrachromatic color vision, with sensitivity extending into the ultraviolet range. It seems that all salticids, regardless of whether they have two, three, or four kinds of color receptors, are highly sensitive to UV light (Peaslee & Wilson, 1989). Some species (for example, Cosmophasis umbratica) are highly dimorphic in the UV spectrum, suggesting a role in sexual signaling (Lim & Li, 2005). Color discrimination has been demonstrated in behavioral experiments.
The principal, anterior median, eyes have high resolution (11 min. visual angle), but the field of vision is narrow, from 2 to 5 degrees. However, the retina at the back of the tube-shaped anterior median eye can move to inspect objects off the direct axis of vision. This dynamic adjustment is a means of compensation for the narrowness of the static field of vision. It is analogous to the way most primates move their eyes to focus images of interest onto the fovea centralis. Such movements within the jumping spider's eyes are visible from outside when the attention of the spider is directed to various targets. It is not clear whether they use their lateral eyes for anything but reaction to threats, but they certainly do react to movement well to either side, and apparently out of view of the anterior median eyes.
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