Jumping spider's eyes. Coloured environmental scanning electron micrograph (ESEM) of simple eyes (ocelli) on the front of the head of a jumping spider (family: Salticidae). Spiders have eight eyes (ocelli) for vision and sensitivity to light levels. A jumping spider has these four eyes on the front of the head for binocular vision. The other eyes (not seen) allow sideways and backwards monocular vision. Ocelli cannot focus or see in colour, but are very good at detecting differences in light intensity. A spider's main sensory apparatus is the system of sensory bristles (setae) covering its body.