Jumping spider's head. Coloured environmental scanning electron micrograph (ESEM) of simple eyes (ocelli) on the head of a jumping spider (family: Salticidae). Its mouthparts and jaws are also partially seen. Spiders have eight eyes (ocelli) for vision and sensitivity to light levels. A jumping spider has four eyes on the front of the head for binocular vision. The other eyes 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.