Spider's head. Coloured environmental scanning electron micrograph (ESEM) of simple eyes (ocelli, blue) on a spider's head. The spider's legs, mouthparts and jaws are partially seen. The arrangement of eyes differs widely between spiders, but in general they have eight eyes: several large ocelli for vision, and several smaller ones to increase sensitivity to light levels. 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.