Scanning electron micrograph of the simple eye of a springtail. Springtails are primitive hexapods, once regarded as insects, now classed as Collembola. They evolved 400 million years ago. The picture shows one of a pair of simple eyes situated on top of the head. It consists of five ocelli, about 8 microns in diameter; surface regions comprising a lens, beneath which are photosensitive dye molecules and opsin, a light sensitive protein that conveys nerve impulses to the brain. The ocelli are very sensitive to light, but cannot form an image. The early evolution of compound eyes (as in insects) or "camera" eyes (as in humans) is a matter for conjecture due to the poor fossil record in the pre-Cambrian era. Springtails live in leaf litter or underground, so have little call for acute vision. They are nonetheless very successful animals; suitable habitats may hold 100000 per square metre