A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most Vertebrata. In this sense "simple eye" is distinct from a multi-lensed "compound eye", and is not necessarily at all simple in the usual sense of the word. The terminology is not rigid, and must be interpreted in proper context; for example, the eyes of humans and of other large animals such as most Cephalopoda, are camera eyes and in some usages are classed as "simple" because a single lens collects and focuses light onto the retina or film. By other criteria, the presence of a complex retina distinguishes the vertebrate camera eye from the simple stemma or ommatidium. Additionally not all ocelli and ommatidia of invertebrates have simple photoreceptors; many, including the ommatidia of most insects and the central eyes of Solifugae have various forms of retinula, and the Salticidae and some other predatory spiders with apparently simple eyes, emulate retinal vision in various ways. Many insects with unambiguously compound eyes consisting of multiple lenses (up to tens of thousands), but achieve an effect similar to that of a camera eye in that each ommatidium lens focuses light onto a number of neighbouring retinulae.