ocellus, ocelli, (anocellate)

From Latin, ocellus, a little eye.

ocellus noun, plural ocelli, adjective ocellate

1. Simple eyes, small extra eyes, usually situated on the top of the head. The cuticle covering the eye is thickened like a lens. Below the lens there is a layer of transparent cells, continuous with the adjacent epidermal cells. Most insects with complete metamorphosis (holometabolous) have three ocelli on top of the head, arranged in a triangle. In some groups, such as most lepidoptera, ocelli are absent (a condition called anocellate). In other groups, or only one or two are present. The presence or number of ocelli is helpful in differentiating some groups, such as the families of Megaloptera.

Larval forms of holometabolous insects usually have simple eyes on the sides of the head corresponding to the location of the compound eyes in the adult, and these are not are not structurally homologous with ocelli. Common entomological usage (1) is to call these "lateral ocelli", but a more specific term is stemmata (singular stemma), since they are not homologous with ocelli (2).

Diagrams and photos showing ocelli:

Photos of lateral ocelli (also called stemmata) of larval insects:

Photos showing the anocellate (lacking ocelli) condition:

2. An eye-like spot of color, consisting of annuli of different colors, enclosing a central spot or pupil, as present on the wings of some lepidoptera. Photos of the eye-spot type of ocelli: