Ocellus

Part of speech: noun

Pronunciation: /əʊˈsɛləs/

Definitions

1. A tiny, eye-like feature typically present in certain animals, especially insects, that plays roles such as visual perception or camouflage
2. A miniature ocular structure found in various invertebrates, capable of sensing light changes but lacking the ability to create complex images
3. A small, simple eye or eye-like spot found in some species, particularly among arthropods, serving functions related to light detection or aiding in survival strategies

Etymology: Ocellus is a Latin diminutive, formed from oculus, "eye," with the suffix -ellus, so it means "little eye." Latin speakers used the diminutive in this way for many small things, and the word passed into English as a technical term for the small eye-like marks found on some animals. Its rise is a learned one: English borrowed it directly from Latin, and it belongs to the vocabulary of naturalists describing the anatomy of insects and other creatures, rather than everyday speech. The most striking thing about the word is how little it has to do with eyes in the strict sense. The same basic idea, "a small eye," covers two quite different structures in insects. A simple eye (the ocellus, plural ocelli) is a light-sensitive spot that detects brightness and changes in light, sitting alongside the large compound eyes on the head. Ocelli are also used as the name for the eyespots on butterfly wings and peacock feathers, where they serve to startle or deter predators.