The ommatidium (soft structure beneath the lens-unit of a compound eye) is very simple in both Scorpio and Limulus.
ommatidium -ia: one of the elements of which the compound eye is composed.
The cells of the ommatidium are a good deal larger than the neighbouring common cells of the epidermis.
The ommatidium is from the first segregate and consists of few cells.
Each of these facets is the external window of an eye element or ommatidium.
One of the tiny light-sensitive parts of the compound eye of insects and other arthropods. An ommatidium resembles a single simplified eye. See more at compound eye.