Ommatidia pl. : of Ommatidium

Example:
The repeating units of a compound eye are called?

Taxonomy
ommatidium, ommatidia, pigment cells, free encyclopedia, elements, the eye, cells, the brain, pigment cell, transparent cornea, cross section, species, compound eye, support cells, picture element, search, Unsourced, horsefly.

Defination from the web:
ommatidia in the eye depends upon the type of insect and ranges from just a handful in the primitive Archaeognatha and Thysanura to ... this arrangement is that it increases the number of picture elements by a factor of seven, without increasing the number of ommatidia. ...