om·ma·tid·i·um 
(ŏm′ə-tĭdē-əm)
(ŏm′ə-tĭdē-əm)n. pl. om·ma·tid·i·a (-ē-ə)
One of the optical units, consisting of photoreceptors and usually one or more lenses, that make up a compound eye of an insect or a crustacean.
[New Latin, diminutive of Greek omma, ommat-, eye; see okw- in the Appendix of Indo-European roots.]
om′ma·tidi·al (-ē-əl) adj.