"The whole set of eye-units (ommatidia) of Musca vomitoria are enclosed in a chitinous capsule. [Note] The term ‘ommateum’ was introduced by Lankester in his memoir on the eyes of Scorpions to signify the entire soft parts of the non-segregate (unicorneal) eye of Arachnida and Hexapoda.‥ The similar term ‘ommatidium’ is introduced in this paper to signify the units consisting each of a retinula and a vitrella, together with their sheath of pigment cells, into which the ommateum of the multi-corneal (polymeniscous) eye of Arthropods, is segregated" (OED).