ommatidium

Japanese: ommatidium

...A general term for pigment compounds derived from tryptophan, such as xanthommatine, an insect eye pigment produced from L-tryptophan via kynurenine and 3-hydroxykynurenine. The name is derived from the ommatidia, where this substance is most prominent. Ommatidia are the individual small eyes in the compound eyes of arthropods, and ommochromes are contained in the photoreceptor units located on the posterior surface of the cornea.

From [Compound Eyes]

…The pair of lateral eyes found in arthropods such as insects and crustaceans is made up of a number of ommatidium, called compound eyes. Each ommatidium has a lens, a small number (8-9) of photoreceptor cells underneath, and pigment cells between adjacent ommatidae. …

*Some of the terminology explanations that mention "ommatidium" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…L‐トリプトファンからキヌレニン，3‐ヒドロキシキヌレニンを経て生成する昆虫の眼色素キサントマチンxanthommatineに代表されるトリプトファン由来の色素化合物の総称。この名称は，最も顕著にこの物質が存在している個眼ommatidiumにちなんでいる。個眼は節足動物の複眼にある個々の小眼で，オモクロムはその角膜の後面にある光受容の単位に含まれている。…