ommatidium is the smallest unit of a rom pound eye in periplaneta.

Explanation:

The smallest unit of a rom pound eye in periplaneta is Ommatidium. A pair of dark kidney shaped compound eyes is present on the dorsolateral sides of the head, one on each side of it. Each and every compound eye consists of about 2,000 functional units known as ommatidia which are optically independent. The outer surface of the compound eye is divided into 2000 hexagonal areas known as facets.