Why are arthropod eyes called "compound"?
The arthropod eyes are called compound eyes because they are made up of repeating units, the ommatidium, each of which functions as a separate visual receptor.
The compound eye of cockroach are large, black kidney-shaped structure situated dorso laterally on the head. Eye consists of about 2000 similar hexagonal units called ommatidium.
Each ommatidium consists of :
1) a lens that is the front surface and makes up a single facet.
2) a transparent crystalline cone.
3) light sensitive visual cells arranged in a radial pattern, like the sections of an orange.
4) pigment cells that separate the ommatidium from its neighbours.
Light entering the ommatidium parallel to its long axis reaches the visual cells and triggers nerve impulses. Thus each ommatidium contributes information about only one small area in the field of view. The composite of all their responses is a mosaic image - like a pattern of light and dark dots. The compound eye is excellent at detecting motion.