What are ommatidia?

Hint: Ommatidia is commonly found in insects, crustaceans and millipedes. It is most commonly used in the case of cockroaches in their sense organs which present on its head and help in sensing the environment.

Complete answer:
Ommatidia is also known as compound eyes which are made up of a large number of visual elements. It is formed by the clusters of photoreceptor cells which have pigmented cells. Externally, ommatidia consists of a transparent covering known as the cornea. Each part of ommatidia makes an image which together forms a complete image inside the brain. Ommatidia form a hexagonal shape that gets attached and forms the complete eye. In the case of ommatidia, pseudocones are formed which help to make focus on light. Each ommatidium has a minimum of one whereas a maximum of 9 photoreceptors, which is also known as primary or secondary pigment cells.

Additional Information:
In the ommatidia of flies, rhabdoms are present which get separated into seven individual rhabdomeres. The rhabdomeres lie adjacent to the ommatidia which have the same view as between that of ommatidia. Rhabdomeres help to increase the sensitivity of ommatidia seven times greater than the previous one. The six pigment cells in ommatidia help to concentrate the light pigment and it also helps to prevent the light pigment from diverting into other angles. The pigment cells also help to enhance the direction of light in low light condition or lower resolution.

Note:
Different organisms have a different number of ommatidia in their eyes like Glyptonotus antarcticus consist of 5 ommatidia whereas nearly 30,000 ommatidia are present in dragonflies. Dragonflies have the biggest arthropods.