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Basic unit in the eye of cockroach or insect is
A)Retina
B)Rhabdome
C)Corneal facet
D)Ommatidium

Answer
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Hint: The visual organ in the arthropods is called a compound eye. It comprises small photoreceptor units which have cornea, lens and photoreceptor cells which help in distinguishing brightness and color.

Complete answer:
To solve this question, we must know about the structural and functional unit of the eye of arthropods.

The eye of arthropods is called the compound eye. The vision by the compound eye is called mosaic vision. It is composed of several units called ommatidium. It is present in the central part of the compound eye. Ommatidia are composed of clusters of photoreceptor cells surrounded by support cells and pigment cells. The pigment cells prevent the light from seeping into the other units. The lens of ommatidium focuses light on some light sensitive cells without any adjustments.
Retina- It is the photoreceptor unit in the simple eye. It comprises photoreceptor pigments.
Rhabdome- It is the light sensitive receptor present in the eye of arthropods.
Corneal facet- it is a small flat depression on the outer surface of the cornea. It is formed due to the loss of the tissues in the cornea.

Hence, the correct answer is option (D)

Note: The compound eye of cockroach is composed of around 2000 ommatidia which are separated from each other by a pigment sheath. The cockroaches are unable to see in red light but they show excellent vision in green light.