Facettögon

Facettögon, also known as compound eyes, are a type of eye found in many arthropods, including insects and crustaceans. They are composed of numerous individual visual units called ommatidia. Each ommatidium functions as a tiny independent eye, containing its own lens, photoreceptor cells, and pigment cells.

The arrangement of ommatidia in a facettöga is typically hexagonal, allowing for close packing and efficient use of space. The lenses of the ommatidia focus light onto the photoreceptor cells. The brain then integrates the information from each ommatidium to form a mosaic image of the visual field. This mosaic vision is characterized by a low resolution compared to the single lens eyes of vertebrates, but it offers a very wide field of view and excellent motion detection.

There are two main types of facettögon: apposition eyes and superposition eyes. In apposition eyes, each ommatidium is surrounded by dark pigment cells, preventing light from one ommatidium from reaching its neighbors. This results in a bright image with good contrast, especially in bright light. Superposition eyes, on the other hand, have less pigment. Light from adjacent ommatidia can converge on a single photoreceptor cell, leading to a brighter image that is more effective in low light conditions, but with potentially less contrast.

Facettögon are highly sensitive to movement and can detect changes in light intensity across a wide area simultaneously. Their wide field of view allows many animals to detect predators or prey approaching from various directions. The specific structure and number of ommatidia can vary greatly depending on the species and its ecological niche.