Arthropod Eye - Eyes and Functions

Eyes and Functions

Most copepods have a single compound eye in the middle of their head, but this copepod, of the family Corycaeus, possesses two large cuticular lenses paired to form a telescope. Like most spiders, this wolf spider has eight simple eyes, two main eyes at the front and six smaller secondary eyes. The main eyes form images. The secondary eyes detect peripheral movement, and have a reflective tapetum lucidum which improves night vision. Holochroal eye of an Early Devonian trilobite Paralejurus sp. Schizochroal eye of a Devonian trilobite from the family Calmoniidae Horseshoe crabs have two primary compound eyes and seven secondary simple eyes. Two of the secondary eyes are on the underside.

Most arthropods have at least one of two types of eye: lateral compound eyes, and smaller median ocelli, which are simple eyes. When both are present, the two eye types are used in concert because each has its own advantage. Flying insects can remain level with either type of eye surgically removed, but the two types combine to give better performance. Ocelli can detect lower light levels, and have a faster response time, while compound eyes are better at detecting edges and are capable of forming images.

Further information: Compound eye Further information: Ocellus