What separate functions do a vampire spiders eye play?

The separate and unique function, the eye of a vampire spider play is to act as organ of senses to catch its prey apart from vision.

Explanation:

The vampire spider possesses large eyes with the help of which it detects the presence of mosquitoes, mainly female mosquitoes as they carry blood and jumps upon them to feed on. This spider have very well developed senses,  they not only detect blood laden mosquitoes simply by smell, but also with their eyes and acuity of vision. The pairs of eyes in the front are large and contains about a thousand of receptors. In other words the vampire spider eyes make them the precise predator to be able to finely select their prey, that is, female mosquitoes laden with blood.

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