This spider’s retinas burn out every morning and regrow every night
Gah! The Rufous Net-casting Spider (rufous just means reddish) has six regular spider-eyes and two massively enlarged ones that point forward, like searchlights. This gives it very good aiming ability with its net.
“At night they build a rectangular, postage-stamp-sized web, made with wool-like, entangling silk threads. These little nets are made among low vegetation, usually above a surface across which prey animals are likely to walk (e.g. a broad leaf , a tree trunk or even a house wall). After spinning its web the spider deposits some spots of white faeces on this surface to act as aiming points.
The spider hangs head down from a trapeze of silk, holding the net in its front pairs of legs;and there it waits, its enormous eyes watching for prey movement across the white aiming spots. When an insect passes over the white target spots, the spider opens the stretchy web to two or three times its resting size and lunges it downward over the unsuspecting prey.”
“The Net-casting spiders’ large eyes provide outstanding low-light night vision. Their compound lenses have an F number of 0.58 which means they can concentrate available light more efficiently than a cat (F 0.9) or an owl (F 1.1). The image is focused onto a large, light-receptive retinal membrane (which is destroyed at dawn and renewed again each night).” Source
This incredible night vision, by the way, means they can see stars and galaxies in the night sky that we can’t.
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This would be an awesome feature to give a vampire if you happen to be writing a vampire story. Fits with the mythos, based on a ‘scary’ animal, the star thing is appropriately poignant, and it’s a weakness ( which is important, both heroes and villains need a a weakness) but it's somehow a weakness that’s more upsetting than its strengths.
Alternative: an order of creepy astronomer monks.