The legs of some spiders detect airborne vibrations of approaching insects thanks to specialized vibration-sensitive hairs, called trichobothria, on certain leg segments.

References

"Spiders can detect vibrations traveling through the air from sources far away. They can do this thanks to specialized vibration-sensitive hairs, called trichobothria, on certain segments of their limbs. These hairs are able to move in any direction, and tell the spider the direction from which an object is approaching and its size. They are so responsive to airborne vibrations that they can even detect those caused by the wings of insects in flight, alerting the spider to the approach of a potential victim as it heads toward the spider's web." (Shuker 2001:36)

Book
The Hidden Powers of Animals: Uncovering the Secrets of NatureJanuary 1, 1970
Dr. Karl P. N. Shuker

Organism
SpidersAraneaeOrder