The spider's leg is obviously very hairy. There are several types of sensory hairs. You can identify them by the way they stand in their socket. The large tactile hair above emerges obliquely. They are extremely sensitive to air currents or low frequency air vibrations.
Spiders can sense a flying insect from several centimetres.
Some spiders are able to detect and catch a flying butterfly in mid-air. There are also chemosensitive hairs, the tip of the leg contains a series of these hairs the spider uses to taste. There is also a whole variety of other receptors. With these sensilla the spider senses it's own movements but they also enable it to detect sound. Each slit has a thin membrane in which we can see a small dot.