this is an old question. People have been thinking about it since a long time ago, but the more definite answer was obtained only a few years ago.

in 2012, some researchers photographed the behavior details of spiders during web weaving, and analyzed the three magic weapons of spiders' anti-sticking:

① is first of all the bristles on the spiders' feet as dense as brushes. These structures reduce the contact area between the spider and the sticky material in the spider web.

② spiders pay attention to their movements carefully, which also helps keep its contact area with sticky substances to a minimum. There has been speculation in the past that spiders rely on sticky parts that do not touch the web at all, but video observations show that this is not the case. However, when in contact with sticky parts, spiders will still be careful of their "feet" and "feet" movements.

③ spiders also have an "anti-adhesive coating" on their feet. To prove the role of surface anti-adhesive substances, the researchers also cleaned the feet of spiders with water and organic solvents. It turned out that washing off the anti-sticking material did make the spider's feet and silk stick more tightly.

finally, put a picture recorded in the study. Here you can see the legs of Nephila clavipes detached from the sticky silk, and you can clearly see the hairy structure on it.

[do you need an early warning here? ]

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[it's actually quite cute. ]

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the original paper is here, published in a journal whose name I can't pronounce.

R.D. Brice ñ o and W. G. Eberhard. 2012. Spiders avoid sticking to their webs: clever leg movements, branched drip-tip setae, and anti-adhesive surfaces. Naturwissenshaften. DOI 10.1007/s00114-012-0901-9. Published online: 1 March 2012.