Spider silk is a simple silk.

There is a lot of silk pulp in the spider's stomach, and there are small holes in its tail. When making a web, the spider spits out silk pulp. Silk pulp condenses into sticky silk when it meets air, and the net formed by it, no matter what flying insects, can't run away once it hits.

Spiders often secrete a layer of oil on their bodies and feet, which is not sticky. But generally flying insects don't have this layer of oil, so spider webs can firmly stick to flying insects but not spiders.

Spider silk is famous for its powerful physical properties:

The strength (tension per unit cross-sectional area) of spider silk is greater than that of high-quality steel, and it is equivalent to many artificial aromatic polyamide fibers, such as Teveron fiber or Kevela fiber. More importantly, the mass of spider silk is extremely small: the mass of spider silk that can circle the earth is less than 500 grams.