Spiders can control the amount of stickiness of their webs by adding tiny dots of what I call "Spiderweb glue." The web's spokes and the central part of the web where the spider stays doesn't have the sticky substance. Insects that get caught in the "glue" become entrapped, when the spider detects a struggle she runs up and wraps them in a blanket of silk. (see next photo)

This spider (Cyclosa spp.) was camouflaged to look like a bird dropping, it is the whitish "blob" in the center! It had its dried food in the middle of the web. I found it on a window at school.