Spiders are able to spin webs because they can produce silk from structures called spinnerets, which are glands at the base of their abdomens. A spider constructs its web as its spinnerets produce silk, and the kind of web it builds depends on its species.
There are seven different types of spinneret, though no species of spider has all seven. Each kind of spinneret makes silk for a specific function in web construction, such as sticky capture silk or fluffy capture silk. Spider silk is not only very strong but is elastic as well, capable of stretching to many times its length.
Depending on species, spiders spin one of four different kinds of web: orb, sheet, funnel and tangle. Once a spider chooses a habitat, it spins a web that serves as both home and hunting ground. Just as they are programmed genetically to choose the best cross-bracing and supports for their webs, spiders also automatically know what kinds of silks to use during different phases of construction. Accordingly, they block out areas of tough, dry silk so that they can move over the web without sticking as they gather food from the even stronger sticky traps they spin to capture their prey.