I love watching and taking photographs of spider webs. They are a bit like lightning. You never know what you will get. Here is some background on how and why.
A Typical Spider Web
An orb-spinning spider puts its elegant traps together pretty quickly, proceeding easily from step to step according to the instruction manual pre-programmed into its brain.
It walks across the thread, releasing a looser thread below the first one. It attaches this thread on both ends and climbs to its center. The looser strand sags downward, forming a V-shape. The spider lowers itself from this point, to form a Y-shape. This forms the core support structure of the web.
After building all the radius threads, the spider lays more nonstick silk to form an auxiliary spiral, extending from the center of the web to the outer edge of the web. The spider then spirals in on the web, laying out sticky thread and using the auxiliary spiral as a reference. The spider eats up the auxiliary spiral as it lays out the sticky spiral, resulting in a web with non-sticky radius threads, for getting around, and a sticky spiral for catching bugs.
Many species can also distinguish the characteristic vibrations of dangerous insects, such as wasps, from their preferred prey.
When the orb web has deteriorated and is no longer useful, many spider species will destroy it, eating up all the threads so it can recycle the raw silk material. Spiders may leave the heavy bridge thread so that they can easily rebuild the web at a later point.
Not all spiders catch their prey by spinning a web.
Clancy's comment: Being a photographer has helped me to learn amazing things about nature, by viewing things much closer through the lense.