I think they are just old spider webs aren't they? And if that is the case they just come from spider's butts. Right?

Not thinking enough. Actually I think it's old spider webs that then collect dust so they are very visible.

Cobwebs come from the Cob Spider. This spider is a fairly large spider with thick, snow-white fur. Its double, sometimes triple and quadruple, fanged bite is exceedingly painful though not fatal, and is largely avoided by intelligent life. Cobs can live up to 50 years! Cob Spiders can grow over two palm spans in length, and one palm span in height. Cob spiders usually hide in dark cool places such as basements and under frigerators and other appliances. They are nocturnal and move very quickly (for their size), this causes them to be very rarely seen by humans. They wander from their daylight hiding places at night in search for food. They enjoy feeding on dead skin cells which makes a bedroom mattress one of their favorite feeding places. Sometimes at night they can crawl under the covers of a sleeping person and peel the dead skin from his or her legs, feet and toes. Sometimes you may feel one pulling on your leg... much like I'm pulling your leg right now. LMAO! :P Seriously... The Middle English name for a spider web. Origin of the word is coppeweb, coppe pronounced 'cobbie' being the Middle English word for spider, which was introduced to England by Dutch invaders and originally pronounced 'kab' in the Netherlands. Contrary to popular belief, cobwebs only form when dust collects on wayward pieces of silk produced by spiders or other insects. Popular myth indicates that dust alone is capable of forming a chain, with each piece connecting to the next piece, finally forming intricate lines in corners and across furniture. This belief is erroneous. Hope this helps. :)

It is the result of a Spiders food traps and dust. The spider will live in the web and wrap up what it traps to save for later.