To give them their proper name, stabilimenta are quite literally web decorations. Some spiders, once their webs are complete, choose to further enhance them. While there are competing theories as to why this is done, the jury is still out for the final verdict. They do, however, look fantastic.
In the early nineteen fifties the children’s author EB White was struggling to come up with ideas for his second novel. One day he noticed the additional decorations on the web of a Banded Garden Spider – much like the one above. It was from this natural inspiration that he would come up with the idea of a writing spider and would go on to write one of the world’s most cherished children’s books, Charlotte’s Web. Although anecdotal this story serves as a fine introduction to this most peculiar of insect habits.
http://scienceray.com/biology/zoology/the-spiders-that-decorate-their-own-webs/
From the Upcoming ueue, submitted by taliesyn30.
I hate to be a pedant, but spiders aren't insects, they are arachnids...