orb weaver spider in her web, showing the stabilimenta....

from http://animaldiversity.ummz.umich.edu/accounts/argiope/a._aurantia$narrative.html:

The function of web stabilimenta is controversial. At least 78 species of spiders add these structures to their webs, originally named "stabilimenta" because they were thought to provide structural stability. One study of Argiope spiders supports the idea that these bright white structures attract flying insects (Tso 1998). Contrary to this "prey attraction hypothesis," hungry spiders build fewer or smaller stabilimenta, and webs with stabilimenta capture fewer prey (Blackledge 1998, Blackledge and Wenzel 1999). A competing hypothesis is that the highly visible threads prevent birds from flying through and destroying the webs. Spiders of another species, Octonoba sybotides , vary their stabilimenta in order to control thread tension. Different tensions allow a spider to detect prey of different sizes. However, this mechanical hypothesis doesn't explain why only diurnal spiders use stabilimenta. (Milius 2000).