One day this week while sitting next to my trailer I noticed a commotion between the trailers siding and my seat. Inside a rather messy looking spider web three large, long-legged spiders were applying white silk to a captured deerfly - one that earlier I'd been swatting at. I got out my little Golden Nature Guide called "Spiders" and identified my companions to family level, then on the Internet I figured out the species. They were Marbled Cellar Spiders, HOLOCNEMUS PLUCHEI, and you can see one at http://www.ipm.ucdavis.edu/PMG/H/I-AR-HPLU-AD.001.html.
Cellar spiders aren't normal spiders. For one thing, their legs are so long and slender they almost look like harvestmen, or daddy longlegs. But, insects have three body segments, spiders have two, and harvestmen have only one, so harvestmen aren't even considered to be spiders. These creatures next to me were definitely two-sectioned spiders. Sometimes cellar spiders are also called daddy longlegs, but that's confusing two entirely different kinds of critter. You can compare the above spider picture with harvestmen images at http://www.fcps.k12.va.us/StratfordLandingES/Ecology/mpages/harvestman.htm.
With other spiders usually you just see one spider in a web, though sometimes a male and a female share a web, or the female has her babies there. But here were three adult-size spiders apparently cooperating in subduing a fly. One spider would be adding silk, seem to get tired and draw back a little, then another would approach and bite the fly through the silk. They took turns doing things. When I left, all three had their fangs in the fly. I can't recall having ever seen such cooperation among three full-size spiders in a single web.
Google tells me that this is an introduced spider, one brought into California during the 1970s, and since then it's done a good job displacing the once-common Longbodied Cellar Spider, Pholcus phalangioides, which also was an invasive species. So now we're at the point of invasive species pushing aside invasive species...
Anyway, there's bad news and good news about this spider. The bad news is that its venom may be as deadly as any spider venom in the world. The good news is that cellar spiders bear such short fangs that they could never puncture the skin of a finger, so they can be considered harmless to humans.