In Texas, it seems that heavy spring rains bring lots (I mean LOTS) of late-summer spiders. For instance, in 2007 a giant spider web popped up at Lake Tawakoni, made from the labor of thousands of Tetragnatha guatemalensis individuals (with possible help from some other species). The running hypothesis for these massive spider outbreaks can be simplified to this: heavy rains cause huge blooms in insect populations, spiders feed on these insect populations, and so, spider populations grow to larger-than-normal sizes. Although orbweavers such as T. guatemalensis do not normally cooperate in web-building (that is, they are not social spiders, per se), they may be more tolerant of others and even share web space when prey populations are large enough (i.e. competition is low). Joe Lapp (or, Spider Joe) carefully documented this outbreak, and you can read more about it (and watch some interesting videos of spider behavior), here on his blog.
I wasn’t living in Texas in 2007, and so have only heard about these occasional massive outbreaks of spiders. Although this is the stuff of most peoples’ nightmares, it is the stuff of an arachnologists’ dreams. So, when I heard that this August (2015), there was another outbreak in Northern Texas, I was very excited to be able to see it, and take a few pictures to share with others. Unfortunately a few days before I could go to the site at Lake Ray Hubbard, officials had sprayed pesticides around the area, in an effort to control mosquito populations. Thus, the webs that I saw were not as pretty and pristine as those shown on the news (see links below), and many of the spiders had died as a result of pesticide exposure. Still I got a few good images and it was really a wonder to see the sheer number of spiders that could all live in one place.
You can read more about the 2015 outbreak (and find some pre-spraying pictures of the webs) by following the links below: