Anyway, I'd like to introduce you now to a common kind of spider, but one which you might know nothing about. It's called a Cribellate Orb Weaver. We've already examined other orb weavers such as Neoscona crucifera, but cribellate orb weavers are quite different. For one thing, they have no venom. This means they must capture their prey using silk, which these particular spiders create in their cribellum. The cribellum differs from the spinnerets in that it produces very fine fibers which are combed out by the spider's calamistrum into a woolly silk, from which prey cannot escape. The spider doesn't need any other adhesive, but simply encases the prey in this silk substance and eats the contents. Second, they build horizontal webs, not vertical ones, with a sticky snare in the center. Third, and probably most obviously, they have two very long forelegs and prefer to sit in their webs upside down, with their body extended horizontally, kind of like a twig. Unlike Neoscona crucifera, which hunts alone, you can find several cribellates together in a tier of webs. Of course, that doesn't mean there aren't any territorial battles.
Though unconventional, cribellate orb weavers are indeed true spiders (Araneomorphae). They belong to the Entelegynes section of this infraorder, and form the family Uloboridae. The specimens I photographed are members of the genus Uloborus, and the species is most likely glomosus. Most cribellate orb weavers are found outside the US. There are only 4 species of Uloborus in this country.
BugGuide.Net provides a good description of Uloborus: "A small species with suboval carapace [top of the cephalothorax]. Posterior eye row strongly recurved. Legs of female usually with brushes of hairs on front tibiae. Abdomen variable, usually well elevated in front third, with a pair of humps at highest point." You can see these features in the photos below.
Specimen #1, feeding:
Specimen #1, after meal:
Specimen #2, in a nearby web:
References:
http://ednieuw.home.xs4all.nl/Spiders/InfoNed/webthread.html (web & silk)
http://jeb.biologists.org/content/209/16/3131.full.pdf (mechanical properties of cribellate silk)
http://ednieuw.home.xs4all.nl/australian/uloboridae/uloboridae.html (good info on Uloboridae, with macro pictures of each genus)
http://bugguide.net/node/view/1956 (Uloboridae)
http://bugguide.net/node/view/1995 (Uloborus)
http://bugguide.net/node/view/289073 (Uloborus glomosus)
I have a spider that looks identical to these spiders and its web looks the same, but it is vertical. I wonder if it just built it that way since the spot its in.
ReplyDeleteHm...yes, the web should be almost horizontal. I don't think I've seen one that's vertical. Could you send me a picture of your spider?
DeleteIt is in between posts in the railing on my porch and it is vertical and horizontal. I creates almost like a cube. I would love too, but I only have my phone for pictures right now and it doesn't take clear enough pictures of "Sally" to see anything more than a black dot. Sorry :/
ReplyDeleteIf it's a cribellate orbweaver, the web should be on a single plane: just one single web. From what you've said about the cube, you might have another kind of orbweaver; there are several species which build an orb and a secondary, messier web to trap insects and make them fall into the orb. Others build a separate web in which to wait for prey. When you look at those webs, it's hard at first to recognize the orb. Since the pictures didn't turn out, you could try to describe the spider to me in more detail. I might be able to identify it that way. Whatever you prefer :)
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