Sunday, October 16, 2011

Spider Sunday: Triangle Spiders

The following is a “reprint” of a blog entry for September 10, 2009.

An outing to the Westfield River in Hampshire County, Massachusetts last Sunday, September 6, included a stop at the Knightville Dam, where a unique arachnid awaited my discovery. There, among the goldenrods, asters, and ornamental black locust trees I found two triangle spiders, named for the shape of their web: a triangle that is essentially the sector of an orb web. The reduced size of the snare is just one puzzling feature of these amazing spiders.

Triangle spiders belong to the genus Hyptiotes (pronounced Hip-tee-OH-teez) in the obscure family Uloboridae. They are part of a larger group of arachnids called “cribellate spiders.” Cribellate spiders all share one feature in common: an extra spinning organ called a cribellum, located adjacent to the normal group of spinnerets. The cribellum issues a special type of silk that the spider literally “fluffs up” using a comb-like organ called a calamistrum, located on each hind leg.

Perhaps even more amazing than the “accessories” that uloborids have is what they lack. These are the only spiders in North America that do not have venom glands. That’s right, they are non-venomous spiders. So, you ask, how then do they subdue their prey? That is a great question for which I have not the foggiest answer. They probably do an extra-good job of wrapping their prey in silk, but not just any silk.

The cribellate silk threads in the part of the web designed to trap insects is not sticky like you would expect. Instead, it is tangled, and this is apparently just as effective as little droplets of glue.

Once it has erected its snare, the triangle spider sits on the thread near the tip of the twig or grass stem to which the apex of the triangle is secured. Depending on which book or article you believe, the spider either bridges a gap in this anchor thread, or simply perches there and reels in the slack line to render it taut. When a prey insect impacts the web, the spider then instantly releases the anchor thread, causing the web to rebound, further entangling the prey.

This feat of engineering and strength is performed by a very small animal. Even an adult female Hyptiotes is only 3-4 millimeters long. Males are 2-3 millimeters at maturity. Simply spotting one of these spiders is cause for self-congratulations for any naturalist.

There are four species of Hyptiotes in North America, three of which are chiefly western in their geographic distribution. The one shown in the image here is Hyptiotes cavatus, the sole eastern species.

Keep a sharp eye out for triangle spiders. The webs are mostly built about waist-high in weeds, on bridges and other structures, and twigs of trees and shrubs.

1 comment:

  1. When I have observed the related Uloborus subduing prey, they spent a really long time wrapping it, like, 10 minutes. They do this even with small, innocuous prey such as pomace flies, completely obscuring the little critter in glossy swaddling. Smallness and innocuous may characterize all their prey. But eventually, no doubt, the injection of digestive enzymes does some serious damage to the immobilized prey.
    Just wondering -- Do we know if they use strands exuded through the cribellum for prey wrapping, or is it only used for the tangley capture threads of the webs? By the way, watching Uloborus construct the tangled spiral of their orbs is only slightly more fun than watching paint dry -- Each little tangled span between spokes is drawn out very slowly through the cribellum, then "ratted' like a 1960s hair style. Building the whole spiral takes hours!