Spider Sunday: Featherlegged Orb-weavers

A surprising number of spider families spin the familiar wheel-like webs we see in our yards, gardens, and parks. While most orb webs you see are produced by “ecribellate” spiders that spin glue-studded silk to catch prey, some orbs are spun by “cribellate” spiders that use “hackled,” non-sticky threads to ensnare victims. The most common cribellate orb weavers in North America belong to the genus Uloborus, family Uloboridae. They are known as “featherlegged orb-weavers” for the tuft of hairs midway down each front leg that several species display. They are also called “hackled band orb-weavers.”

What exactly is a cribellate spider? Cribellate spiders possess two anatomical structures that ecribellate spiders do not. Chiefly, the cribellum is a plate-like organ on the underside of the spider’s abdomen just in front of the spinnerets. The cribellum produces a special type of silk that can be manipulated into a tangle of threads that snags prey as effectively as the sticky glue produced by ecribellate spiders. A cribellate spider also has a special row of bristles, called a calamistrum, on the outer surface of the metatarsus of each hind leg. This structure acts as a comb to fluff the cribellate silk into those tangled strands.

The Uloboridae also includes the triangle spiders I wrote about previously. These are very small arachnids. Mature Uloborus are only 3-6 millimeters in body length, males slightly smaller. Five species in the genus occur in North America, collectively distributed across the entire U.S. and southern Canada.

Webs of Uloborus are relatively small, usually horizontal orbs that may sport a stabilimentum (thickened, often zig-zag patterned band of silk). The stabilimentum may take the form of an auxiliary spiral, or a linear band running through the center of the web. The web shown above is one I discovered inside the greenhouse at the Tucson Botanical Gardens where the seasonal “Butterfly Magic” exhibit was in full swing. While the spiders cannot capture the large butterflies in such an exhibit, the silk can still incapacitate the insects and render them incapable of flying or feeding properly. I have also found featherlegged orb weavers at the mouth of culverts, and a variety of other outdoor situations.

Female Uloborus construct small, flattened, papery egg sacs (see image below, with recently hatched offspring). The egg sac is usually near the periphery of the web, and there can be several in a row.

The hatchlings build an orb web immediately after they disperse, but their webs differ greatly from those of more mature individuals. This is because they are born without their trademark attributes: a cribellum and a calamistrum. Consequently, the webs of recently hatched spiderlings have many additional radii (spokes), and the auxiliary spiral is retained at the hub (center). After its second molt, the spiderling is able to spin a “normal” web (Foelix, 2011).

There are competing theories on to whether orb webs evolved independently among cribellate and ecribellate spiders, or if the orb web came about only once. The convergent theory that proposed the orb web evolved twice held sway for many decades, but recent studies of spiders, and the molecular analysis of their silk, has now given weight to the assertion that the orb developed only once in the evolutionary history of spiders (Foelix, 2011).

These are truly remarkable arachnids. Remember that all members of the family Uloboridae lack venom glands, so are unable to quickly subdue prey by biting it. Instead, they kill by constriction. They wrap the victim so thoroughly that the silk binding compresses the softer body parts, and can even break antennae, legs, and other appendages. I suspect the silk may also shrink as it dries, further tightening around the insect until it dies of asphyxiation.

I leave you on that happy note to go seek for yourself the little wonders of the arachnid world around your own home and garden.

Sources: Foelix, Rainer F. 2011. Biology of Spiders (3rd Edition). Oxford: Oxford University Press. 419 pp.
Ubick, D., P. Paquin, P.E. Cushing, and V. Roth (eds). 2005. Spiders of North America: an identification manual. American Arachnological Society. 377 pp.

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.