Eight-Legged Engineers

Orb-Weaver Spiders Are Masterful Builders

Roughly half of all spiders ambush or use snares and traps to capture prey. The rest spin webs to catch their meals. There are many different types of webs — sheet, funnel, cob, mesh, etc. Several families of spiders create orb webs (flat webs with sticky spirals that catch insects).

One of the most common of these families, and the most diverse, is Araneidae. While not every spider in this family spins an orb web, the vast majority do. You are probably familiar with at least one or two species in this family. E.B.White’s “Charlotte” belongs to this family, as does the black and yellow argiope, or garden spider, that is quite common. While they come in all sizes, colors and shapes, members of this family all have eight eyes in two rows, eight legs and three-clawed feet. Identification down to species often requires microscopic inspection of genitalia, so you’re doing well if you can determine the genus of an orb-weaving spider with your naked eye.

Every orb web begins with a single thread, which forms the basis of the rest of the structure. The spider climbs up vegetation — a tree branch or plant stalk — and releases a strand of silk into the wind. If and when the silk gets caught on something, the spider tightens the strand, attaching it to the starting point. It then walks across this thread, releasing a looser thread below the first one. It attaches this thread on both ends and climbs to its center. The looser strand sags downward, forming a V-shape. The spider lowers itself from the bottom of the “V” to form a Y-shape. This is the core structure of the web.

As the spider grips the silk with special serrated claws, a smooth hook and a series of barbed hairs on the end of its legs, it lays more frame threads among various anchor points. Then it starts laying out radius threads from the center of the web to the frames. Thus far, all the silk the spider has used is non-sticky. After building all the radius threads, the spider lays more non-stick silk to form an auxiliary spiral, extending from the center of the web to the outer edge of the web. The spider then retraces its steps on this non-sticky spiral, eating it while simultaneously laying out sticky thread, using the auxiliary spiral as reference. The finished web has non-sticky radius threads, which the spider uses to get to its prey, and a sticky spiral that snares prey.

One feature of the webs of some orb-weavers is a thick, white band of silk through the center of the web, called a stabilimentum. The web of the black and yellow argiope usually includes such a band. There are many theories about its function, including a lure for prey, a marker to warn birds away from the web, and camouflage for the spider when it sits in the center of the web. Recent research suggests that the stabilimentum actually decreases the visibility of the silk to insects, resulting in more food for the spider.

Most orb-weaver webs are vertical, and the spiders usually hang toward the center of the web with their heads downward, monitoring the radius threads for vibrations. Occasionally, a spider can be found hidden off to the side of the web, but with at least one foot in contact with it.

Orb-weaving spiders typically have poor eyesight, as they are not dependent upon their eyes for spotting prey. Their feet, not their eyes, receive information through web vibrations which tell them whether a leaf, a dangerous insect such as a wasp, or an edible insect has been caught. If it’s suitable for eating, the spider rushes out and paralyzes the prey by biting it and then rapidly wraps it in silk. Sometimes the prey is eaten (actually drunk, after enzymes have liquefied the innards) then and there, but sometimes the spider will return at a later time to consume it.

Most orb-weavers are nocturnal, but some are active during the day. Many build a new web every 24 hours. Generally, in the evening, the spider consumes its old web, rests for an hour or so and then spins a new web in the same general location. For this reason, unlike the webs of other spiders, early in the morning orb webs are pristine — without holes or the dried-up exoskeletons of insects that have been caught and devoured by the web spinner.

Mary Holland is the author of several books, both on natural history and for children. She has a natural history blog at www.naturallycuriouswithmaryholland.wordpress.com