Amazing Arachnids: Orb Weavers

Adapted from pages 184-186 of Amazing Arachnids:

A delight to the eye and an engineering marvel, the orb web epitomizes the stereotypical spider web. It is built in a vertical plane, with strong, nonsticky silk radiating out from a central hub like the spokes of a wheel, supporting a spiral of evenly spaced sticky silk threads. A gap in the sticky silk near the hub allows the orb weaver to rapidly climb from one side of the web to the other, depending on which side of the web a flying insect has blundered into. Some orb weavers wait in the center of the web, legs stretched out in contact with the radiating silk lines that convey the vibrations of a struggling insect. Others build a little retreat at one side of the web, maintaining contact with the radiating lines via a signal thread leading to the hub. Lying in wait in the retreat, the spider rests with one leg touching the signal line. At the first indication that an insect has been caught, the spider moves into the web and tugs at the radial lines, testing to see the general location of the prey. It then uses the nonsticky radial lines as a quick pathway leading to the insect. Once the prey is reached, the spider uses large amounts of silk to wrap and immobilize it prior to settling in for the meal.

Many orb weavers build a fresh web every night and eat the silk by the next morning. Experiments with radioactive labeling have shown that spiders are the ultimate recyclers; up to 90 percent of the old silk is recycled into the new web, and such ingestion and reuse of the silk protein can occur in as little as 30 minutes. The spiral silk of the orb weavers owes its stickiness to the addition of little beads of viscous glue along its length, like the beads of a necklace. Neither the radial threads nor the hub threads have this glue, allowing the spider easy and rapid access to all parts of its web.

An orb weaver spider

Surreal in color and form, the spiny orb weaver, Gasteracantha cancriformis, builds its web in trees and other tall
vegetation. This genus occurs primarily in the tropics; however, this particular species is also found across the
southernmost states in North America.

Some orb weavers build a web that remains in place for more than one day. Among these diurnal spiders are some that incorporate a special structure into the web, called the stabilamentum. The stabilamentum is composed of a thicker kind of silk, frequently appearing as a conspicuous white area in the web. It may look like a lace doily, or like one or more heavy zigzags in the web. Another type of stabilamentum consists of a line of silk above and below the resting spot in the hub of the web. The empty husks of insect prey are attached to this line, forming irregular clumps of detritus. Sitting motionless in the open spot in the middle of this detritus, the orb weaver Cyclosa appears to be just one more clump of debris in the stabilamentum. Camouflage protects the spider against predation by birds. Yet a different type of protection from birds may be derived from the presence of stabilamenta.

Orb weavers are more flexible in their ability to react to different circumstances than one might imagine. They build larger webs when they are hungry or if they are in areas of low prey availability than when they are well fed or in areas of high prey availability. Both web design and the timing of its construction are synchronized with the type of prey and its availability, requiring the adjustment of the spider’s circadian rhythm. In addition, orb weavers modify their approach to different types of prey in the web depending on whether the prey is potentially dangerous or not. They seem to know what kind of prey has been captured (perhaps based on the vibrations transmitted from its struggles) even before the spider physically makes contact with the prey. Some undesirable prey, such as stinging insects, are deliberately cut loose and released from the web. Other prey, like stink bugs, may be carefully wrapped so as to avoid eliciting a release of defensive chemicals until the killing bite can be administered in safety.

Amazing Arachnids coverAmazing Arachnids
By Jillian Cowles

The American Southwest is home to an extraordinary diversity of arachnids, from spitting spiders that squirt silk over their prey to scorpions that court one another with kissing and dancing. Amazing Arachnids presents these enigmatic creatures as you have never seen them before. Featuring a wealth of color photos of more than 300 different kinds of arachnids from eleven taxonomic orders–both rare and common species—this stunningly illustrated book reveals the secret lives of arachnids in breathtaking detail, including never-before-seen images of their underground behavior.

Amazing Arachnids covers all aspects of arachnid biology, such as anatomy, sociality, mimicry, camouflage, and venoms. You will meet bolas spiders that lure their victims with fake moth pheromones, fishing spiders that woo their mates with silk-wrapped gifts, chivalrous cellar spiders, tiny mites, and massive tarantulas, as well as many others. Along the way, you will learn why arachnids are living fossils in some respects and nimble opportunists in others, and how natural selection has perfected their sensory structures, defense mechanisms, reproductive strategies, and hunting methods.

  • Covers more than 300 different kinds of arachnids, including ones new to science
  • Features more than 750 stunning color photos
  • Describes every aspect of arachnid biology, from physiology to biogeography
  • Illustrates courtship and mating, birth, maternal care, hunting, and defense
  • Includes first-ever photos of the underground lives of schizomids and vinegaroons
  • Provides the first organized guide to macroscopic mites, including photos of living mites for easy reference
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