Spider silk is not a single, unique material--different species produce various kinds of silk. Some possess as many as seven distinct kinds of glands, each of which produces a different silk.
Why so many kinds of silk? Each kind plays particular roles. All spiders make so-called dragline silk that functions in part as a lifeline, enabling the creatures to hang from ceilings. And it serves as a constant connection to the web, facilitating quick escapes from danger. Dragline silk also forms the radial spokes of the web; bridgeline silk is the first strand, by which the web hangs from its support; yet another silk forms the great spiral.
The different silks have unique physical properties such as strength, toughness and elasticity, but all are very strong compared to other natural and synthetic materials. Dragline silk combines toughness and strength to an extraordinary degree. A dragline strand is several times stronger than steel, on a weight-for-weight basis, but a spider's dragline is only about one-tenth the diameter of a human hair. The movie Spider-Man drastically underestimates the strength of silk¿real dragline silk would not need to be nearly as thick as the strands deployed by our web-swinging hero in the movie.
Dragline silk is a composite material comprised of two different proteins, each containing three types of regions with distinct properties. One of these forms an amorphous (noncrystalline) matrix that is stretchable, giving the silk elasticity. When an insect strikes the web, the stretching of the matrix enables the web to absorb the kinetic energy of the insect¿s flight. Embedded in the amorphous portions of both proteins are two kinds of crystalline regions that toughen the silk. Although both kinds of crystalline regions are tightly pleated and resist stretching, one of them is rigid. It is thought that the pleats of the less rigid crystalline regions not only fit into the pleats in the rigid crystals but that they also interact with the amorphous areas in the proteins, thus anchoring the rigid crystals to the matrix. The resulting composite is strong, tough, and yet elastic.
M. Dawn of Brandon, Miss., asked the related question, "Why doesn¿t a spider get stuck on its own web?"
Over the years, three explanations for this phenomenon have surfaced . The first invokes an oil, secreted by the spider, that serves as an anti-stick agent. The problem with this hypothesis is that such an oil has yet to be discovered.
The second scenario is based on the diversity of silks. Many webs include strands made of silks that are much less sticky than the others are. The non-sticky strands appear in the hub of the web, the radial spokes and the threads by which the web hangs from plants or other supports. Some researchers have thus posited that the arachnids use only these strands when navigating their webs. If you watch them in action, however, you see will see that although they do seem to prefer the non-sticky strands, the spiders are able to move around freely, touching many of the strands, including the very sticky ones that spiral out from the hub.
The third explanation appears to solve the sticky-strand problem. In short, the legs of at least some spiders feature a disengaging mechanism that enables the arachnid to detach itself instantly from a sticky strand. This mechanism involves a clever anatomical adaptation. Each leg ends in a pair of "walking claws" that grasp vegetation, among other functions, but a third claw collaborates with associated spiny, elastic hairs to detach the leg from a sticky web strand. This third claw grasps the strand, pulls it against the elastic hairs, and pulls them further, cocking the mechanism. When the claw relaxes, the hairs rebound vigorously, throwing the strand away and springing the leg free.
Police, the military, physicians, and other groups are eager to obtain large quantities of dragline silk, which can be woven or compacted to make bulletproof clothing, replacement ligaments, medical sutures, fishing line, ropes for rock climbers, tethers to snag planes landing on aircraft carriers and myriad other products. It is impracticable to harvest sufficient quantities of silk from spiders due to their territorial nature, so biotechnologists have turned to other sources. The Canadian company Nexia has demonstrated that goats and cows can be genetically engineered so as to produce dragline silk in their milk. Using a clone of such goats, Nexia aims to produce a modified dragline silk, which they call BioSteel, to meet the many demands.
WHY A GARDEN SPIDER
DOES NOT GET STUCK IN ITS OWN WEB
Ben Prins, Netherlands
(Note: this article is best viewed at a screen resolution of 800x600 or higher.)
Of the many spiders living in our neighbourhood the garden spider (Araneus diadematus) is one of the best known. Especially in autumn the large and heavy females catch the eye, their abdomen swollen with eggs. Hanging head down in the centre of their magnificent orb webs they are waiting motionless for insects getting caught by the mass of sticky threads. As soon as that happens, the spider orients its front into the direction of the disturbance and moves without hesitation along one of the threads radiating away from the hub, to rapidly secure its prey .
This action of the spider is not risky in itself, since all the silk used for making the hub, the radiating spokes and the strong threads between which the web is hanging, is not sticky. Only the silk produced to construct the spiral thread connecting the radii, is covered with a strong glue. Yet, when we observe the spider walking at high speed along one of the spokes, it regularly touches the sticky spiral. And once having reached its prey, it does not hesitate to seize the threads of the sticky spiral with its legs, when approaching the victim as close as possible in order to immobilise it by wrapping it in masses of silk and giving it a fatal, venomous bite. Evidently the garden spider is not afraid of getting stuck in its own web and walks easily along the non-sticky as well as along the sticky threads. How does it manage to do so?
To answer this question, we are forced to catch a garden spider and to investigate the tip of one of its legs at high magnification. What we see at first glance are the two dark, serrated walking claws (see figure A). They are used for getting a firm grip on the relatively smooth surface of the branches and leaves, between which the spider constructs its web, and for moving over the soil. In front of them there is a somewhat smaller, strongly hooked third claw, that is surrounded by a number of bent hairs. The striking difference between these hairs and the other hairs elsewhere on the spiders legs is not only that they are conspicuously curved, but also that they are provided of quite a number of slender spines. These hairs and the third claw play together a crucial role in the ability of orb spiders (Family Argiopidae) to move around in their own webs.
TIP OF A LEG OF THE GARDEN SPIDER
We shall trace what happens with help of figures B, C and D. When the spider places the tip of one of its legs against a thread (see B) the hooked third claw is tilted backwards, its pointed end directing obliquely up. The thread, which always consists of two or more components of at most 0.005 mm across ( 5 micrometres), is pushed against the elastic, spiny hairs. Then the third claw rotates forward (see C), the hook seizes the thread, and forces the thread and the elastic hairs backwards. The leg is holding the thread now in a firm grip with only a minimal surface area of the thread in touch with the spines on the hairs and with the inner margin of the third claw. To loosen the thread the hook of the third claw is simply lifted and the rebounding elastic hairs, retaining their original upright position, propel the thread away from the tip of the leg (see D). In this way even a sticky thread becomes easily detached.
TIP OF A LEG OF THE GARDEN SPIDER
Although this mechanical device alone may be sufficient for a garden spider to move freely through its web without running the risk of becoming entangled, yet another provision may play a role. In a number of popular books it is stated that an oily secretion covers the legs, preventing the spider from being glued to the sticky threads. However plausible this possibility may look, not a single scientific publication could be traced as yet, which supported this statement. For the moment, therefore, the only answer to the question raised in the title, is the very special way, in which the tip of the legs of any web weaving spider is built. But an oily cover of the legs may represent an (unproven?) additional necessity for orb spiders, the only spiders producing a spiral thread covered with a glue, to avoid being glued to their own web.
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