The spinning of silken webs affords spiders an enormous lifestyle advantage over many other creatures................ In fact spiders are the largest group of animals that are exclusively predators........... Their numbers alone speak of success.............. Calculations of spider populations in various natural environments range from eleven thousand to almost three million per acre, depending on the type of habitat.........The predation strategy of spiders can be summed up as trapping insects in webs, delivering a poisonous bite, tying them up, and then consuming them. ..................The combination of web and venom allows spiders to overpower prey, often several times their size, without life-threatening combat; or, for that matter, an exhaustive output of energy................. While venom has evolved several times in the animal kingdom (as in snakes, centipedes, and scorpions), nowhere else is silk used to capture prey

A Tale of Silk and Venom

by Lilian Shen | northernwoodlands.com

 

A spider, fat from consuming a summer’s worth of insects, hangs suspended outside my window in the corner of an orb web bigger than a dinner plate. Its legs grasp the silken strands like a puppeteer’s fingers controlling the strings. For many, the beautifully symmetrical orb web is one familiar sign of a spider’s presence; less welcome are the untidy cobwebs festooning indoor corners.

The spinning of silken webs affords spiders an enormous lifestyle advantage; in fact spiders are the largest group of animals that are exclusively predators – living by catching other creatures. Their numbers alone speak of success. Calculations of spider populations in various natural environments range from eleven thousand to almost three million per acre, depending on the type of habitat.

The predation strategy of spiders can be summed up as trapping insects in webs, delivering a poisonous bite, tying them up, and then consuming them. The combination of web and venom allows spiders to overpower prey, often several times their size, without life-threatening combat; or, for that matter, an exhaustive output of energy. While venom has evolved several times in the animal kingdom (as in snakes, centipedes, and scorpions), nowhere else is silk used to capture prey.

Spiders manufacture in glands within their bodies a liquid that turns irreversibly to silk when extruded. How silk forms is a mystery that has not yielded to studies by scientists hoping to replicate and exploit this phenomenon. It’s known that spider silk is composed of proteins, and that some of the protein molecules bond tightly to each other when silk is extruded as a thread. These core proteins are embedded in another, amorphous type of protein with rubbery qualities. In his book, The Biology of Spiders, Rainier Foelix summarizes the properties of silk as, “stronger than bone or tendon, and about half as strong as the highest quality steel of the same thickness.”

Spiders make different kinds of silk to support their lifestyles. Primitive spiders make but one type of silk; wandering spiders make four types, and orb web spiders, the most highly developed, make seven to eight types. Each type of silk comes from its own gland, or spinneret. If you wonder why spiders stick to your clothes and can’t be shaken off, it’s because they trail a silken drag line from one type of gland and glue it down periodically with sticky material from a second gland.

Silk for swathing the unfortunate prey is from yet another gland type, and this is distinct from silk for egg cocoons. In addition, orb web spiders have glands that make the structural spokes of their webs plus others for the sticky spiral part. The sticky strand is actually the product of two glands working simultaneously, one to make silk, the other, glue.

Spider capturing a bee

An orb web in place, carefully tailored to take advantage of surrounding support, is a marvel of engineering. Orb weaver spiders have tiny eyes and are thought to perceive their surroundings primarily by touch, through a myriad of sensory hairs on their legs and bodies. They first create the web’s outer framework, using the drag line to swing themselves from one support to the next, often aided by breezes. They add the radial spokes and then make a temporary spiral to support their weight and act as a guide while they lay down the all-important sticky spiral.

What’s more, they often tear down and rebuild their webs daily, thus keeping them free of dust and debris. Spiders don’t waste the old web; they methodically eat it. Practically all the old web is recycled into new, fluid web within the spinnerets in the remarkably short time of thirty minutes. It is a perpetual mystery that spiders don’t get stuck in their own webs. Apparently these near-blind predators are able to step only on the non-gluey radial spokes and not the sticky spiral thread.

Other web designs exist, for instance sheets and funnels, and experts can identify spiders down to the species by their web structure. A few aberrant spiders have done away with webs. The keen-sighted wolf and jumping spiders chase their prey down; crab spiders camouflage themselves in flowers and kill unsuspecting pollinators. However, even web-less spiders use the universal drag line to break their falls and silken cocoons to protect their eggs. And baby spiders find transport to new homes by extruding long silken lines that catch the breeze and carry the spiders aloft. Spiders have been retrieved from thousands of feet up in the air.

Little wonder that spiders enjoy a global distribution and began colonizing Mt. St. Helens a few months after the slopes cooled. These relatively weak and vulnerable creatures have conquered all terrestrial habitats, thanks to the marvel of spider silk.