The Outside Story

The Outside Story: Spider silk

By Rachel Sargent

There is an all-natural material, produced at room temperature, that can be used to build homes, to make protective coverings, to hunt and trap, and even to swing through the air. It’s hypoallergenic, antimicrobial, and waterproof. On a per-weight basis it’s stronger than steel and more elastic than nylon or kevlar.

What is this remarkable material? Spider silk.

If it sounds impossible that a single material can be used for so many purposes, well, in a way it is. Depending on how you want to count them, there are seven or eight kinds of spider silk in the world, and any given spider species may make as many as six different kinds.

All spider silks are made of proteins. They are produced by special glands that connect to spigots on the spider’s spinnerets–the finger-like projections at the end of a spider’s abdomen.

Unlike the superhero Spiderman, spiders cannot actively eject lines of silk; the silk must be pulled from the spinnerets by an external force. Most often, the spider will either attach the end of the silk thread to a surface and walk away, or it will pull a silk thread from its spinnerets using its back legs.

Each kind of spider silk is produced by a different type of gland. The aciniform gland (aciniform means “shaped like a bunch of grapes”) produces aciniform silk. This is one of the two silks that spiders use for the critical job of protecting their eggs. Aciniform silk makes a soft cushion around the eggs, while tough cylindrical silk (from the cylindrical gland) is used to create an impermeable outer barrier. If you ever see a mother wolf spider or nursery web spider carrying an egg sac, that grayish shell is made of cylindrical silk.

The spider silk that is perhaps most impressive to people is dragline silk–it’s the silk that is stronger than steel on a per-weight basis. If you’ve ever startled a spider into dropping away from you by a rappelling thread, that’s the dragline silk. It is also the silk that little spiderlings use to go ballooning.

The beautifully geometric orb webs you find built between fence railings or the tall grass in a field are composed of four different and highly specialized silks. Dragline silk forms a strong frame for the web and super stretchy flagelliform silk makes up the capture spiral. Attachments between the frame and the capture spiral are held together with piriform silk, a kind of silk cement. Finally, the spider adds droplets of glue-like aggregate silk to give the web its remarkable stickiness.

Some spiders can make a different type of “sticky” silk called cribellate silk. They have a special organ called a cribellum in addition to their spinnerets. A cribellum has spigots like a spinneret, but there are thousands of these spigots and they are arranged on flat plates. These spiders comb the silk with a set of special hairs on their legs as they pull it from the cribellum, which gives the thread a woolly texture. That fuzziness makes the silk stick to spikey insect body parts, just like a wool sweater sticks to burs.

From dragline silk to cribellate silk fuzz, different spider species use these same silks to make different types of webs. My favorite web type is the “gum-footed” web built by many cobweb spiders, a group that includes the infamous black widows and the ubiquitous common house spiders. The spider builds a three dimensional web in a quiet cranny, such as an attic window. The main web is the spider’s retreat. The spider then runs taut dragline threads down to the ground or out to the side, like rubber bands under tension. The spider adds blobs of the glue-like aggregate silk to the ends of the draglines and then returns to the main web to wait. When a passing insect steps in the glue and trips one of these lines–snap!–the silk retracts back into the web, taking the hapless insect with it.

The bolas spider has yet another creative way of using silk: it makes bolas of dragline silk with a blob of aggregate glue on the end. As a sneaky but effective hunting strategy, adult female bolas spiders make pheromones that mimic the pheromones of female moths. When a male moth approaches, the spider whips the silk bolas around, sticking the insect with the blob of glue, and reels in its catch. At least one species, the cornfield bolas spider, is known to live in the Northeast, but these spiders are rare and nocturnal (and bear a remarkable resemblance to a fresh bird dropping), so you will need a lot of luck and a flashlight to spot one of these unusual hunters.

Rachel Sargent is an editor for a pharmacology journal, as well as a freelance nature writer and illustrator. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation:

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