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The Spider's Silk

Spider silk in general is remarkable for its strength and elasticity, but there are many different kinds, all with special properties, and each used for a specific purpose.

a strand of sticky silk, enlarged more than 20 times
a strand of sticky silk, enlarged more than 20 times

How Spiders Use Silk

Wherever a spider goes, it spins a silk thread behind itself. This thread is called a dragline and is essential to the spider as a "lifeline." If danger threatens a spider in its web, it can drop from the web on its dragline and hide in the grass, or it can simply hang in the air until the danger has passed. Then it climbs back up the dragline into its web. Hunting spiders use their draglines to swing down to the ground from high places. When floated through the air to entangle or catch on distant objects, a dragline can serve as the spider's bridge over ground or water. Sometimes a dragline, caught up by the wind, can balloon a spider over tremendous distances.

Spiders also use silk to spin tiny masses of sticky threads called attachment discs, which are used to attach their draglines and webs to various surfaces.

enlargement of an attachment disc
enlargement of an attachment disc

A different kind of silk is used to swathe a spider's "victims" in a kind of strait jacket to prevent their escape and, in some cases, to store them for future consumption.

an orange garden spider wraps one of its victims in silk to 'store' it for a future meal
an orange garden spider wraps one of its victims in silk to 'store' it for a future meal

All female spiders produce another silk with which to encase their eggs in insulated, water-proof, shock-absorbent sacs. Some spiders lodge their eggs everywhere -- in leaves, on bark, under stones -- but always secured and protected by silk. Others built tent-like webs as a nursery for their young; these webs may also capture enough insects for the newly hatched spiderlings to get their first meals.

the cave spider's snow-white egg sac is a delicate tear-drop-shaped case
the cave spider's snow-white egg sac is a delicate tear-drop-shaped case

Each kind of spider builds a different type of silk nest as its home. Some spiders line a folded leaf with silk to make a nest. Others dig burrows in the ground and line them with silk. Still others build nests in the center of their webs.

How Spiders Make Silk

Spiders as a group have seven kinds of silk glands. No species of spider has all seven kinds, but all spiders have at least three of them and most have five. Each kind of gland produces a different type of silk that the spider uses for a specific purpose. Some silk glands produce a liquid silk that becomes dry outside the body. Other glands produce a sticky silk that stays sticky.

The liquid originating in the glands flows through minute tubes to external organs called spinnerets, which are on the underside and near the rear end of the abdomen. Spinnerets are composed of a group, usually three pairs, of fingerlike organs. Each is a movable, directional bundle of hundreds of tubes, ending in flexible nozzles which cover the tip of each organ. These tips can be used singly or in combination, depending on the specifications of the material. Thus, threads of spider silk are usually composed of different types of liquid silk, from different glands, brought together and blended by the spinnerets.

profile of the humped orb weaver showing location of the spinnerets
profile of the humped orb weaver showing location of the spinnerets

the underside of a humped orb weaver showing the location of the spinnerets
the underside of a humped orb weaver showing the location of the spinnerets

spinnerets enlarged, showing the hundreds of silk-extruding tubes
spinnerets enlarged, showing the hundreds of silk-extruding tubes

enlargement of the spinneret profile
enlargement of the spinneret profile

The solidification of the liquid silk occurs as the spider draws out a strand with its foot, or attaches the silk to an object and pulls away. The formation of the thread itself results from stretching and compressing the liquid mixture as it is extruded through the nozzles. The faster a thread is drawn, the thinner and stronger it becomes. The strength, thickness, or viscosity of the filaments can be controlled, not only by the mingling of the different silk secretions from the glands, but also by the different shapes and openings of the nozzles of the extrusion tubes.

PRINT SOURCE
Laura Barr Lougee. The Web of the Spider. Bloomfield Hills, MI: Cranbrook Institute of Science, 1964

SEE ALSO
Spider Webs

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The Robinson Library >> Science >> Zoology >> Arachnids >> Order Araneae

This page was last updated on April 21, 2017.