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On the Trails: Silken threads

Posted: November 16, 2012 - 1:02am
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Crab spiders lurk in flowers and ambush flower visitors. They do not build webs, but this one is using silk threads to tie down a bumblebee.  Photo by Bob Armstrong
Photo by Bob Armstrong
Crab spiders lurk in flowers and ambush flower visitors. They do not build webs, but this one is using silk threads to tie down a bumblebee.

If you are the first one down a trail in the morning, you probably have to swipe a few silken threads off your face. I like to have a tall person ahead of me, to clear away those tickling filaments. A tiny caterpillar or a baby spider may dangle from those long threads, using the thread like a kite to “fly” to a new site. Or perhaps, an enterprising adult spider began a web that stretched across the trail. That is a very small sample of the uses of natural silk, which turns out to be used in a great many ways, not only by the animals that produce them, but also by other organisms.

Natural silks are produced by insects and spiders. The most famous silk is produced by the silkworm moth, long domesticated by the Chinese for human use. But silk production has reportedly evolved, independently, at least 23 times among the insects and who knows how many times among the spiders. In some cases, a single kind of creature may extrude several kinds of silk from different silk-secreting organs, each silk with a particular function. Many different organs are used to produce silk — some in the mouth area, some derived from excretory or reproductive organs, even some on the legs.

Silks are varied in structure and function, but they all consist of protein — long chains of amino acids. The particular amino acids, their sequence in the chain, and the conformation of the chain all vary, giving each kind of silk unique characteristics suited to its function.

Insects and spiders use their silks for many different purposes. Some insects use silk in reproduction: to wrap nuptial gifts for females, to help transfer sperm from male to female, to support, anchor, or cover eggs. Many insects and some spiders use silks to construct shelters: lining tunnels, building retreats, nests, or nest-supports, or making cocoons in which a larva transforms into an adult. (Cocoons of the silkworm moth were unraveled, traditionally after killing the larva, to yield long silken fibers for making elegant garments.) Spiderlings of some species and the offspring of some insects use the gossamer threads for dispersal. Some insects, and many spiders, use silk to capture prey with sticky threads or nets, and to wrap prey until it is eaten. Silk is indeed very versatile!

Here are some of the clever ways that insects and spiders capture prey, using silken threads.

Stream-dwelling caddisflies make silken nets to filter prey from the water. Different species of caddisfly make nets of different mesh sizes: small meshes for filtering tiny drifting organisms from relatively slow waters and larger mesh sizes for catching larger prey from faster waters.

The famous “glowworms” of New Zealand are really fly larvae that live in silken shelters on the ceilings of caves and dangle long, sticky, silken threads to capture small flying insects. The glowworms have light-producing organs on their rear ends, which attract prey that fly up toward the ceiling and get caught in the hanging threads.

Undoubtedly the best know silk traps are those of spiders. We sweep their webs from the corners of our rooms but admire their delicate beauty when we see a dew-bespangled orb-web back-lit by the sun. Strung on radiating silk threads between twigs and stems, with a spiral of sticky or fuzzy silk spread out on the radii, orb webs intercept the flight of insects that generally become the spider’s meal. Up to four kinds of silk may be involved with such a web. Many spiders take down their webs after one night and often eat it (it’s protein, after all), but other maintain a web for days. These persistent orb webs commonly have some conspicuous thick-stranded white patches on the web. Although this is called a “stabilimentum,” it has little to do with stability. Instead, it typically protects the web from flying birds, which see the web and veer away.

Other spiders in various parts of the world dangle a curtain of silk threads into a pond to capture aquatic insects, or throw a silk line with sticky glob at the end to snag a moth. Another kind of spider holds a drop-net trap in its legs, and pops it over a passing ant or other unwary bug. Spiders are such inventive beasts!

The silk made by insects and spiders is useful to other animals, too. Several species of birds use spider silk in their nests: Hummingbirds often decorate the outside of their tiny nests with silk and bits of lichen. In other parts of the world, spider-hunters use silk strands as pop-rivets to anchor a nest to the underside of a broad leaf. Tailorbirds make yarn out of silk and plant down, and then use the yarn to stitch two leaves together, making a pouch in which to put a nest.

By the way, some molluscs, such as mussels, spin strong byssal threads to anchor themselves to the substrate. Although the amino acid composition and the structure of these threads are different from that of insects and spiders, byssus threads are sometimes called sea silk.

One could write at least one book about the making of silks and their uses!

• Mary F. Willson is a retired professor of ecology.

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