Silken threads

nature’s multi-purpose material


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 twenty-three 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 bird 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!

Mate choice (1 of 2)

…it’s complicated!

When a female wood frog, ready to mate, arrives at a small pond full of singing males, she is jumped by the nearest male, who grabs her around the neck and locks his thumbs together, so he cannot be dislodged. She apparently has little or no choice in the matter—it is first come, first served.

But in many vertebrates, females can and do choose their mates. Sometimes choice is based, at least partly, on the male’s property—his territory, defended against other males and providing nest sites or food sources or protection from weather. For example, male yellow-headed blackbirds carve out territories in a marsh. When females arrive, they cruise around, checking out each male and his property. One of the factors determining their choice of mate is the availability of suitable nest sites at the edges of clumps of cattails or bulrushes—the more edges he owns, the more females he gets.

Males of a wide variety of insects present females with nuptial gifts of food, and females select males on the basis of the size of the gift. The female gains energy for egg production, at least, and if the ability of males to find good gifts is hereditary, she also may get good genes for her offspring.

Sometimes females base their choices on the qualities of the male himself. It might be his song, or the vigor of his courtship dance, or his colors. For example, in the Lower Forty-eight, the size and brilliance of male plumage pattern is the basis for choice by female house finches. Male house finches have red feathers on the head and chest. Females prefer males with intense red coloration and large red chest patches. The red pigment is carotenoid-based, and carotenoids come from the bird’s diet, so the red depends on what the male has eaten (and his ability to convert components of food to red pigment). Thus, house finch females may be choosing males that are the best foragers or have the most efficient metabolism.

A small warbler called a yellowthroat nests in marshes and shrubby swamps. Males have black masks and yellow chests. Careful research has shown that yellowthroat females have marked preferences, but that these preferences differ from region to region: In Wisconsin, females like males with bigger black masks, but in New York, they like ‘em with bigger yellow chest patches. What the females get from making these choices is not clear.

Female preferences also affect the ability of males to obtain extra-pair copulations (many socially monogamous birds engage in very active mating activity outside the pair bond). Mountain bluebirds, for instance, vary in the intensity of blue plumage, and males with brighter blues are more successful in attracting extra-curricular females. Similarly, intensely colored male tree swallows and yellow warblers are preferred by females that copulate with males outside their pair bond.

The bowerbirds of New Guinea and northern Australia have gone a step farther, by transferring the signals to females from themselves to elaborate structures (bowers) that a built solely for the purpose of attracting and courting females. Different species construct bowers in differing shapes—avenues, towers, huts—and decorate them with colorful objects. Each species uses different kinds and colors of objects; some like blue, some like white or yellow, and so on. Within each species, female bowerbirds cruise around and visit the bowers of the males and judge each male on the construction and decoration of his bower. After she makes her choice and mating takes place, she goes off to build a nest and rear chicks by herself.

What’s the payoff to females for making these choices? In some species, there is a direct benefit in terms of resources such as nest sites or food. In other cases, males with brighter colors turn out to be better providers for the offspring. When males do no parental care, females might at least get good genes for their offspring. And in other cases, choosing a ‘sexy’ male might mean that the sons of the choosing female will also be sexy and successful in attracting females. The so-called sexy-son hypothesis obviously requires that the attractive characteristics are genetically based and inheritable.

Of course, male animals make choices too. Males of some species choose on the basis of color or pattern, just as females do. For instance, for whatever reason, male barn owls prefer to mate with females that have lots of lovely black spots on their white breasts. However, in many vertebrates, males seem to be somewhat less choosy than females.

When female creatures choose males, we do not suppose that the choices are conscious in the human sense. (Nor can we say that all human female choices are necessarily conscious!). All that is required is that there is variation among males and that females can discern the differences and act accordingly.

The result of mate choice (and of the competition to be chosen) is, in the big picture, an incredible diversity of color and form in the animal kingdom—diversity that is not directly related to making a living or simple survival but, instead, is related to mate selectivity and being attractive to the opposite gender.