Silk

Insect versatility demonstrated

            Silk was "discovered" in China in the late Neolithic age when people first began unraveling the cocoon of a large, wild saturniid moth.  What use they first made of this fiber is not clear--- thread, fishing line, bowstring, or something else --- but by almost 6000 years ago, silk had been woven into fabrics in China.  For millennia thereafter, silk has conjured up images of luxury, wealth, and indulgent beauty.  So desirable was this material that as long ago as 4000 years, the ancestors of today's Chinese had domesticated the large-winged, far-ranging wild moth, Bombyx mandarina into a flightless, stubby-winged descendant, Bombyx mori, whose fat caterpillars spent their young lives relentlessly eating mulberry leaves until it was their time to pupate. When that time arrived, they voided the contents of their gut, found a hiding place, and began to spin the cocoon that would protect them while their body transformed from a caterpillar into a moth. 

Silkworm cocoons from a sericulture village near Bangalore, India

            Silk is essentially congealed spit, a product of the caterpillar's pair of (very large) salivary glands. Each gland produces a continuous flow of a fibroin protein gel, like toothpaste out of a tube, and these two streams of gel are pressed together and forced through a narrow tube to produce a single fiber that hardens into silk as it exits the mouth and contacts air.  The caterpillar attaches this fiber to the walls of the cavity in which it is hiding, and by moving, twisting, and rotating, spins the cocoon around itself. By the time it is finished, it has formed a hollow, oval chamber around itself from a single silk fiber 1200 meters long.  Its labor done, the caterpillar metamorphoses by forming the pupal skin and body inside the skin of the last caterpillar stage.  Left alone, the pupa sheds the larval skin inside the cocoon a week or two later, then forms the adult skin and body inside the pupal skin.  When the adult sheds the pupal skin, it dissolves a hole in the cocoon to emerge as an adult moth.

            But in the silk industry, as in the sericulture village I visited near Bangalore, India, the hapless pupae are not left alone.  Only a small proportion of them are allowed to emerge as adults to produce the next generation of silkworms and the next harvest of silk.  The fate of most of the pupae, hidden in their cocoons, is to be plopped into boiling water to kill them and to soften the glue that binds the silk fiber into a cocoon.  The entire village was dedicated to farming silk--- women and children harvested the mulberry leaves and tended the feeding silkworms, more children, their hands red from hot water, reeled off the silk, women spun it into skeins, and presumably the men marketed the silk to wholesalers and others, to be sold for making a multitude of silken products.

            The human exploitation of silk by silkworms was developed from one case of something that thousands of species of insects do--- produce silk.  Indeed, of the 500 species in the family Saturniidae (to which the domestic silkworm belongs), most spin some kind of cocoon.  A few of these species are exploited for "wild" silk derived from post-emergence cocoons in which the continuous thread has been severed, so that the silk must be processed like cotton by carding to align the fibers before spinning into thread. 

This bounty of silk among the insects traces back to the fact that all insects are champions of secretion, for almost every surface of their bodies is covered with a secreted, protective, and supportive exoskeleton composed of protein, chitin and waxes, hardened into a tough skin.  Some parts of this marvelous exterior have been specialized to secrete different compounds, and among them are the labial (or salivary) glands that open into the mouth cavity. Their original, and still most common function across insect species is to secrete saliva to aid in feeding.  Saliva contains proteins, such as enzymes that begin the digestion of food, and from this basic synthetic ability, insects evolved other proteins for different functions--- anti-coagulants in blood-feeding insects, paralyzing toxins, digestive enzymes that liquefy the insides of prey, and, for the subject of this essay, silk to be used in a wide range of insect species for purposes far beyond spinning cocoons. 

            For example, some communally feeding tent caterpillars follow silken trails to feeding areas;  some caterpillars dangle by a thread at night to protect themselves from roaming predators, or let themselves down to the ground to pupate; among the ants, larval silk not only produces cocoons for pupation, but is used by adult tree ants to sew leaves together into a nest; bark lice cover large areas of tree trunks with protective silk webbing so they can eat fungi, algae and lichen in peace; some solitary bees line their brood cells with silk; blackfly larvae living in swift streams let themselves downstream on silken "lifelines" then spin patches of silk on rocks onto which they fasten themselves with a circlet of hooks so they can catch drifting food with the fans on their heads; caddies fly larvae construct fishing nets from silk that is sticky even when wet; they also use it to build portable houses for themselves using tiny pebbles or pieces cut from leaves that have fallen into their aquatic realm. 

This caddis fly larva has cut leaf circles and sewn them together into a protective case, using silk .

  

          But in another demonstration of the inventiveness of insects, not all silk is secreted by labial glands.  In some beetles, the accessory glands of the genital organs produce silk, while in the Embioptera (web spinners) and Empidae (dance flies), the silk glands are in their feet.  The Embioptera spin themselves a chamber in which they live, and empid males capture prey, spin a cocoon around it and offer it to females to entice them to mate.  In some dance fly species, males present an empty cocoon, an extreme example of the evolution of symbolic gesture from an initially multi-purpose behavior.  In some beetle, ant lion and lacewing larvae, the silk is produced by the excretory Malphigian tubules, voided into the hindgut, and spun into a cocoon through the anus.  Lacewings lay their eggs at the end of a silken stalk to hide them from the roaming ants (below).   

            Having evolved an exoskeleton formed from gel-like fibrous protein (along with chitin) that is hardened into a tough outer skin, it is only a “minor” jump for insects to harden some of this gel-like fibrous protein into tough strands of silk by means of specialized glands.  Silk, like the insect exoskeleton, is a miraculous substance of a thousand uses, and has contributed to the unrivaled success of the insects.  My cocoons from India remind us that even humans can benefit from the marvelous capacities of insects.