Lifeform of the week: A silkworm’s work is never done
This week many American employees got a three-day weekend courtesy of Labor Day – a celebration of the important contributions of workers to our society. But as hardworking citizens in one country kicked back with beers and burgers, an unheralded insect on the other side of the globe was toiling away to crank out the raw materials for our fancy textiles.* Bombyx mori, the domesticated silkmoth, produces most of the world’s commercial silk. These larvae perform this task with no benefits package, no annual time or sick leave and they’re generally boiled alive well before retirement age. So if you own a silk tie, silk shirt or perhaps some of that silk-enhanced thermal underwear, why not take a moment to learn about the curious little worm that made these garments possible.
A bug’s life cycle
Silkworm is just one of the rapidly-changing identities this creature takes on during its brief life. It’s the larval stage. The animals start off as tiny eggs laid by the adult female silkmoth. These hatch in 10 to 14 days, and the first incarnation (or instar) of larvae emerge. They are black and furry at this point. Additional molts will yield the white silkworm image we’re more familiar with. The larvae eat pretty much constantly for the next month or so – with white mulberry leaves their preferred meal. They increase their girth 10,000-fold from the scant starting weight of a half a milligram to a sturdy five grams. They grow to about four centimeters in length (about an inch and a half) as they cycle through all five instars.
Upon reaching full size the silkworms are ready to spin their famous cocoons. The silk emanates from glands and is pushed through spinnerets near the critters’ mouths. A single thread of silk, up to 4000 feet long, forms the cocoon. Inside each cocoon, a vulnerable pupa prepares for its grand debut as a silkmoth. But, unfortunately for these pupae, the cocoons are more valuable to the silk industry if the single thread of silk remains uncut. To prevent the moths from breaking through their cocoons and ruining perfectly good silk, the cocoons are generally boiled and the thread carefully unfurled.†
The lucky few of the species who are permitted to emerge from their cocoons (for breeding purposes) get to extend their lives only slightly. After freeing themselves from the protective casing, it’s all about reproduction. Even without the aid of hot water, both male and female moths die shortly after the female’s eggs are deposited.
Freak of nurture
Bombyx mori has been bred for its cocoons for thousands of years. While originally native to China, they no longer exist in the wild anywhere. The lengthy stint as domesticated silk producers has altered these insects physically. Adult silkmoths cannot fly, and larval silkworms have lost the adaptation that would otherwise allow them to hang from the mulberry leaves that comprise their diet. They must now be provided with the leaves by their keepers.
Bombyx mori isn’t the only moth that spins a silk cocoon. Its closest relative, Bombyx mandarina, the wild silkmoth, lives a traditional outdoor moth’s life throughout parts of China, Korea and Japan. However, the cocoons of wild moths have proven difficult to unravel due to a mineral coating, and thus wild moths have been largely spared the silk industry indignities visited upon their domesticated relatives. But this may change, as researchers have discovered a “demineralizing” technique that would overcome such hurdles.
Meanwhile, others are taking up the challenge of producing silk fabric without killing the insects that supply the materials. Entrepreneur Kusuma Rajaiah has patented a technique for weaving silk from cocoons after moths make their natural exit. Of course this severs the silk thread, but the resulting fabric is said to be softer and more breathable, though it also has less of a sheen than conventional silk.
But if you are making silk the old-fashioned way, what are you supposed to do with all those un-fulfilled silkworm pupae? Well, one option is eating ‘em. Outside of Europe and the U.S., edible insects are a pretty common item on the menu, and not necessarily because of food scarcity. Such dietary leanings are often driven by taste preference. Silkworm pupae are described as “nutty” in flavor, and they’re full of wholesome buggy protein. Yum?
Before you dismiss the domesticated silkmoth as just a helpless slave to its human overlords, note that it’s also a favorite model organism for scientific research (not the most glamorous life either, but it’s a good résumé builder). One of the animal’s more interesting contributions in this field is in the subject of pheromones – those externally secreted hormone-like chemicals used for communication, such as attracting mates. The term pheromone was introduced in 1959 by Peter Karlson and Martin Lüscher to describe this phenomenon. Later that year another scientist, Adolf Butenandt, identified the first pheromone – bombykol, the sex pheromone of Bombyx mori. Female moths of the species secrete the chemical from glands in their abdomen. Any Bombyx mori male within smelling distance finds this pretty exciting and shows his approval with a “flutter dance” (a lot of wing flapping). Egg fertilizing soon ensues.
More recently (June of 2011) researchers demonstrated that a single receptor in the male moth responsible for detecting the female-produced bombykol is all it takes to provoke the sexy dance. An entire mating ritual hinges on one chemical and one receptor. And, no, this pheromone doesn’t work on humans.
* Let me preemptively state that I am aware that it takes more than insect labor to make silk fabric, but Homo sapiens is not the life form I’m focusing on this week.
† Up to 3,000 cocoons are needed to produce one pound of silk. Those numbers come from PETA, who, as you probably guessed, are not too happy about this.