Silkworm Moth

Jun 29

Bombyx mori

A silkworm wraps itself in a fluid substance from its silk glands that hardens to become a cocoon — this cocoon can be unravelled to a single strand reaching lengths of 1.5 km (1 mile). Inside, it "digests" its own larval body (called histolysis) before building up the body of a silkworm moth.

A caterpillar sidles across the branch of a mulberry tree. Its body is a yellow-tinted white, each segment marked with a dot of black along its sides. Sparse light hairs cover its head, back, and underside. A little spike juts out from its rear like the thorn of a rose. It undulates as it propels itself forward, passing by the sweet-tart black and red berries. It heads for the delectable green leaves instead. Its three pairs of pointed front feet grasp at the leaf and it chomps down with an alienesque mouth. Here, the silkworm will spend the next month gorging itself on mulberry leaves.

The Ancient History of Silk

Stacked racks of mulberry leaves on a silkworm farm.

Thousands of years ago, this scene would have played out along the temperate banks of the Yangtze River in Central China, where mulberry trees grew wild, attracting hungry wild caterpillars. Nowadays, the only leaf you're likely to see a silkworm feast on would be found at a silk farm. Although it once existed in the wilds throughout Asia, mainly in China, it has long since been domesticated and made unfit for survival in its once native habitat. On the farms, silkworms live on stacked racks of mulberry leaves or artificial feed, where they are sped towards their metamorphosis. This is all so humans can get their hands on that opulent prize: their silk cocoons.

The first instances of silk production elude an exact date, given the short-lived nature of the product. Many site the Neolithic Yangshao culture of the 4th millennium BCE (over 5,000 years ago) as the pioneers of the practice. Others the Longshan period (3500 - 2000 BCE). While some evidence plunges further into prehistory, with 8,500-year-old tombs bearing evidence of silk products. Whatever the exact date is, it can't be understated how long the relationship between humans and silkworms has lasted.

Silky Cocoon

This relationship is a pretty one-sided one. For while humans may provide the caterpillars copious amounts of leafy food, the silkworms give humans their organic riches and often their lives. Once a silkworm has had its fill of food, usually after about a month to six weeks, it begins to secrete a fluid from its silk glands, pushing it through a spinneret on its mouth. An industrious insect, it can produce about 30 cm (1 ft) of this gummy liquid per minute — impressive for a 4 cm (1.6 in) caterpillar. As the fluid emerges, it hardens into a fine silk strand. The silkworm is both a weaver and sculptor, shaping its threads into a cocoon that encases its entire body.

For two weeks the cocoon lies still and unmoving, with the silkworm wrapped inside. However, this is anything but a peaceful sleep. For the larva, it is a time of tumultuous transformation. Inside the sealed cocoon, its body is essentially "digested" by the juices that it itself has released. Its caterpillar form, with its nubby legs, fleshy segments, and voracious mouth is broken down to allow a new body to develop. This gruesome breaking down of tissues, called 'histolysis', is a process undergone by most insects in their pupal states, and what makes such drastic transformations — like a caterpillar to a butterfly — possible. In the case of a silkworm, it emerges from its cocoon as a fluffy moth.

From Legend to Industry

A fan from Qing dynasty (1644–1911) China depticting silkworms feeding on mulberry leaves.

A Chinese legend tells of the discovery of silk. As Empress Leizu — the wife of the mythical Yellow Emperor — was having tea, a silkworm cocoon somehow happened to drop into her teacup. As she pulled it out, it unravelled into a delicate silk strand of extraordinary length that she could weave into a fine material. Upon this discovery, she began her study of silkworm lifecycles and instructed her court in her knowledge, thus inventing the practice of sericulture — the raising of silkworms for silk production.

Of course, this is just legend. The first person to pluck a silkworm cocoon from a mulberry tree and realize its revolutionary textile potential is lost to history. But the discovery of this crafty caterpillar and its cocoons had a large impact on the commerce of the ancient world; greatly contributing to China's wealth and trade connections, and eventually giving its name to perhaps the most famous trade route in all of history: the Silk Road. So important was the silkworm industry to China, that upon having created and monopolised it, the state made any smuggling of these caterpillars out of the country punishable by death. Evidently, some people had risked the potential consequences, as nowadays silk is produced in countries like India, Thailand, Vietnam, Tajikistan, Brazil, and others — although China remains the dominant producer.

A single silkworm cocoon, unravelled, can produce a strand that reaches up to a kilometre and a half in length (1 mile). In 1989, 74 thousand tons of silk were produced. In 2021, 146 thousand tons of raw silk was produced — that is about 24,300 male African elephants of silk. But in most cases, as the silk piles up in tons, so do the bodies of the pupating caterpillars, killed for their cocoons. Before the silkworm can complete its metamorphosis and dissolve a hole in its cocoon to get out — damaging the threads in the process — the whole thing is thrown into boiling water to kill the pupa inside. Around 3,000 silkworms are killed for about half a kilogram (1 lb) of silk. Considered in yearly tonnage, the number of deaths is unimaginably large.

Mating & Metamorphosis

When a silkworm pupa is allowed to complete its metamorphosis, it transforms into a completely different creature. Bombyx mori (the silkworm's/moth's binomial name) practices 'holometabolism', also known as complete metamorphosis. Its life cycle consists of four stages: egg, larva, pupa, and imago (or adult), famously exhibited by lepidopterans (butterflies and moths), the order to which Bombyx mori belongs — despite its name, the silk"worm" is a caterpillar, the larval form of a moth. But many other insects undergo this process; including ants, wasps, and bees, beetles, flies, fleas, and more.

The silkworm moth emerges looking positively cute. It trades the patchy hair of its caterpillar form for a heavy covering of scales that give it the appearance of wearing a fuzzy buff-coloured coat. Its six legs and big bulbous thorax are similarly "furred". It sees with its large, black compound eyes and two comb-like antennae hang from the sides of its head like long pigtails. Its mouth is tiny. It has done all the eating it will ever do as a caterpillar. Brown-lined wings drape its back, but thanks to thousands of years under the influence of human rearing, they no longer serve their function. With the moth's inability to fly, surviving in its original wild habitat is a near impossibility.

While this species may not survive in the wild any longer, a close cousin — Bombyx mandarina (the wild silk moth) — does. This wild silk moth might be the species all domestic variants descend from. More drably coloured, in shades of brown, with a more slender body, it is perhaps less aesthetic than its domesticated relative. Its wings are strangely shaped, with a wide span and large gap above the thorax, but at least they're functional. It can fly between the mulberry trees of inland China, further ranging into Korea and Japan. Outwardly very different, it has not diverged too far genetically from the coddled domestic species as the two can still interbreed to produce hybrids.

Beautiful as Bombyx mori's moth form may be, this stage of its life is fleeting; devoted to reproduction and little else. A female's thorax is tipped with a gland that releases pheromones irresistible to males. The female's pheromones are so concentrated, it has been suggested that if she were to release them all at once, a trillion males would come flocking to her chemical call — a hypothetical of course, for a trillion male silkworm moth males don't exist. Picking up on the "love" scent, the males that do exist, begin a wing-fluttering dance. The female will often pick the largest, most plumed male to mate with. Once fertilized, she lays as many as 500 minuscule, yellow eggs, then dies very shortly after.

Peace Silks & Pheromones

A closeup of a silkworm moth’s head, with its antannae clearly displayed. ¹

The story of Bombyx mori seems like a lesson in the transience of beauty but also a tragic waste of life. It is a fact that most mass production of silk leads to an inconceivable number of lives lost — insects though they may be. However, more ethical forms of silk production, although practised on a smaller scale, have been developed. Examples of these "peace silks" include Ahimsa Silk from India and Eri Silk from North East India and some parts of China and Japan. Both are made from cocoons of already emerged moths. This requires additional working of the severed strands and leads to an increase in price for the final product, but it produces a silk that didn't take the lives of tens of thousands of silkworms to make. Aside from industry, silkworms have provided other benefits for humanity. In Australia, they are brought to schools to educate a new generation in biology. Their presence in the lab has yielded knowledge in both genetics, as their genome is being mapped and studied, as well as animal communication; given the females' chemical endowment, Bombyx mori was the first animal in which pheromones were discovered.

**Where Does It Live?**

⛰️ No longer found in the wild.
📍 No longer found in the wild.

Size // Tiny
Length // 75 mm (~3 inches) as a worm, 100 mm (4 in) as a moth
Wingspan // 40 to 50 mm (~2 inches)
Weight // N/A
Activity: Diurnal ☀️
Lifestyle: Solitary 👤
Lifespan: 6 - 8 weeks (total lifecycle), 5 - 10 days (adult/moth lifespan)
Diet: Herbivore
Favorite Food: Mulberry Leaves 🍃
Phylum: Arthropoda
Class: Insecta
Order: Lepidoptera
Family: Bombycidae
Genus: Bombyx
Species: B. mori

The silkworm moth (Bombyx mori) is now considered a domesticated species — no longer found in the wild (it was once found throughout Asia). On farms, silkworms typically live on stacked racks and feed on mulberry leaves or artificial feed. Apart from bees, silkworms are the only insects domesticated by humans.

The silkworm moth (Bombyx mori) can no longer fly due to its deteriorated wings. Its close cousin, the wild silkworm moth (Bombyx mandarina) still lives in the wilds of China, Korea, and Japan, and retains its ability to fly. The two species can still interbreed to produce hybrids.

To get out of their cocoons, silkworm moths dissolve themselves an exit hole. To avoid damaged silk strands, most farms boil the cacoons in water, killing the silkworm before it completes its metamorphosis. Around 3,000 silkworms are killed for about half a kilogram (1 lb) of silk.

The production of "peace silks" involves using the cacoons of already emerged moths.

The silkworm moth practices 'holometabolism', also known as complete metamorphosis. Its life cycle consists of four stages: egg, larva, pupa, and imago (or adult).

The mouth of a silkworm moth is tiny, as it no longer eats in its adult form.

The purpose of the imago moth form is to mate. The female can release a potent pheromonal cocktail from her thorax so strong, it is speculated, that if she released all of it at once, a trillion males would come flocking (if they could).

This silkworm moth was the first animal species in which pheromones were discovered.

A female silkworm moth can lay as many as 500 tiny, yellow eggs and will die not long after.