The sticky juice paints purple splotches on my forearms, and I balance on one foot, stretching as high as I can to reach a particularly plump-looking berry in the branches above. The large leaves of the mulberry tree shade us from passers-by, and yet an onlooker still manages to stumble off her bike as her inquiring backward glance at two twenty-somethings in a tree stretches a little too far. The canopy stretches wide, housing us underneath its huge umbrella structure. Every second berry lands in the bottle used as a makeshift container, satisfying both our bellies and the plan to dry some for later. Every now and then, one slips out of my hand and plops to the ground.

Legend has it that in the third millennium BCE, Lei Zu Princess of China, was drinking a cup of tea, when something plopped into her drink. A silkworm cocoon had fallen from a nearby mulberry tree, and as she flicked it out, she noticed that the cocoon had started to unravel from the heat of her tea [1]. The human discovery of silk fiber prompted the domestication of Bombyx mori L., the mulberry silkworm, the only entirely domesticated invertebrate in the world [2]. It is likely that this species was domesticated from the wild silkworm Bombyx mandarina M. some 5000 years ago [2].

Like all butterflies and moths (order Lepidoptera), silkworms go through a four-stage cycle of metamorphosis: egg, larva, pupa and adult. It all starts with the egg. Or rather, three to four hundred eggs, from which caterpillar larvae hatch [3]. The larvae munch on a lot of mulberry leaves. Silkworms particularly like white mulberry, Morus alba L., for the high protein content of the leaves. Conversely, humans tend to favor black mulberry, Morus nigra L., for its higher concentrations of flavonoids in the berries [4]. During their 2-3 week cycle, larvae molt four times, progressing from eating chopped-up leaves to whole ones as they rapidly grow [3]. During the larval stage, caterpillars develop long, sac-like silk glands on the sides of their bodies. These glands, which are actually modified salivary glands, produce the silk used for spinning cocoons. The spinneret, a hook-like structure, extrudes the silk from the inner glands [3].

In the pupal phase, the once voracious caterpillar stops its feeding frenzy and starts to secrete a sticky fluid through the spinneret, which hardens on exposure to air to create a fine filament of silk [3]. The larva spins the silk around itself, weaving a snug cocoon in just a matter of days [5]. The inner filament of the cocoon is a continuous single thread which can measure up to 900 meters long! Inside the refuge of its newly created house, the caterpillar larva undergoes a metamorphosis, silently turning into a moth in just 2-3 weeks. When ready, the adult moth secretes an alkaline fluid to soften one end of the cocoon and emerges out [3]. Not long after emergence, the adults mate, lay eggs and die. 

Fig. 1: Silkworm larvae on a mulberry leaf surrounded by fruits (Gorkaazk. 2010). Free source picture from Wikimedia Commons.

This incredible natural process creates a fiber whose use has profoundly impacted the world. Beyond clothing a vast Chinese population, silk production during the Han Dynasty (circa 130-100 BCE) gave rise to trade routes across Central Asia and into Europe [6]. The trade routes were later named the Silk Road by Ferdinand von Richthofen in 1877 CE, owing to their prominent use for silk trade [6]. The Silk Road played an important role in the exchange of religion, ideas, and commerce between East and West. You could say the humble mulberry-silkworm duo helped initiate an important exchange between distinct cultural and geographical worlds. Protruding from the spindling spinneret came a product for a globalization enriched with humanity [7].

At first, and for a long time, silk production was a strongly guarded secret. Traitors and spies could be killed, if they spilled the beans on it [6]. Then, around 440 CE, a different Chinese Princess stuffed silkworm eggs and mulberry seeds into her headdress. She travelled westwards to a kingdom on the edge of the Taklimakan desert, where she handed the keys to silk production to the king of Khotan [8]. She was not the only one to spread the secret. Silkworm smugglers in the form of Nestorian monks brought the eggs to ancient Greece around 550 CE [6]. Silk production, or sericulture as it is referred to, had also already moved to Korea, Japan, India, Iran and Islamic kingdoms around the Mediterranean [6]. By 900 CE, sericulture was up and running in Europe. Important centers for sericulture were established in Italy in Lucca, Genoa, Florence and Venice using white mulberry orchards to feed the silkworms [6].

From global trade and valuable feed to berry delight and artistic heed (The Mulberry Tree was drawn by Vincent van Gogh during his time at an asylum), the umbrella canopy of the mulberry tree stretches wide. A study from the Free University of Bolzano on urban greening highlights the black mulberry as a particularly valuable species to cool the town down, thanks to the temperature-regulating effect of their crowns [9]. From the high branches of the riverside mulberry, I spot a lady down below, feasting on the berries of the neighboring tree, mouth gaping like a fish, the addictive juices of the berry fueling the smacking of her lips. In the sunshine, eyes shaded by the wide brim of a few leaves, she looks at peace, face directed up towards the drooping fruity drops. Even the birds are happy, as a flock of doves flutter around below, happy to join in on the feast.

References

Related articles:

Cover image: Mulberry trees along the Isarco in Bolzano. Bolzano, Jul 2024. Jennifer Lüdtke.

Preview image: Silkworm larvae on a mulberry leaf surrounded by fruits (Gorkaazk. 2010). Free source picture from Wikimedia Commons.