The Advent of the Giant Silkworm
The moutling process starts with the silkworm losing interest in its food. The caterpillar then has to become still as the new skin is formed beneath the old one. At the same time the old skin must separate from the body; some of this is reabsorbed to form the new skin, while the rest becomes hard and brittle and eventually slips off. During this time the silkworm is vulnerable. The new skin must be 'cured' and finished before the old skin is sloughed.
But what causes the silkworm to lose its skin? It turns out that it almost the same thing that causes it to pupate. It is the result of some fairly simple chemicals coursing around the silkworm's body. I have been finding out a little more about their discovery - it makes dramatic reading.
In February 1941 a Polish scientist called Stefan Kopec was arrested. He was an eminent entomologist and several years earlier his work had caused a stir. In 1915, he had discovered that two hormones were essential for pupation. However Kopec clearly did not confine his thoughts to insect physiology. He was also a member of the Polish Underground. For this liberal-thinking Kopec and his son would be shot. His death, in March, was a great loss to science and reported in Nature the following year.
Kopec's work had already led to Muroga's experiments (that I mentioned in an earlier post), and also Fukuda's in 1940. Between them they worked out the origin and sequence of hormones required to turn a silkworm into a moth.
Like all living things, insects respond to their surroundings. Silkworms react to the shortening of the day or a drop in temperature by secreting a hormone from the brain which then travels to a gland in the thorax. This causes another hormone, a steroid similar to cholesterol, to be released and this circulated throughout the insect's body. This steroid-hormone tells the tissues of the insect to change - but the 'change' that results (either a moulting or pupation or metamorphisis into adult) depends on the levels of another hormone, called the 'juvenile hormone'.
The 'juvenile hormone' is secreted directly to the insect's body from a gland close to the insect's brain. This 'juvenile hormone' stops the larvae growing up. A high level of 'juvenile hormone' prevents metamorphosis and the steroid simply causes the caterpillar to moult; with lower levels of 'juvenile hormone' the caterpillar becomes a pupa, and then with another burst of steroid-hormone, the insect metamorphoses from the pupa to adult insect.
Although Kopec had discovered that these two hormones were involved long before the start of World War Two it was not until the mid-nineteen fifties that scientists managed to extract the hormones from the silkworm and determine their chemical structure.
As soon as Kopec's two insect hormones were isolated scientists started to experiment with them. An American scientist called Williams found that if a silkworm was dosed with sufficient 'juvenile hormone' it could be prevented from pupating indefinitely. The silkworm would simply keep eating and growing until it became enormous. If he then gave this giant caterpillar the steroid-hormone it would immediately start to pupate and start to spin a cocoon large enough to envelop its huge bulk. The cocoon that resulted was as large as a hen's egg. In this way silk production could be controlled - just by juggling those two chemicals Kopec had found fifty years earlier.
But these hormones are not just found in insects. Interestingly, a very similar hormone to the moulting and pupating steroid is found not only in other animals - Crustacea and Arachnida - but in plants too (the Togariha-maki, a type of black pine, and the root of the Hinata inokozuchi, a variety of Achyranthes). Astonishingly, it was found that when this plant hormone is injected into a silkworm, the silkworm begins to moult then too. Furthermore, if a tray full of larvae in their last instar is sprayed with the moutling steroid, all the caterpillars begin to spin their cocoons at the same time. This is very useful, and to be honest I wish I had a little of that chemical now. The house is beginning to smell of mashed-up mulberry leaf, and I am starting to wish that my dear little silkworms would just get on with things.