Several species of moths produce silken cocoons, but those which produce the greatest amount of silk and are kept commercially are mulberry silkworms, or Bombyx Mori. As their name suggests, they eat mulberry leaves - in fact they devour huge quantities of them! The mulberry tree comes in three types. Black Mulberry is the slowest growing, and has the thickest leaves, which have the disadvantage of being tough, so using up more of the larvae's energy to eat, but the advantage that they will remain fresh for longer than all others so there is less wastage. White Mulberry is the type grown for commercial silk production as it grows the fastest and has softer, finer, more easily chewed and digested leaves. It also thrives when grown as shrubs or hedging, so the leaves are easily picked. Red Mulberry is a small to medium sized tree with leaf quality between the other two, but slower growing than white. If no mulberry is available the larvae can be brought up on artificial chow, but they never do as well and tend not to reach maturity. I've taken the photographs below using a British one pence piece to give an indication of size.
Eggs are laid on a flat surface, where they stick firmly. This might be a leaf or bark in natural surroundings, but for convenience cultivated moths will usually be given sheets of paper to lay on. These can then be refridgerated until needed. Once brought into a warm room they will develope within a few days and hatch into the almost black larvae pictured below. Note that they are not worms, but caterpillars. Once hatched they will quickly crawl off in search of their first meal. Leaves sliced into strips will attract them several inches, and once they are eating, the strips can be picked up with tweezers with the larvae clinging tightly to them. Note that anything coming into contact with them must be clean and dry - they are easily infected by bacteria.
The hatchlings grow at an incredible rate. They move only as far as they need to reach the next mouthful of leaf. Like most caterpillars, their bodies are little more than a very efficient digestive system, and they don't waste energy walking about aimlessly! Given food, they will remain almost exactly where you put them, so they can be kept on uncovered plates or trays.Remember that larvae such as these are very high in protein and aren't built for speedy getaways or counterattacks! They make tasty, highly nutritious meals , and are easy targets for hunting birds, mammals and even insects. Their best defence is simply to grow and mature as fast as possible. They are short on table manners; soon their constant munching of leaves will become clearly audible. Depending on temperature and quality of food, they will be fully grown within about 6 weeks. By this time they will have shed or moulted their skin four times. The ones pictured below are in their 5th and final instar.
Note the shape of the head of the one moving towards the bottom of the photograph. It becomes more pointed when the time for metamorphosis is close, and the skin will become tight and slightly translucent, with a yellow tinge. Now the larvae have eaten their fill, and for the first time in their hungry lives they will think of something other than food. They need a place to anchor themselves firmly and spin that wonderful silken cocoon. Given a choice of materials, they will select a dead leaf or branched twig. They need to be able to suspend themselves and the cocoon within a protective framework. :eaves will be curled around them, and they will usually want to climb. Every year I have a group of ambitious breakaways that will spin in the corners of the room, against the ceiling, or the underneath of a shelf. Instinct tells them that most predators will attack from above. Incidentally, most commercial silkworms lack the black banding that this variety has, and produce a pure white cocoon. The yellow colouring is actually not the silken strand itself, but the gum produced to cement the strands together.
Having made a roughly spun network, the larvae will begin to turn in a tight figure of eight in its centre, exuding the liquid silk which solidifies into a fine strand as it contacts the air. As the bulk of the silk leaves its body space becomes available for the cocoon to be built up in thickness from the outside inwards,
Finally the cocoon will have been built up enough to form a thick, hard, weatherproof protective layer , within which the magic of metamorphosis takes place... At this point commercially produced cocoons will be tossed into near boiling water, The larvae will be killed by the heat, the gum softened, and the silk will be unreeled and twisted with several other strands to produce a continuous thread that is ready to weave or spin. The larvae are usually removed from the water to provide food for birds, fish, small mammals or - wait for it - humans. Its hard to see how the larvae can actually fit inside the cocoon. Below are two larvae about to pupate, alongside two completed cocoons.
However, there is a growing opinion that the exploit of this gentle eating machine to produce fine fabric is not a good thing, and there is a move towards a product known as Peace Silk, which is what I produce. In this case the moths are permitted to leave their cocoons to lay eggs. When the new moth is ready to face the harshness of its adult world, it produces acid that will burn a hole through one end of the cocoon, efficiently cutting a mile or two of silken strand into two or three inch pieces as they do so, as can be seen below.
The moths don't have the life they might have chosen for themselves. With no mouth-parts at all, they are no longer able to eat or drink, and their predecessors lost the ability to fly thousands of years ago. They mate on the ground or clinging to a leaf or empty cocoon, lay eggs and die.
Future posts will give more scientific detail regarding the lives of Bombyx Mori and other silk-producing moths, as well as looking at the production and use of the silk itself. The photographs used here were taken by myself, and are linked to my smallholdings site, where they have also been used.