The Control of Pupation in the Silkworm

In 1939, while the rest of the world was at war, a Japanese scientist called Muroga was busy performing some peculiar experiments on silkworms. He was trying to find out what caused the silkworm to pupate - and his experiments were simple but conclusive.


What he did was to tie pieces of thread (presumably silk since it is so strong) around various parts of a silkworm's body. The silkworm in question was on the point of making a cocoon, that is at the end of its fifth instar. When he ligated the head from the body, so that no fluids could pass from one to the other, he found that the lower, posterior part of the animal pupated as normal (as in d above). However, when he ligated the head and the first segment down of the thorax, he found that the animal failed to pupate at all (as in b). The source of the hormone that controlled pupation was clearly somewhere in the first segment - and not the brain.

Left: front view of head of silk with cutaway section showing brain (green) and corpus allatum (red) and subesophageal ganglion(blue). Right: view from side.

As you can see from my crude diagrams (I hope) there were several contenders. To find out which was responsible he ligated another silk worm, again separating the head and first segment from the rest of the body, and transplanted each organ in the lower part of the body in turn. Given how the size of these caterpillars this must have been skilful work. He found that only when he transplanted the subesophageal ganglion (shown in blue) did the caterpillar pupate. Therefore the hormone that controls pupation is in this subesophageal ganglion.

There was more to come. Similar experiments on younger caterpillars showed that the subesophageal ganglion was implicated in moulting too. In fact both the subesophageal ganglion and the Corpus allatum was necessary for moulting, but the effect of the Corpus allatum died away as the larva in its fifth and final instar matured. Perhaps this is why pupation can occur. Maybe the hormone from the Corpus allatum dies away leaving the hormone from the subesophageal ganglion in charge and the caterpillar can now turn into a pupa without another moult.

There were some even more bizarre experiments done on some more hapless silkworms involving brain transplants however I shall have to leave those for now. Just now I have to show Hodmandod Minor how to feed the silkworms.

Silkworms at 15 days old

Still my silkworms grow. They shed their skin leaving an image of themselves impressed there like the shroud of an icon or a holy relic. Then they eat again and grow.


Silk worms at 15 days old. The scale is in centimetres.

According to the book I received yesterday, which I am going to have to go through carefully since it is translated from the Japanese in the style usually reserved for instruction manuals, it says that moulting depends on a hormone secreted from a pair of small organs called Corpus allutum. These protrude backwards from the insect's brain like a pair of internal antennae inside its head. I have drawn a crude little sketch here:

Sketch of silkworm's head showing the location of the brain and Corpus allutum.

The fact that they are not integral to the brain was of interest to a Frenchman called Bouhniol. In 1937 he managed to excise the Corpus allutum from some silkworms in their fourth instar (that is caterpillars before their fourth and final moult) and found that they then changed into pupae without moutling. Similarly, if he removed the Corpus allutum from silkworms in their third instar they missed out two of their moultings.

This experiment was extended by a Japanese scientist called Kin. He removed the Corpus allutum from a silkworm early on in its fourth instar and then replaced it. He found that if he did this quickly enough all was well and the silkworm moulted as usual. However, if he delayed the transplant a few days the moulting didn't occur and the silkworm went straight to pupa. Clearly enough time had elapsed for the hormone that initiated the pupation process to come into play. Futhermore this pupation hormone was not secreted by the Corpus allutum but by something somewhere else.

This result obviously piqued the interest of the entomological world because they then performed some more interesting experiments, but I am still trying to understand these and shall write another post when I do.

Silkworm cycles

Yesterday, at dawn, my last silkworm emerged from her cocoon and promptly started to mate. This morning she has laid some eggs. I have a feeling this may be the last clutch. Other matings seemed to have yielded no eggs at all. I think maybe these held on to each other too long because now they seem to be almost dead with exhaustion.



These are all that are left. The majority have died. They seem to die gently and gradually, life slowly leaving them, and I pick them out and place them alongside the cocoon. How shrivelled and small they seem to be now. All that huge consumption of mulberry leaf extract has led to is this.


And this.


Where does it go? I think the answer must be into the air: starches converted to sugars in the gut of the silkworm and then stored as fat, must eventually be consumed in the business of mating to produce energy and carbon dioxide and water...which the mulberry bush could one day use again to produce more leaf. It is a pretty cycle.

In some parts of china the silkworm-mulberry plantation, together with a fish pond, provide a self-contained ecosystem for a small community. The silkworm feeds on the mulberry and produces cocoons. Its waste is fed to the fish; white the pupa provide useful nourishment to the growers. The sludge from the fishpond is used to fertilise the mulberry plants, and the fish themselves are another source of food. I like the way nothing is allowed to go to waste. An extension of this idea - the use of silkworms as a useful food for astronauts - is reported in March's edition of Scientific American. Pupa not only contain fat, but four times as much protein as eggs and milk. Also 'These insects breed quickly, require little space, food or water, and produce only minute amounts of excrement, which could serve as fertilizer for onboard plants.'

I am not so sure about 'little food' - but I suppose in comparison with most mammals and birds silk worms are indeed very efficient converters of plant into animal.

The Chemistry of Silkworm Eggs

I am getting a satisfactory collection of eggs. These change colour as they age. When they are laid they are a creamy white, they then become yellow, then brown then grey. The seem to harden too. I am keeping them warm and humid, and once all the moths are dead (though only one female has died so far) I am going to treat a few of them with hydrochloric acid.



Silk worms come in several types, and one way to distinguish them is by how many times they breed in a year. In temperate places they tend to breed just once, or more commonly twice, a year to coincide with the leafing of their only food-source, the mulberry. These are called monovoltine or bivoltine respectively. In tropical regions the silkworms tend to be multivoltine. The eggs take only about 10-12 days to hatch because the mulberry is in leaf all year round. Multivoltine silk tends to be inferior to bivoltine silk - being thinner, more variable and shorter. So a common practice in India is to hybridise the silkmoths with the hybrids having the favourable characteristics of both parents.

Unfortunately I have no idea what sort of silk moths I have. If they are bivoltine they might need to be kept for a few months in cool conditions. However it is possible to speed things along by washing them in hydrochloric acid. This destroys the enzyme present in the temperate silkworms that inhibits their development in the egg. So I think I will experiment with just a few and see what happens.

What's in the box...

beneath the lid...


but another box...


covered up like a magician's trick...


to reveal...ta-dah...


another box...of beautiful white cocoons.

I have decided that the pupae are taking to long to emerge, maybe because the air in the airing cupboard is not humid enough. So I have soaked one of Hodmandod Senior's old flannels to act as a humidifier which also effectively keeps out the light.

Cocoons! (part 2)

Just before I went to bed last night (around midnight) I took another look at the silkworms, and again took a short video to show the grubs at work...



It is interesting to compare them with a video taken about four hours before, just here. The silkworms seem to construct a messy support web first, and then make smaller and smaller movements until they are enclosed in a cocoon. At the moment, in this video, the one on the egg carton can still rotate, but soon I expect it will not even be able to do that.

This must seem mundane and boring to some people, I suppose, but to me it is extraordinary.

Of Silkworms, Orchids... and Columbian Rebels.

Silkworms are fussy creatures. Like Giant Pandas they live only on one type of plant: the white Mulberry Bush (not the black one). If they are fed anything else they sicken and die. Mulberries, in turn, are fairly fussy too: the ground must be well-drained, and they prefer a little warmth. They can be grown in the UK but their season in leaf is short - from May to October.

This, more than anything, has dictated the distribution of silk production around the world. The UK, for instance, is generally too damp and cold for effective production, so my pampered creatures are living in our airing cupboard, and they seem to be quite happy in there.

Presumably, the silk farm at Lullingstone Castle Silk Farm in Kent kept its silk worms in a similarly cossetted environment. I was excited to come across it on the internet today, and then disappointed to find that it had closed in 2004. It seems to have had an auspicious history - providing silk for three royal weddings, including Princess Diana's in 1981. I remember that gorgeous voluminous dress of ivory silk and now realise it must have taken the cocoons of tens of thousands of silk worms.

So, the only British Silk farm has closed, which is sad, but there is something equally interesting in its place. The heir to the Castle, Tom Hart Dyke - the twentieth generation of his family to live there - is a gardener of the most adventurous sort. He has established a garden which has plants from all over the world planted in the outline of the continents. It was, apparently, an idea he thought of when one of his orchid hunting trips went a little wrong and he was kidnapped by rebels in Columbia(!) In the account I was reading it seemed like a remark slipped in by a particularly laconic dinner party guest. He has written a book about his experiences called THE CLOUD GARDEN - which looks like it would make interesting reading.

So, where was I? Ah yes, the vagaries of the domesticated silkworm....more on this later. I have some boxes to clean out.

Into the moult

Silkworms, I have noticed, seem to have two sets of legs: six insect legs on their front segments and then eight, more-rudimentary ones on the segments further back. (Added later: I have since discovered that the legs on the front are thoracic ('real' insect legs) on the thorax; while the ones at the back are prolegs ending in suction caps called crochets. These are on the abdomen).

I clean them out every day, removing their tiny black walnut-shaped droppings and the food they haven't deigned to eat. This is to prevent mould, which is dangerous to silkworms.

Their food comes in plastic tubs and has the consistency of butter. It is supposed to be grated into their home, but really it is too soft for that. I usually end up with a few small pieces and hope that will do. I think it does, because if I listen carefully I can hear my silkworms eating. It is a contented munching sort of sound - and I think that if I could amplify this it would be quite relaxing.

Quite often they stop moving altogether and sit with their heads in the air. They are the texture of a creamy limestone and look so much like statues of themselves that I have been tempted to prod them to make sure they are not dead. However, I have just read on a website that this means they are getting ready to moult. One of them - the small pale one in the centre right of this picture is doing that now. His head seems to have changed a little too - his eyes are protruding and yellow. At least I assume they are eyes. (Added later: these are fake eyes. The real eyes are smaller and below the fake eyes).

Now a moulting silkworm should not be touched and I feel I am a little like a midwife or an expectant father, wondering what will happen next. I suppose, like all metamorphosing creatures, the silkworm is bristling with stem cells. Somewhere, perhaps on the inner layers of the old skin, there are cells reproducing and becoming something else as they mature and are bombarded with chemical messages: a part of a wrinkle or pigment, grow a bristle or a pore. After a few days a new silk worm will appear - his skin a little tender and vulnerable but otherwise just as perfect as he used to be - part of the series of little miracles which turns the egg into the moth.