The Control of Pupation in the Silkworm
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.