Silkworms at 15 days old
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.