This membrane, semi-translucid, is the door to the chamber of metamorphosis, its protection against the evil intentions of external creatures.
It is also the only obstacle which the adult will encounter at the moment of exit.

To lessen the difficulty of opening it the grub takes the precaution of gnawing at the inner side of the skin, all round the circumference, so as to make a line of least resistance.

The perfect insect will only have to heave with its shoulder and strike a few blows with its head in order to raise the circular door and knock it off like the lid of a box.

The passage of exit shows through the diaphanous skin of the pea as a large circular spot, which is darkened by the obscurity of the interior.

What passes behind it is invisible, hidden as it is behind a sort of ground glass window.
A pretty invention, this little closed porthole, this barricade against the invader, this trap-door raised by a push when the time has come for the hermit to enter the world.