It profits so greatly and so swiftly by its position that by the time the dog-days have come it is already preparing for its approaching liberation.  The adult is not sufficiently well equipped to open for itself a way out through the pea, which is now completely hardened.  The larva knows of this future helplessness, and with consummate art provides for its release.  With its powerful mandibles it bores a channel of exit, exactly round, with extremely clean-cut sides.  The most skilful ivory-carver could do no better.

To prepare the door of exit in advance is not enough; the grub must also provide for the tranquillity essential to the delicate processes of nymphosis.  An intruder might enter by the open door and injure the helpless nymph.  This passage must therefore remain closed.  But how?

As the grub bores the passage of exit, it consumes the farinaceous matter without leaving a crumb.  Having come to the skin of the pea, it stops short.  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.  Shall we credit it to the Bruchus?  Did the ingenious insect conceive the undertaking?  Did it think out a plan and work out a scheme of its own devising?  This would be no small triumph for the brain of a weevil.  Before coming to a conclusion, let us try an experiment.

I deprive certain occupied peas of their skin, and I dry them with abnormal rapidity, placing them in glass test-tubes.  The grubs prosper as well as in the intact peas.  At the proper time the preparations for emergence are made.

If the grub acts on its own inspiration, if it ceases to prolong its boring directly it recognizes that the outer coating, auscultated from time to time, is sufficiently thin, what will it do under the conditions of the present test?  Feeling itself at the requisite distance from the surface, it will stop boring; it will respect the outer layer of the bare pea, and will thus obtain the indispensable protecting screen.

Nothing of the kind occurs.  In every case the passage is completely excavated; the entrance gapes wide open, as large and as carefully executed as though the skin of the pea were in its place.  Reasons of security have failed to modify the usual method of work.  This open lodging has no defence against the enemy; but the grub exhibits no anxiety on this score.