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.