Sometime in the remote past we received the pea, growing it at first in the prehistoric vegetable garden which already supplied the bean.  It was found a better article of diet than the broad bean, which to-day, after such good service, is comparatively neglected.  The weevil was of the same opinion as man, and without entirely forgetting the bean and the vetch it established the greater part of its tribe upon the pea, which from century to century was more widely cultivated.  To-day we have to share our peas:  the Bruchidae take what they need, and bestow their leavings on us.

This prosperity of the insect which is the offspring of the abundance and quality of our garden products is from another point of view equivalent to decadence.  For the weevil, as for ourselves, progress in matters of food and drink is not always beneficial.  The race would profit better if it remained frugal.  On the bean and the vetch the Bruchus founded colonies in which the infant mortality was low.  There was room for all.  On the pea-vine, delicious though its fruits may be, the greater part of its offspring die of starvation.  The rations are few, and the hungry mouths are multitudinous.

We will linger over this problem no longer.  Let us observe the grub which has now become the sole tenant of the pea by the death of its brothers.  It has had no part in their death; chance has favoured it, that is all.  In the centre of the pea, a wealthy solitude, it performs the duty of a grub; the sole duty of eating.  It nibbles the walls enclosing it, enlarging its lodgment, which is always entirely filled by its corpulent body.  It is well shaped, fat, and shining with health.  If I disturb it, it turns gently in its niche and sways its head.  This is its manner of complaining of my importunities.  Let us leave it in peace.

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.