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.