Behind this compound door, the larva makes its arrangements for the metamorphosis.

The sides of the chamber are rasped, thus providing a sort of down formed of ravelled woody fibres, broken into minute shreds.

The velvety matter, as and when obtained, is applied to the wall in a continuous felt at least a millimetre thick.
(.039 inch .-- Translator's Note.) The chamber is thus padded throughout with a fine swan's-down, a delicate precaution taken by the rough worm on behalf of the tender pupa.
Let us hark back to the most curious part of the furnishing, the mineral hatch or inner door of the entrance.

It is an elliptical skull-cap, white and hard as chalk, smooth within and knotted without, resembling more or less closely an acorn-cup.

The knots show that the matter is supplied in small, pasty mouthfuls, solidifying outside in slight projections which the insect does not remove, being unable to get at them, and polished on the inside surface, which is within the worm's reach.