At an earlier period matters would not have reached the desired stage; at a later period the acorn would be too mature.
The exterior of the acorn gives no indication whatever of the progress of this internal cookery.

In order not to inflict unsuitable food on the grub, the mother beetle, not sufficiently informed by the look of the acorn, is thus obliged to taste, at the end of her trunk, the tissues at the base of the cup.
The nurse, before giving her charge a spoonful of broth, tests it by tasting it.

In the same way the mother beetle plunges her trunk into the base of the cup, to test the contents before bestowing them upon her offspring.

If the food is recognised as being satisfactory the egg is laid; if not, the perforation is abandoned without more ado.

This explains the perforations which serve no purpose, in spite of so much labour; the tissues at the base of the cup, being carefully tested, are not found to be in the required condition.