Sometimes, but more rarely, the hole is drilled through the cup itself.
Let us select those acorns which have been recently perforated: that is to say, those in which the perforation is not yet surrounded by the brown ring which appears in course of time.

Let us shell them.

Many contain nothing out of the way; the Balaninus has bored them but has not laid her eggs in them.

They resemble the acorns which for hours and hours were drilled in my laboratory but not utilised.

Many, on the contrary, contain an egg.
Now however distant the entrance of the bore may be, this egg is always at the bottom of the acorn, within the cup, at the base of the cotyledonary matter.