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Monthly Science: Chestnut Weevil Damage
The dried chestnuts looked undamaged in their husk, but three groups of weevil grubs surely left some damage behind:

Chestnut husk – dried Gingerly prying the seeds out revealed holes in all three:

Chestnut weevil damage – exterior The weevils converted the nut meat into what looks like solid frass:

Chestnut weevil damage – interior Having eaten themselves out of house and home, they moved on to the next plane of existence.
For most of them, that would be bird food.