[Photo © M.Hoskovec]

The left picture shows a hollow in the trunk of horse chestnut (Aesculus hippocastanum) with several exit holes of Rhamnusium bicolor bicolor.

The right picture shows a detail of the exit holes, some of them are filled with a fibrous frass. This frass (created by the last instar larvae) protected pupae and/or immature adults of Rhamnusium bicolor bicolor before their emergency.