Possible Clues:

• Bone cavities
• Sinus cavities
• Anatomical cavities
• Cavities in bones
• Bony cavities, anatomically
• Some subterranean travel in natural hollows
• With an inversion of painting one knows where they are, by the sound of them
• Bone cavities in human trapezia
• Natural cavities in bone; ratan (anag.)
• Sounds as if one knows they're near to the insect artist