domatium

noun1

1. •
   A chamber produced by a plant in which insects, mites, or fungi live.

   • A domatium typically takes the form of a hollow under a leaf, or a system of tunnels in a thorn or stem. Ideally, it is a mutualistic adaptation and should not be confused with simple damage by a borer or gall-forming pest, although commonly there is no sharp distinction between domatia of value to the plant and galls caused by harmful aphids and mites, for example.