domatium

from The Century Dictionary.

• noun A shelter formed on certain tropical plants for the protection of beneficial insects, especially mites and ants. See acarodomatium.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun entomology, botany A chamber produced by a plant in which insects, mites, or fungi live. It typically takes the form of a hollow under a leaf, or a system of tunnels in a thorn or stem. Ideally a domatium is a mutualistic adaptation, not to be confused with simple damage by a borer or gall-forming pest, though commonly there is no sharp distinction between domatia of value to the plant and galls caused by harmful aphids and mites for example.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a part of a plant (e.g., a leaf) that has been modified to provide protection for insects or mites or fungi