inquiline

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An animal that characteristically lives commensally in the nest, burrow, or dwelling place of an animal of another species.
• adjective Being or living as an inquiline.

from The Century Dictionary.

• noun In zoology, an animal that lives in an abode properly belonging to another, either at its expense, as certain insects that live in galls made by the true gall-insects, or merely as a cotenant, as a pea-crab which lives in an oyster-shell, or a sea-anemone growing on a crab's back; a commensal. See cut under cancrisocial.
• Having the character of an inquiline; commensal.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) A gallfly which deposits its eggs in galls formed by other insects.

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

• noun biology An animal that lives commensally in the nest, burrow, gall, or dwelling place of an animal of another species.
• noun biology An organism that lives within a reservoir of water collected in the hollow of a plant stem or leaf.