Armillaria is a genus of fungi that includes the A. mellea species known as honey fungi that live on trees and woody shrubs. It includes about 10 species formerly categorized summarily as A. mellea. Armillarias are long-lived and form the largest living organisms in the world. The largest known organism (of the species Armillaria ostoyae) covers more than 3.4 square miles (8.8 km2) in Oregon's Malheur National Forest and is between 1900 and 9600 years old. Some species of Armillaria display bioluminescence, resulting in foxfire.