A genus of tissue-invading flies that cause myiasis in sheep; the head botflies in the family Oestridae. Oestrus ovis (a nose fly) is a grayish brown, robust, hairy, beelike botfly, imported from Europe, and now a serious pest in parts of the U.S.; larvae are deposited by the adult fly in the nostrils of sheep, and inch-long larvae develop in the paranasal sinuses, causing considerable mucous discharge and distress in old or weak sheep. [G. oistros, gadfly]