Catholic Encyclopaedia:
The flabellum, in liturgical use, is a fan made of leather, silk, parchment, or feathers intended to keep away insects from the Sacred Species and from the priest. It was in use in the sacrifices of the heathens and in the Christian Church from very early days, for in the Apostolic Constitutions, a work of the fourth century, we read: Let two of the deacons, on each side of the altar, hold a fan, made up of thin membranes, or of the feathers of the peacock, or of fine cloth, and let them silently drive away the small animals that fly about, that they may not come near to the cups. Its use was continued in the Latin Church to about the fourteenth century... Among the ornaments found belonging to the church of St. Riquier, in Ponthieu (813), there is a silver flabellum, and for the chapel of Cisoin, near Lisle, another flabellum of silver is noted in the will of Everard (died 937), the founder of that abbey. When, in 1777, Martène wrote his Voyage Littéraire, the Abbey of Tournus, on the Saône in France, possessed an old flabellum, which had an ivory handle two feet long, and was beautifully carved; the two sides of the ivory circular disc were engraved with fourteen figures of saints. Pieces of this fan, dating from the eighth century, are in the Musée Cluny at Paris, and in the Collection Carrand... The inventory, taken in 1222, of the treasury of Salisbury, enumerates a silver fan and two of parchment. The richest and most beautiful specimen is the flabellum of the thirteenth century in the Abbey of Kremsmünster in Upper Austria. It has the shape of a Greek cross and is ornamented with fretwork and the representation of the Resurrection of Our Lord.