When the man of God had leisure to read his holy books, the fly would trot up and down his codex: and should some one call him, or he had to go about other business, he would instruct the fly to sit down upon the line at which he had halted, and keep his place until he should return to continue his interrupted reading: which the fly infallibly would do.
--Story told of Saint Colman mac Duach of Ireland, "Saint Colman and the Cock, the Mouse, and the Fly," from John Colgan (d. ca 1657), Acta Sanctorum Hiberniae [Acts of the Saints of Ireland] (folio, 1645) 1.244a, as cited in Plummer (ed.) Vitae Sanctorum Hiberniae (Oxford, 1910), p. cxliv. I believe the source of this English version of the story is Helen Waddell's Beasts and Saints (London, 1934, repub. 1996).
Quotation found online in sidebar in The Continuum Encyclopedia of Animal Symbolism in Art by Hope R. Werness (pub 2004)