The fly in heraldry is a small but striking charge, usually shown as a stylized insect with wings displayed so it reads clearly on a shield. Compared with lions, eagles, or even bees, flies are uncommon, and when they do appear they often point to identity through wordplay rather than a universal “virtue symbol.” Heraldic writers note flies appearing in canting arms, where the charge echoes a surname or place name, such as Fieramosca in Venice, Vliege in Flanders, and Mouchard in France (Heraldica, “Insects in Heraldry”). That tendency makes the fly a very practical emblem, it helps viewers remember a name instantly.

Meaning still accrues around the image. A fly can imply persistence, alertness, and tenacity, since it is hard to drive away and quick to return, and it can also signal humility by choosing an intentionally modest creature. In blazon, you may see flies described simply as “flies,” or more specifically as gadflies or harvest-flies, and they are commonly shown volant (flying). Fox-Davies, in his classic discussion of insect charges, records coats using gadflies and a “harvest-fly … volant en arrière,” and notes how such insects were sometimes blazoned inconsistently as bees or flies in older compilations (A Complete Guide to Heraldry, Ch. 17 (Wikisource)).

For concrete examples, Heraldica highlights the French parliamentary family de Thou, bearing Argent, a chevron between three flies sable (Heraldica, “Insects in Heraldry”). For image browsing, Wikimedia Commons collects depictions across civic and family arms in Category:Flies in heraldry, and you can widen the net via Category:Insects in heraldry.