Fly

From Dwarf Fortress Wiki

Flies are vermins that will occur in swarms. They will rot your unprotected food (including unpicked farm crops) and they cannot be trapped. They will spawn from pools in any non-freezing biome. All your dwarves will just hate them. Unlike most other kinds of vermin, cats can not kill them.

For some unkown reason, flies will spawn at the edge of magma as well as where magma is melting stones. They will stop spawning from it after a while though.

The Acorn fly is a type of fly only found in savage biomes.

Game Object Data
[CREATURE:FLY] [NAME:fly:flies:fly] [TILE:250][COLOR:0:0:1] [VERMIN_MICRO][VERMIN_ROTTER][VERMIN_GROUNDER][FREQUENCY:100][VERMIN_HATEABLE] [FLIER][VERMIN_NOTRAP] [SMALL_REMAINS][NATURAL][NOBONES] [SPEED:2900] [NOT_BUTCHERABLE] [PREFSTRING:ability to annoy] [DIURNAL] [NO_SLEEP] [BIOME:NOT_FREEZING] [BIOME:ANY_POOL] [STANDARD_FLESH] [POPULATION_NUMBER:2500:5000] [CLUSTER_NUMBER:100:200] [MUNDANE]