Fissure: a crevice: a narrow longitudinal opening: a slit.
Fissus: cleft: longitudinally divided nearly to base.
Fistula: a slender tube: specifically applied to the channel formed by the union of the two parts of proboscis in Lepidoptera.
Fistular: like a slender, cylindrical tube.
Flabellate: with long flat processes folding like a fan.
Flabelliform: fan-shaped.
Flabellum: a fan: a leafed structure: the transparent lobe at the end of the glossa in bees: also used as = flagellum; q.v.
Flabs: the lobes at the tip of the dipterous mouth:= labella; q.v.
Flaccid: feeble: limber: lax.
Flagelliform: whip-like; applied to a process.