Noun
A small beaklike process or extension of some part; a small rostrum; as, the rostellum of the stigma of violets, or of the operculum of many mosses; the rostellum on the head of a tapeworm.
Source: Webster's dictionaryA small splinter of wood or a grass stem is used to lift the rostellum or move the flap upward, so the overhanging anther can be pressed against the stigma and self-pollinate the vine. Source: Internet
When the pollinator enters another flower of the same species, the pollinium has taken such position that it will stick to the stigma of the second flower, just below the rostellum, pollinating it. Source: Internet