Greenhood orchids look plain but they are little marvels of nature, with the flower being an elaborate trap that forces a gnat or mosquito into a chamber where it brushes past the pistil (female bit) and anther (male bit) to pollinate it. The trapdoor is a modified petal called the labellum.
The dorsal (upper) sepal and petals are fused to form the hood containing the column that forms the tube.
The lower (ventral) sepal together make the landing platform in which, a spring loaded petal called the labellum closes the trap when gnat passes over it.
I dissected a Dark-banded Greenhood Orchid to show how it works. The gnat is attracted to the flower by the promise of a nectar feed and possibly pheromones that smell like a female. When the gnat crosses the labellum it snaps shut against the column. To escape, the gnat climbs up the light-filled hood, through the tube in the column and squeezes past the pollen. Hairs at tube entry stop the gnat moving backwards. Arrows in the image below show the gnat's journey through the flower.