Horizontal galea Pterostylis concava Greetings fellow Foxies,
Greenhood orchids (Pterostylis species) look plain but their flowers are little marvels of nature, which trap insect pollinators (mainly Mycetophilidae and Sciaridae families, commonly known a fungus-gnats and dark-winged fungus-gnats respectively). The hood called a "galea" consists of the dorsal sepal fused with two lateral petals. The galea curves forward to form a light-filled chamber containing the column. The column, which contains the sexual parts of the flower (stigma and pollinia) has wings which fold around to form a tube.
The lower sepals are partially joined make a landing platform on which, the lower highly modified petal, the labellum closes the trap when gnat passes over it.
Greenhood orchids (Pterostylis species) look plain but their flowers are little marvels of nature, which trap insect pollinators (mainly Mycetophilidae and Sciaridae families, commonly known a fungus-gnats and dark-winged fungus-gnats respectively). The hood called a "galea" consists of the dorsal sepal fused with two lateral petals. The galea curves forward to form a light-filled chamber containing the column. The column, which contains the sexual parts of the flower (stigma and pollinia) has wings which fold around to form a tube.
The lower sepals are partially joined make a landing platform on which, the lower highly modified petal, the labellum closes the trap when gnat passes over it.
There are two basic galea orientations. Local examples are
- Horizontal galea with a short labellum:- dark banded greenhood orchid P. sanguinea, frog greenhood orchid P. sargentii, cupped greenhood orchid P. concava, and painted rufous greenhood orchid P. picta.
- Semi- vertical galea with long labellum:- jug orchid P. recurva, red-veined shell greenhood orchid P. hamiltonii, hairy stemmed snail greenhood orchid P setulosa, bird orchid P. barbata.
 Vertical galea P. hamiltonii |  Vertical galea P. setulosa |  Vertical galea P. recurva |  Vertical galea P. barbata |
Male gnats are attracted to flowers by a female pheromone scent, and walk over the labellum, which snaps shut and traps them. Light from the translucent galea above guides them through the column tube where they press against the stigma and pollinia to fertilise the flower then attach a pollen sac to the gnat's back.
The labellum is a miracle of evolution: a petal which has changed into a tiny trapdoor with an attachment on its base called a lure, and a hinge attachment. The lure is a counter-weight and pheromone emitter. I noticed that jug and shell orchid lures had pinned and killed unfortunate gnats that triggered them.
The labellum is a miracle of evolution: a petal which has changed into a tiny trapdoor with an attachment on its base called a lure, and a hinge attachment. The lure is a counter-weight and pheromone emitter. I noticed that jug and shell orchid lures had pinned and killed unfortunate gnats that triggered them.
 Pterostylis sargentii |  Pterostylis sanguinea |  Pterostylis picta |  P. hamiltonii interior of flower |
When a 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 pollenia. Hairs at tube entry stop the gnat moving backwards. Arrows in images below show a gnat's journey through the flower.
If the labellum shuts and the flower is not pollinated, it reopens in ten to thirty minutes.
If the labellum shuts and the flower is not pollinated, it reopens in ten to thirty minutes.
 Pterostylis concava |  Pterostylis recurva |
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