I had the first mini breakthrough of my PhD last month! I've been working with Pterostylis longifolia, a common, multi-flowered greenhood which flowers during winter in the wetter forests of south-eastern Australia. I've been 'presenting' the flowers in various locations around the country-side, a method known as 'baiting', hoping to attract pollinators to the flowers...and it worked!
Take a look at the video below...can you guess what's happening?
A gnat is attempting to mate with the orchid's labellum!
Basically, we suspect the orchid flower is releasing chemical signals which mimic the sex pheromones of the female gnats. The males, attracted by the chemistry, arrive at the flower. The males then start fluttering their wings, a behaviour known as 'wing-fanning', which is part of the copulatory routine of many gnats. This appears to be directed towards the labellum of the orchid, which the gnat may mistake for a female. Not all gnats are convinced, but if they decide the labellum is actually a female, they proceed to attempt to mate with the labellum by curling their abdomen, probing for the corresponding female genitalia. One poor dude (gnat) spent over one minute trying to mate with the labellum.
One of the coolest things about greenhood orchids is that the labellum is like a loaded spring, and flips upwards when something disturbs it. In the video above, the labellum remains down and the gnat eventually gets bored and leaves. But have a look at what happens when the gnat triggers the labellum, below.
After the labellum catapults the gnat into the 'hood' of the greenhood, the gnat is essentially in a trap, with only one way out. It has to find its way through a small passageway, moving towards the tip of the hood where it brushes past the pollen and receives a package of pollen on its back. Kind of like a backpack. In the video below, you can see the gnat struggling around inside the flower, before figuring out his escape route.
An orchid gets pollinated when one gnat visits another orchid flower, gets trapped, escapes with a backpack of pollen with it, then visits another flower, gets trapped and deposits the precious little bundle. In the final video, a gnat arrives with a backpack of pollen but unfortunately the labellum doesn't flip and the pollen isn't transferred.
[A small footnote: we're fairly confident this is what is going on. This pollination method is known as sexual deception, and has been confirmed in other orchids, including one other species of greenhood orchid. I am currently in the process of confirming that only males are attracted to the orchids, and that males of only one species are involved. The next step will be to determine exactly which chemicals the orchids use to attract the gnats].