Creature Feature: Orchids and Wasps

April 24, 2022

I've decided to start an irregularly-recurring segment, tentatively entitled Creature Feature, where I word barf about interesting critters, flora, and fungi. I'm doing this because I know more weird facts than is healthy for one person and have a captive audience to share them with! We all win! (Or not, if you're not really keen on listening to me prattle excessively.)

We're starting off with an installment featuring wasp-pollinated orchids, collectively making up the tribe Drakaeinae, and their respective thynnine wasp pollinators. I warn you that I'll be talking a bit about bug sex in this one; I have no legal liability for any emotional distress that may occur if you keep reading. All images require you to click a button before you see them, so you can read this even if you don't like seeing wasps (or plants that look like wasps).

Drakaeinae is native to Australia and made up of 5 genera: Caleana (duck orchids), Chiloglottis (wasp orchids), Drakaea (hammer orchids), and Arthrochilus and Spiculea (both commonly referred to as elbow orchids).

Photos of the genera in Drakaeinae
Left to right: Arthrochilus, Caleana, Chiloglottis, Drakaea, Spiculea

While most plants reward their pollinators in some form, these orchids ensure successful pollination by, if I may be crude, tricking male wasps into fucking them. Understanding why this strategy works requires a discussion about the wasps.

Thynnine wasps are solitary wasps where many, but not all, species have extreme sexual dimorphism. (When I refer to thynnine wasps from here, I'm talking about this subset of them.) The males are significantly larger than the females, and the females can't fly.

Photos illustrating the sexual dimorphism in thynnine wasps
Thynnine wasps
Left: A male thynnine wasp carrying a female attached to his abdomen. Right: A female thynnine wasp waiting for a mate on a blade of grass.

Since the female wasps can't fly, all they can do to get men to notice them is crawl up on a blade of grass or stem, emit pheromones, and wait. When a male wasp finds a female, he picks her up by sticking his genitals in her and and flying off. (There was no nicer way to phrase that. Sorry!) The wasps mate in the air, then the male carries his woman (who, I will remind you, is still hanging off his penis) to a flower. Depending on the species, the female will either help herself to nectar, or the male will feed her... also through his penis. After all this, he takes her back to the ground to lay her eggs.

The labellum of an orchid is the part of the flower that attracts insects. The orchids in Drakaeinae have extremely specialized labella that resemble a female wasp in some form. The visual resemblance isn't very precise, but it doesn't need to be because the orchids attract male wasps by mimicking female pheromones rather than visual cues. Once a male arrives at the orchid, he may grab onto the fake wasp and try to mate with it. As he does this, he'll try very hard to pull the labellum off of the flower. The orchids have mechanisms that take advantage of the male's momentum to dunk him into their reproductive organs.

In Arthrochilus, the male's weight pushes him headfirst into the column of the flower. The wings around the column have small hooks that hold him there long enough to collect or distribute pollen.

In Caleana, the labellum is attached to the column by a flexible piece of stem that bends underneath the weight of a wasp. When a wasp lands on the labellum, it bends down and the wasp gets held at the orchid's reproductive organs by a pair of wings around the column.

In Chiloglottis, when the male attempts to pull the labellum off, his weight will dip his front half down, pushing his backside up to come into contact with the column. If the wasp already has pollen on his back, he'll pollinate the flower. If he doesn't, he'll get a sticky film put on him by the rostellum. Then, his movement will open the anthers, which will deposit pollen on his thorax.

In Drakaea, the stem attached to the labellum suplexes the wasp in the direction of the rest of the flower, forcing his thorax to come into contact with the column. Unlike many of the other orchids here, Drakaea doesn't use physical restraint to ensure the wasp picks up or leaves pollen.

In Spiculaea, the column is located above the labellum. When a wasp tries to fly off with the labellum, he'll go up into the column, where the column wings hold him there long enough for his abdomen to come into contact with the flower's reproductive organs.

Drakaea pollination
Drakaea pollination
A hammer orchid (Drakaea) getting pollinated by a wasp. His weight has pulled his head into the flower, with his thorax brushing against the column.

The vast majority of orchid-wasp relationships are one-to-one (in other words, one species of orchid is pollinated by one species of wasp). In order for the orchids to be convincing enough for a wasp to mate with them, they had to put all their eggs into one wasp basket; thynnine wasps vary so much in size and mating mechanics that it's effectively impossible to be a generalist. A wasp may try to mate with an orchid from the species he doesn't normally pollinate, but he will quickly realize something isn't right and leave before any pollen can stick to him.

The orchids are a bit of a problem for the wasps, especially females. The males waste time and energy in the mating attempt, but otherwise aren't harmed; it doesn't appear that wasting time on orchids affects a male's future reproductive success. On the other hand, females have a big problem, as they rely on their mates for food and transportation.

Still, male wasps generally try to avoid the inconvenience of the orchids by modulating their response to pheromones depending on how many orchids are in an area. In an orchid-dense location, male wasps may visit the site of pheromone emission, but the chance that they'll actually go through with a copulation attempt is significantly lower than it is in a place with fewer orchids. It's hypothesized that males can learn and remember orchid locations to avoid. However, they can't distinguish between the real and mimic pheromone; in orchid-dense areas, they will indiscriminately avoid trying to mate with any source of pheromones, wasp or orchid. It's currently uncertain how females cope with the orchids due to their relatively low amount of agency in the mating process. One possibility is that they also avoid orchid patches.

If you're wondering how this relationship persists despite the wasps very obviously getting the short end of the stick, wasp-on-orchid mating attempts are actually pretty rare. Pollination via sexual deception has managed to stay stable despite relatively low rates of contact between the plants and their pollinators; orchid pollen is bundled into single units, so it only takes one wasp for pollination to be successful. Thus, the orchid and the wasp are able to coexist without too much friction.

One last factoid for this very factoid-filled blog post: thynnine wasps are parasitic themselves! The females lay their eggs on ground-dwelling beetle larvae after paralyzing them, and a newly-hatched wasp will emerge to a beetle feast. This is wonderful for the wasp baby, but not so fun for the beetle baby. In life, sometimes you're the wasp, and sometimes you're... also the wasp.

For further reading, click here to see the list of references I used to write this.