Thynnids are seriously sexy wasps
Updated: Mar 19, 2019
You may have seen some medium to large, chunky-bodied, often dark-coloured wasps visiting nectar-rich plants such as flowering ti-trees and leptospermums. If you’re observant and lucky you may have seen a winged male visiting flowers while copulating with a wingless female. If you've seen this, you've probably seen a thynnid, a remarkable and seriously sexy group of wasps.
There are at least 2000 species of thynnid wasps in Australia, although only around 600 have been named so far. All species are parasitoids – insects whose larvae feed on and eventually kill their host. (True parasites live in or on but rarely kill their hosts, but things always end badly for the hosts of parasitoids. The alien creature in the Aliens movie is a parasitoid, and could well be modelled on a thynnid wasp.)
When a female thynnid hatches from a pupa underground, she has three tasks: get mated, get fed, then lay eggs. She climbs to the end of a twig or blade of grass, releases a scent (a pheromone) that wafts on the breeze to form an olfactory trail for any males in the vicinity, and waits. A male will follow her scent trail, pick her up (literally), then fly off with her. Mating begins immediately and lasts for several hours, during which time he will take her to nectar-rich flowering shrubs where she has the only meal in her life. Sex and food are closely coupled for thynnid wasps.
After mating, the male drops the female to the ground, where the serious business of egg-laying begins. Almost all species hunt the grubs of scarab beetle (Christmas beetles and their relatives), using powerful front legs to dig for the grubs in their underground burrows. One unusual species of thynnid, the so-called blue ant (Diamma bicolor) hunts mole crickets instead.
When a female finds a suitable host for her larvae, she paralyses it with a sting, then lays a single egg. The beetle, unable to move, comes to a grisly end. The wasp larva eventually pupates in the beetle's underground chamber, before hatching next season to repeat the whole process.
Of course, in biodiversity there's always more to the story, and this is no exception. A remarkable facet of the thynnid story is that an unusually high number of cross-species matings seem to occur. For a thynnid taxonomist, catching a mating pair is like gold - the males and females are quite different-looking, and before DNA fingerprinting it was often difficult to match the sexes into one species. So thynnid taxonomists are always keen to catch mating pairs.
Which is why it's possible to say that unusually often a female will be caught mating with a male from another species Strictly speaking, this shouldn't happen, and for most species it usually doesn't.
The reason, of course, is eminently sensible. Remember that a female thynnid can only feed in copulo. If a female fails to attract the right male, it's possible that she subtly changes the chemistry of her pheromone trail to pick up anyone who may be around. She gets fed and it's possible that, recognising the mismatch, she rejects the male's sperm but lays unfertilised but viable eggs anyway, giving rise to larvae that are clones of herself. There is a suspicion that something like this is involved because, even though extra-species couplings are known to happen quite a lot, true hybrid wasps are rarely found.
Of course, the extreme case of an extra-species coupling is when individuals from two different phyla mate. And this also happens with thynnids, when the males get seriously confused between a female thynnid and an orchid flower. We'll write that story out soon.