More bonkers inflatable lepidopteran organs! Only these are called osmeteria, and they’re meant to drive things away when they annoy caterpillars rather than attract things that moths want to sex.
See, when something irritates or tries to eat a caterpillar, they inflate their little osmeterium with caterpillar-blood.
Above: Harassing invertebrates for science!
This turns the organ inside out and exposes their stink glands, which they can then use to stink really aggressively at whatever it is they’d like to go away. (This is actually a really popular defense mechanism in the animal kingdom. And the plant kingdom. Things don’t like to eat things that smell like ass, really.)
So what’s up with the “You want some?” posture?
It is actually a “You want some?” posture. Lots of caterpillars have defensive mechanisms that involve rearing up and biting at things that piss them off. If you recall what I said about the gumleaf skeletoniser (and its incredibly fancy hat), you’ll remember that it can actually bite the legs off parasitic wasps.
Caterpillars can be pretty violent, when they want to be. Lots of other caterpillars will try to startle predators by flicking their heads or tails around wildly, and let’s face it, caterpillars don’t exactly have wings to get their stench-chemicals out there, so waving their heads around is a good way to spread the pain.