Black Swallowtail "mating avoidance flight pattern"

A couple weeks ago I was lucky enough to pick a day for a hike when huge numbers of butterflies were out.  Sulphurs had emerged from alfalfa fields by the thousands, Monarchs were migrating, Pearl Crescents were on the goldenrod, but best of all the Black Swallowtails were out in force.  I usually consider myself lucky to see one or two; that afternoon I saw over a dozen in one field.

And as I watched, I saw a phenomenon I'd never encountered before.   Mating behavior is pretty easy to spot in this species, because they are large butterflies, and there's a subtle color diffference between male and female, and they are not very territorial, so when you see a "chase" you know it's a prelude to (attempted) procreation.

That afternoon I watched a female rebuff a male by flying away from the thistle where he had approached her.  He gave chase.  Instead of flying horizontally, she started to go vertical.  Totally vertical.  She was flapping herself up and up and up and up - to I would guess over a hundred feet in the air.  He was following along right behind her.  Understand these are large, powerful fliers who can move like the wind horizontally, but the vertical ascent was much slower and more laborious.  I watched these two black specks get smaller in the sky, and then...

Then she plummeted to earth.  I don't mean that she flew down.  She fell like a rock.  It was hard to see exactly, but it appeared that she just folded her wings above her head and let gravity take her down, because I didn't see even a flutter.  The poor male was left hovering a hundred feet in the air wondering where in the hell she had gone.  I would guess his pursuit is based on pheromones rather than visuals, so her sudden descent, probably right through her previous pheromone trail, might be totally undetectable.

I didn't see this just once.  I saw it four times with at least three different groups of BSTs, so I'm confident it was a standard avoidance maneuver rather than an accident or the quirk of an individual.  Later I spoke to a friend who's much more knowledgeable about butterfly behavior than I am, and he indicated that this type of flight pattern is well recognized, and has been documented among other butterflies, some of whom use it as a predator avoidance mechanism.  He suggested that I should document this somewhere - so that's why I'm writing this blog post.