I have been writing about Cairns Birdwing butterflies quite often but there are still more things to say (and our visitors always seem delighted and fascinated by them) so I will keep adding to the story.
Pupation, the process of forming the chrysalis, changing inside it and emerging as an adult, is one of the most baffling processes in nature. I’m not even going to speculate about how an insect could have evolved such a bizarre practice but want to share my observations of the beginning and end of it.
Birdwing caterpillars eat until they are big enough and then leave the vine they have been feeding on and attach themselves to a nearby plant, often a metre or two off the ground (sometimes they drop from the vine to the ground to crawl up the stem of another plant). The one in my first photo chose the twig of a rose bush and I was intrigued to see that it reinforced the rose with strands of silk running down from the branch, over the leaf and around the twig – a sensible precaution against the leaf falling off, of course, but again one has to wonder how it evolved (as usual, click the photo to see it better). It then made a silk sling from the plant around what would be its shoulders if it had shoulders, and hung tail down for a surprisingly long period – a couple of days.
But how do they get into their chrysalis? I have to admit to relying on vague memories of silkworms, but those memories put me on the wrong track entirely because they don’t “get into” the chrysalis at all. Rather, they form it under their skin, then shed the skin just as they have already done several times during their growth as a caterpillar.
I actually woke up to that – and to my own ignorance – when I saw a fresh chrysalis, not yet hardened, flexing to shrug off the last part of its skin. In this second photo (not the same caterpillar, of course) you can see the crumpled skin, complete with head-shell and legs, still barely attached to the chrysalis.
A related point is the difference between a chrysalis and a cocoon. As Museum Victoria point out here, “the skin that emerges from under the old skin is a chrysalis. Both butterflies and moths develop a chrysalis in their pupal stage, but the chrysalises of moths are usually contained inside a woven structure made of silk – a cocoon.” Silkworms are the caterpillars of moths so they form cocoons around their chrysalises in just this way – see Wikipedia for more information.
The really mysterious stuff happens in the chrysalis, over the next month or so in the case of our Birdwings, and then the chrysalis splits and a damp butterfly emerges with crumpled wings. As soon as they straighten and harden, she – or he, as seen below – is off and away in search of nectar or a mate.