It’s mid-autumn in Sydney, and I’m still seeing butterflies and moths flitting around and laying eggs, usually on our grapevine and lime tree. One of the egg-layers was this Orchard Swallowtail, Papilio aegeus aegeus:
If you look carefully at the enlargement below you can, with a bit of help from Photoshop, make out the spherical creamy-brown egg at the tip of the butterfly’s abdomen:
This egg is the beginning of another orchard swallowtail lifecycle. After a week it hatches into a caterpillar. At first, the caterpillar’s colour is brown with white patches, possibly as camouflage as a bird dropping.
A caterpillar’s skin doesn’t grow with the rest of its body; it needs to be regularly shed and replaced with a new larger skin. The gap between each skin-shedding is called an instar. In its later instars, an orchard swallowtail caterpillar changes its predominant colour from brown to green:
The orchard swallowtail caterpillar is typical in having six true legs (the brown ones at the front) and ten pro-legs (the white ones below the middle and rear of its abdomen), and six simple eyes. Less typical is its main defensive mechanism. Behind its head, the caterpillar has a forked gland called an osmetarium, capable of producing a strong citrus smell. When it is disturbed, the caterpillar turns the osmetarium inside out to produce what looks like a pair of red horns, and rears up on its pro-legs in a defensive threat display:
Caterpillars can be described as eating machines, and the Orchard Swallowtail caterpillar is no exception. It munches on green leaves such as citrus, and grows rapidly. After a month, a 0.5mm egg has become a 60mm caterpillar ready to perform the final moult where it changes into a chrysalis within which it can pupate. Before pupating, the caterpillar spins a sticky silk pad which it attaches to a branch. The chrysalis adheres to this pad with velcro-like hooks, and is stabilised by a little loop of silk from the middle of its body. The chrysalis may be camouflaged green, brown or grey:
Depending on the season, it takes between one and six months before the adult imago emerges from the chrysalis. At first, the wings are crumpled, and have to be straightened by pumping a liquid (haemolymph) into the wing veins, a process that takes several hours. The resulting imago, if it’s female, looks like this:
And like this, if it’s male:
The relative sizes in the last two photos are deceptive – the male’s wingspan is about 12cm and the female’s 14cm. Although orchard swallowtails are classified as members of the swallowtail family of butterflies, they don’t actually have swallowtails on their hind wings.
Like most butterflies, the imagos have just two interests, nectar and sex. For the nectar, the coiled proboscis is straightened to probe a flower like a drinking straw. For the other, the males tend to patrol regular beats until they see a female. The male then puts on an aerial display, hovering above and around the female with rapid wingbeats:
He then pauses to assess his reception:
It’s a positive one:
And with the female fertilised, we’re back to the first picture.