Curious Caterpillars

Skyrail Nature Diary: May 2008

The Cairns Birdwing (Ornithoptera priamus euphorion) is one of the most beautiful butterflies in Australia; and with a wingspan of up to 18cm, it is also the largest.

Of particular interest is the large, spiny caterpillar of the Cairns Birdwing. Striking in appearance, the caterpillar grows up to 9cm in length and is dark with bright orange spines running along its back.

These caterpillars are quite sluggish and rotund, spending most of their time lazing around on the underside of leaves. They are also very hungry creatures: just a few Cairns Birdwing caterpillars can devour the foliage of an entire plant!

Despite their insatiable appetites, these caterpillars are very picky eaters: feeding only on the vigorous, native large-leafed vine known as the Dutchman’s Pipe (Aristolochia tagala).

Even though the leaves of the Dutchman’s Pipe are poisonous to humans, the caterpillars eat them without trouble and store the poison in the orange spines on their backs. These toxic spikes are the caterpillar’s inbuilt defence mechanism: protecting them from being eaten by birds!

As it grows, the Cairns Birdwing caterpillar will shed its skin several times in a process known as ‘moulting’. When the caterpillar is fully grown, it will moult a final time to become a ‘pupa’ or ‘chrysalis’. After spending around a month in the pupa stage, a beautiful Cairns Birdwing butterfly will emerge.

Whilst the male and female butterflies of this species look completely different, they are equally impressive.

The male butterfly is slightly smaller in size than the female and has rich upper wing markings in an attractive combination of jade, gold and black. The female is black and white in colour, with distinctive yellow markings on her hind wings.

Life is short for the Cairns Birdwing butterfly: they only live for four or five weeks.

To ensure the survival of the species, it is imperative that the butterflies find themselves a mate as soon as possible. In fact, the female butterfly often mates as soon as she emerges from the pupa.

When the female butterfly is ready to lay her eggs, she searches for the correct host plant for her caterpillars: the Dutchman’s Pipe. To find the right plant, the female butterfly will fly from leaf to leaf, “tasting” each one with the chemical receptors in her forelegs.

Whilst the Cairns Birdwing caterpillars rely on the native Dutchman’s Pipe to live, the exotic species of Dutchman’s Pipe (Aristolochia elegans), from South America, is deadly to the Cairns Birdwing. Unfortunately, the Cairns Birdwing butterfly cannot differentiate between the two species; and the female often lays her eggs in the poisonous exotic species. The popularity of this introduced plant is detrimental to the Cairns Birdwing population.

Pictures are courtesy of the Wet Tropics Management Authority