Article and images by Michael Mills, community conservation officer, Sunshine Coast Council
The Richmond birdwing butterfly (Ornithoptera richmondia) is pretty well known these days, most are aware of the very large and colourful male at least, but the caterpillar and pupa are less often in the spotlight; but, no less interesting. At hatching, these spiked little devils start out at about 2mm in length, that’s approximately the width of your standard phone charger cable! They don’t stay that way for long, reaching up to 70mm in six weeks before pupating. These caterpillars are voracious, with a large 10+ year old vine only able to sustain a couple caterpillars at a time. The caterpillar sheds skin to grow during this time, each time changing form and appearance slightly. These forms are referred to as “instars” and the Richmond birdwing caterpillar will go through 4–6 instars.
The host plant Pararistolochia praevenosa contains aristolic acid, making the caterpillars toxic to vertebrate predators, they are still predated on by other invertebrates, including other larvae! In situations where there is not enough foliage of the host plant, the caterpillars may cannibalise one another. Due to this cannibalistic nature, when pupating the caterpillar will often try to leave the host vine to pupate, so as not to be found by another caterpillar in a vulnerable state. For this reason, if planting the vine it is best to try select an area where there is foliage from other plants available.
The pupa attaches with a silk “cremaster” at the tail end. Due to the size and weight of the caterpillar/pupa at this stage in the life cycle, some additional support is required and they attach a silk girdle around the midsection to help hold them up on the underside of the selected leaf.
Richmond birdwings typically have 2 pupation cycles per year, pupas in the summer months can develop to butterfly within 28 days, those pupating during cooler months and over winter can take up to 250 days to develop.