Reviewing my several years of contributions to this esteemed magazine, I realise I’ve been biased towards beetles and moths. The latter usually because I’m late to submit, I’m wondering what to write about, and then fortuitously I see some moth about to be squashed on Helensburgh Station.
Dragonflies and damselflies are not flies at all, of course, but they have large wings hence the name. Dragonflies have wings held rigidly at a 90-degree angle to the body, damselflies can fold their wings along their bodies at rest. The two groups are put in the order Odonata, so biologists refer to them as odonates. Odonates are the oldest of fossil winged insects – they were around in the Late Carboniferous swamp forests 300 million years ago (mya), the same forests that became coal in the UK (our coal is from much later, a mere 220 mya).
Those first odonates were huge, with wingspans up to 70cm, a size never regained in their later history. By comparison, the largest modern odonates are South American damselflies, up to 17cm in wingspan. And the largest modern dragonflies are our very own species of Petalura, up to 16cm in wingspan (P. ingentissima in North Queensland).
Here we are lucky to have one of the Petalura on our backdoor – Petalura gigantea is a frequently recorded species in the peat bogs of the escarpment. However, despite the name, it is only 12.5cm in wingspan.
Petalura belongs to the Petaluridae, a family of only 11 species whose fossil wings have been dated back to 130 mya, peak dinosaur time. So petalurids are ‘living fossils’.
How have they managed to survive so long, especially in this dry continent? The key might be found in their larvae which live in deep burrows (up to 75cm) in wet peaty soil, unlike most modern dragonflies and damselflies, which are truly aquatic.
From these burrows, they ambush prey, which may include small frogs. The larvae live for about five years and grow to about 5cm long. Recent studies of peat bogs in the Blue Mountains after wildfire have shown that the larvae survived in their deep burrows.
Insects breathe by simply allowing air to filter through the body via paired holes (spiracles) in the sides of the body, not unlike the stomata on the surface of leaves. However, plants don’t fly, so it’s probably not important to have such an inefficient system. Insects do, so their size is constrained by that inefficiency, and surface area to body volume ratio. But the largest modern dragonflies are much smaller than their fossil ancestors. How come?
One theory is that there wasn’t anything around to eat them – they were the apex aerial predators of the day. But another is that they could be bigger simply because the Late Carboniferous atmosphere had 20% more oxygen. We are now in the process of depleting the world’s oxygen. Perhaps the dragonflies will shrink.
Good accounts of Petalura gigantea here:
www.aabio.com.au/a-day-in-the-swamps-of-katoomba-chasing-the-giant-dragonfly-petalura-gigantea/