Article by Dr Ian Baird
Petalura gigantea (Family Petaluridae), commonly known as the Giant Dragonfly or Southeastern Petaltail is a very large dragonfly which may have a wingspan up to 12.5 cm. It is recorded from peat swamps, bogs and seepages along the coast and ranges of NSW from near the Victorian border to around the Qld border. It is listed as Endangered in NSW under the NSW Biodiversity Conservation Act 2016, with habitat loss and degradation identified as the main threats. In addition to the large size and widely separated dark eyes, the species (and genus) is characterised by a long pterostigma (darkened cell) towards the end of the leading edge of the wings, and large, petal-shaped, anal appendages in adult males (Figure 1). Adult females lack the conspicuous anal appendages and are somewhat bulkier than males (Figure 2). These features distinguish it from other very large dragonflies such as Austrophlebia costalis. Various Bushcare and Swampcare sites in the Blue Mountains include peat swamp habitat of Giant Dragonflies.
The Petaluridae (“petaltails”) can be traced back to the late Jurassic and currently includes 11 known species around the world, including the endemic Australian genus Petalura with five species. The family is unique amongst dragonflies, in that larvae of nine of those species (including all Petalura species) excavate (sometimes complex) burrows which extend below the water table in peaty soils and which they occupy and maintain for their entire larval stage. The deepest burrow recorded for a petalurid worldwide was a P. gigantea burrow I investigated, which was 75 cm deep. These dragonflies have very long larval stages; extrapolation from recent studies suggests a larval stage of at least six years in P. gigantea, and possibly 10 or more in some situations. Larvae reach a length of 4.5-5 cm. Larvae feed on a range of small invertebrate prey within their burrows, including worms and nematodes, and are likely to act as ambush predators of larger prey from within their burrow entrances, feeding on above ground invertebrates, such as spiders, crickets, cockroaches, and perhaps small frogs, such as Crinia signifera. Larvae with submerged burrow openings in shallow pools can also prey upon other dragonfly or damselfly larvae within those pools. It is possible (but unknown) that they also leave their burrow openings temporarily to forage under suitable conditions, such as at night and during rain. I have recorded above ground chambers above their burrow openings, within litter layers and Sphagnum hummocks, which they may use for foraging purposes. Larvae leave their burrows and climb the nearest shrub or sedgeland vegetation to undergo emergence (ecdysis) to the adult stage, usually leaving their larval skin (exuvia) attached to their shrub or sedge emergence supports. The presence of exuviae confirms a site as a successful breeding site. Emergence may commence in early October in some years in some sites and extend into January, but normally appears to commence during November, at least in the Blue Mountains.
Adults live for a maximum of one summer flying season, which extends into February at least, with occasional late flying individuals having been observed on one occasion as late as mid-March in the Blue Mountains. Adults are predatory upon other flying insects during flight and consume a range of prey, including other dragonflies and damselflies, butterflies, moths, grasshoppers, cicadas, wasps, and various types of flies, including crane flies. Adults typically return to a perch to consume prey. Adults spend much time perched in sedgeland and shrub vegetation, interspersed with generally short flights associated with foraging, mating, and in the case of males, territorial interactions. Adult females typically leave their larval swamp habitat following emergence and only return to a swamp habitat for breeding purposes. There is no courtship behaviour and mating generally occurs in swamp habitat with the pair perched in sedge or shrub vegetation. Egg-laying (ovipositing) involves insertion of the ovipositor into the wet, organic-rich soil substrate, into fissures in the substrate, or amongst or under live or decomposing plant material overlying the substrate. Females typically walk along the soil surface or perch within covering vegetation or on litter while ovipositing. They do not appear to oviposit into substrate covered by more than 1-2 mm of water. Most adults encountered in swamp breeding habitats are males, who are typically territorial in swamp habitat. Predation of adults by birds, skinks and spiders has been observed and dead individuals have been found in spider webs.
Petalura gigantea is considered to be an obligately groundwater-dependent species. Although the burrowing larval habit confers ecological benefits, including increased environmental stability, and some protection from the effects of fire, flood, drought and above ground predators, the species groundwater dependence and restriction to peat swamp habitats places it at increased risk in the event of any reduction in groundwater levels (e.g., due to groundwater abstraction, tunnel or pipeline boring, and longwall coal mining), more intense fire regimes, and the potential compounding effects of rapid climate change. Loss and degradation of habitat as a result of urban and transport infrastructure development, agriculture, forestry, and longwall coal mining continue to threaten the species through loss and degradation of habitat.
Figure 1. Male Petalura gigantea perched on razor sedge, Lepidosperma limicola, in a Blue Mountains Swamp. Note the conspicuous petal-shaped appendages at the end of the abdomen. Photo: Ian Baird
Figure 2. Female Petalura gigantea perched on Acacia ptychoclada in a Blue Mountains Swamp. Photo: Ian Baird