newsCO.com.au | Do dragonflies signal the start of the dry season? Scientists debate Darwin urban legend

March 16, 2018

@newsCOflash

2018-03-16 02:01:30

Posted

March 16, 2018 13:01:30

In Darwin, local folklore holds that the appearance of large groups of dragonflies signals the change of seasons from wet to dry.

And right on cue, coastal areas were filled this week with thousands of the insects as they clung to palm fronds and were photographed by locals.

So are these small winged creatures really weather forecasters?

Not exactly, according to entomologist Graham Brown, who said it was difficult to prove a causal link between the dry season and the insects’ arrival.

“The appearance of the dragonflies is not dependent on the start of the dry,” Dr Brown said.

“That becomes a human analogy, and if that analogy was correct you’d have to argue that we’re not going to get the rain.

“It’s coincidental — it’s not magic, it’s not dragonflies therefore dry.”

More likely, he said, was that the insects’ movements were synced to their life cycles, a journey that began in the wet and continued with migration around this time of year.

So are they BOOKr.VIP to the wet season?

Last year, in a bid to improve local knowledge about the elusive creatures, freshwater ecologist Professor Jenny Davis spearheaded a citizen science project in which participants submitted images of wild dragonflies.

She admitted the results were dependent on people’s enthusiasm for the project, but the results showed a definite spike at the beginning of the project’s four-month window of operation.

“What we found is that in April and May, it’s the dragonflies with the big wings … those were around in quite large numbers and then they sort of dropped off,” Professor Davis, who is the head of the School of Environment at Charles Darwin University, said.

“So we feel it’s one of those answers where you go, ‘Yes, but’.

“So yes, some dragonfly species appear to occur in large numbers as we cross from the wet season into the dry season, but not all.”

There is little concrete data about population movements and a lot of hypothesising.

“The only way we’ll really know is to put tiny trackers on them,” Professor Davis said.

But she disputed their movements were completely independent of the seasons, saying this week’s influx could be linked to heavy rainfall in the wet season.

“The first thing to remember when we see a dragonfly flying is that that’s the adult of that species, and the juvenile or the larva is aquatic, so that lives in the wetland or a river or a stream,” Professor Davis said.

“So the more rain there is, the more extensive the wetland areas are, the more food would be available, the faster everything would be growing.

“The fact that we’re seeing a lot of them tells us that we’ve got some nice, healthy wetlands around Darwin.”

Where do they come from?

Of agreement to both Dr Brown and Professor Davis was that the dragonflies were a long way from home.

Dr Brown said the species was one of three that typically arrived in Darwin around this time of year and they’d also been recorded to the city’s north and south.

“The species that are involved with this start of the dry season concept are quite migratory and they don’t just breed in Darwin; they breed down the track a fair way as well and they migrate [north],” he said.

“There have been sightings of large swarms of dragonflies from people driving on the Stuart Highway, me included, and swarms are recorded out to sea as well.

“We’ve got 103 species recorded in the Territory; most of them wouldn’t migrate, but the ones that we’re seeing in large numbers are definitely known to be migratory species.”

How do they get here?

As part of Professor Davis’s study, researchers examined correlations between insect populations and wind direction recorded in the Bureau of Meteorology’s long-term datasets.

“What we see in about April is a change from the north-westerly monsoon winds dominating into the much drier winds coming from the south and the east,” she said.

“So the theory that I’m currently putting forward is that these species, which do appear to be capable of migrating, are effectively emerging from the wetlands that are south and east of Darwin and they’re being brought here on those winds.”

She checked a weather recording site and found that a similar change in wind pattern was present on Wednesday, suggesting the insects might have blown in from wetlands such as Kakadu’s Alligator River.

“If you can imagine; lots of dragonflies emerging from those wetlands and then happily gliding along the coast, then arriving in Darwin.”

The urban legend may be tested during heavy rainfall expected over the weekend, potentially demonstrating how the purportedly dry season-loving loving creatures survive in very wet conditions.

“A lot of them will get battered to death or certainly blown, displaced long distances,” Dr Brown said.

“It just depends on where they are when the storm hits and which way the wind happens to be blowing at the time.”

Topics:

entomology,

invertebrates—insects-and-arachnids,

animal-science,

science-and-technology,

animals,

weather,

darwin-0800

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