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Sunday, July 22, 2018

Dragonflies: Nature’s pretty,deadly drones

New research suggests the insects may well be the most brutally effective hunters in the animal kingdom

Written by New York Times | Published: April 7, 2013 12:01:00 am


Dragonflies may look dainty,glittery and fun,and they’re often grouped with butterflies and ladybugs on the very short list of ‘Insects People Like’. But they are also voracious aerial predators,and new research suggests they may well be the most brutally effective hunters in the animal kingdom.

When setting off to feed on other flying insects,dragonflies manage to snatch their targets in midair more than 95 per cent of the time,often wolfishly consuming the fresh meat on the spur without bothering to alight.

Next step: grab more food. Dragonflies may be bantam,but their appetite is bottomless. Stacey Combes,who studies the biomechanics of dragonfly flight at Harvard,once watched a laboratory dragonfly eat 30 flies in a row. “It would have happily kept eating,” she said,“if there had been more food available.”

In a string of recent papers,scientists have pinpointed key features of the dragonfly’s brain,eyes and wings that allow it to hunt so unerringly. One research team has determined that the nervous system of a dragonfly displays an almost human capacity for selective attention,able to focus on a single prey as it flies amid a cloud of similarly fluttering insects.

Other researchers have identified a kind of master circuit of 16 neurons that connect the dragonfly’s brain to its flight motor centre in the thorax. With the aid of that neuronal package,a dragonfly can track a moving target,calculate a trajectory to intercept that target and subtly adjust its path as needed.

The scientists found evidence that a dragonfly plots its course to intercept through a variant of “an old mariner’s trick,” said Robert M. Olberg of Union College,who reported the research with his colleagues in Proceedings of the National Academy of Sciences. If you’re heading north on a boat and you see another boat moving,say,30 degrees to your right,and if as the two of you barrel forward the other boat remains at that 30-degree spot in your field of view,vector mechanics dictate that your boats will crash: better slow down,speed up or turn aside.

In a similar manner,as a dragonfly closes in on a meal,it maintains an image of the moving prey on the same spot,the same compass point of its visual field. “The image of the prey is getting bigger,but if it’s always on the same spot of the retina,the dragonfly will intercept its target,” said Paloma T Gonzalez-Bellido,an author of the new report who now works at the Marine Biological Laboratory in Woods Hole,Massachusetts.

As a rule,the hunted remains clueless until it’s all over. Dragonflies are magnificent aerialists,able to hover,dive,fly backward and upside down,pivot 360 degrees with three tiny wing beats,and reach speeds of 30 mph,lightning for an arthropod. In the dragonfly,the four transparent,ultraflexible wings are attached to the thorax by separate muscles and can each be manoeuvered independently,lending the insect an extraordinary range of flight options.

Dragonflies are true visionaries. Their eyes are the largest and possibly the keenest in the insect world,a pair of giant spheres each built of some 30,000 pixel-like facets that together take up pretty much the entire head.

As reported in Current Biology,Steven Wiederman of the University of Adelaide in Australia and colleague David O’Carroll explored how dragonflies single out one target from a chaotic swarm. Working with the two-inch-long Emerald dragonfly often seen darting around Australian ponds,the researchers inserted an electrode about 1/1500th the width of a human hair into a dragonfly neuron known to be involved in visual processing. They then positioned the dragonfly in front of an LCD screen and showed it first one and then two moving targets at a time.

The scientists were amazed to find that the dragonfly attended to multiple stimuli in primate-like style,concentrating first on one target while ignoring the other,and then suddenly switching full attention to Target B,and then back to Target A—rather as we humans can sequentially shift our focus at a busy party from friend to friend,to a wineglass in need of a refill.

“So here we have a simple brain of less than a million neurons behaving like our own brain of 100 billion neurons,” Wiederman said.

The scientists have yet to determine what cues might prompt a dragonfly to decide,ah,there’s the target I will pursue.

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