Grasshopper can turn into locust – Great Moments In Science

G’day, Dr Karl here,

Amazingly, grasshoppers and locusts are the same critter.

Grasshoppers have been around for about 250 million years – so they were here on Earth before the dinosaurs. Today, these ground-dwelling creatures live on all the continents (except for Antarctica), and they can jump up to a meter high with their powerful hind legs. But if the conditions are right, some species of grasshopper can change both their appearance and behavior – and they turn into locusts.

We have records of locust plagues going back thousands of years. They are mentioned in the Koran, the Bible, the Sanskrit Mahabharata, and the Greek Illiad – as well as being carved into ancient Egyptian tombs. Three thousand years ago, the Chinese had official anti-locust officers. In Australia, today we have the Australian Plague Locust Commission to deal with locust plagues.

One of the largest locust plagues on record was the 1875 Albert’s Swarm – in the USA. It covered about half-a-million square kilometers, and involved some 12.5 trillion Rocky Mountain Locusts weighing some 27.5 million tons. As an aside, we’re still not sure why the Rocky Mountain Locust was extinct just a quarter of a century later, by 1902.

Anyhow, a plague of flying locusts can blacken the sky like an umbrella – and when they descend, they devour all the vegetation. Each locust might eat only a few grams – but there are hordes of them. They do this in North Africa, Asia and the Middle East.

A swarm can carry 80 million locusts in each square kilometer, eat as much food as 35,000 people, and can travel 120 kilometers in a day. A swarm the size of Rome can eat as much each day as the entire population of Kenya.

The from relatively harmless grasshopper transition to devastating locust is astoning.

If you have fewer than 25 grasshoppers in each square meter – they’re just shy green grasshoppers, avoiding other grasshoppers.

But when you double the density to about 50 grasshoppers in each square meter, they do this radical change into bigger, gregarious, aggressive and brightly colored locusts – often yellow and black.

And once they reach 75 locusts in each square meter, their behavior changes into a single-minded forward march of relentless eating.

There are at least two triggers for this bizarre change from harmless grasshoppers to marauding locusts.

One is simply that the grasshoppers bump each other’s back legs. Armed only with tiny paintbrushes, scientists worked this out with very delicate and pain-taking labour. And eventually, they found out out that the trigger area on the individual grasshopper was the area over the femur – that section of the leg between the knee and the hip. Touching the grasshopper on the thigh, for as little as five seconds in each minute, and keeping this up for a few hours, was enough to start the transition. In human terms, it’s as though when you sit next to somebody on a train, and your thighs keep bumping into each other for a few hours, both your appearance and your behavious alternate. You change colour, grow physically bigger, love to congregate in groups, become more aggressive and you’re now keen to travel – like a Marvel superhero gone wrong. On the inside, your DNA is still the same, but on the outside, you look and behave quite differently. In the case of locusts, the scientists label this change as “density-dependent phenotypic plasticity”.

Another trigger for the transformation is simply seeing and smelling a locust. At least two chemicals are involved.

One is the hormone serotonin. Perhaps for locusts, it’s a bit like being on anti-depressants or Ecstasy. Another chemical that helps start the metamorphosis is 4-VinylAnisole.

But we’ll probably find that there will be a whole bunch of other chemicals involved. And when we get more information, maybe we’ll control them better.

So, for some grasshoppers, once they break their social isolation and start moving as a large group, the situation changes – and they add new meaning to the phrase ‘just hop to it’.


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