22:00 27 February 2006
NewScientist.com news service
Jeff Hecht

Hunger for protein and salt is what gets millions of Mormon crickets
marching across western North America – that, and a fear of cannibals.

If they want to survive, the insects have to keep marching fast enough
to stay ahead of other hungry crickets that might eat them for their
own protein and salt content.

Mormon crickets – Anabrus simplex – are large flightless
grasshopper-like insects and not true crickets. Millions of them
sometimes march together, with dozens of insects per square metre
moving up to 2 kilometres per day, and covering an area up to 10
kilometres long. Marching troops of the omnivorous insects can cause
serious crop damage, but unlike locusts they do not devour all the
plants in the landscape.

Curious to see if nutrition could explain this behaviour, Stephen
Simpson at the University of Sydney, Australia, studied the crickets
with Greg Sword at the US Department of Agriculture's research
service, and colleagues. They placed samples of foods rich in proteins
or carbohydrates on a cattle trail in the path of the marching
insects.

The crickets showed "a clear preference for protein-containing food,"
with up to 13 crickets clambering over a 4-centimetre-wide dish at
once. They also went after salt.
Eat and be eaten

Foraging crickets prefer protein-rich sources such as seeds, flower
heads, and carrion. But there is not enough to go around. "They get a
lot from eating each other," says Simpson. "Mormon crickets are
walking packages of protein and salt." Any that stop to eat have to
weigh the risks of another cricket stopping to eat them, he adds.

Indeed, the researchers found cannibalism to be common. "We pulled the
heads off to see what they were eating; a substantial portion had
evidence of other crickets inside them," Simpson says.

Some protein-starved crickets devoured the entire bodies of others
their own size. "Some things can swallow other things whole, but this
is a chew-up-and-swallow meal," Simpson told New Scientist.

The crickets do not swarm every year, only when there is not enough
food to go around. They evidently tolerate their cannibal companions
because the risks of going it alone are higher – within two days more
than half of isolated insects were eaten by other animals.

If a person is in motion the crickets do not bother them, but a person
might get in trouble by lying down in front of them, suggests Simpson:
"There's a horror movie in there somewhere."