A swarm advances (video courtesy of Dr Jerome Buhl)
Locust swarms have a unique structure, scientists have discovered.
While fish schools are denser in the middle, and bird flocks more dense at the edges, most locusts gather at the front of their swarm.
This creates a dense band of advancing insects, yet these swarms teeter on the brink of disorder and could dissipate at any time, say the scientists.
The discovery could help researchers predict how swarms will move and find ways to prevent them damaging crops.
Locusts seem to be constantly on the edge of disorder
Locust expert Dr Jerome Buhl
Details are published in the journal Behavioral Ecology and Sociobiology.
Our understanding of how animals aggregate into large groups has mainly come from theoretical studies, which suggest many animals are capable of spontaneously ordering themselves and then cohesively moving together.
Many specialists have predicted that locusts do similar as their swarms do not correlate with environmental cues such as the sun, wind, or type of vegetation.
Wild and chaotic
"We showed that this happens in the field," says Dr Jerome Buhl of the University of Sydney, Australia.
But Dr Buhl's research has revealed much more about the behaviour of locusts, which are renowned for devastating large areas of food crops and natural vegetation.
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"While many fish schools and birds flocks are remarkable for their extreme cohesion, locusts seem to be constantly on the edge of disorder."
Dr Buhl and his colleagues studied a series of locust bands that formed during February 2006 near Topar in New South Wales, Australia.
They filmed these bands and analysed the images to ascertain the dynamics of the locusts forming them, measuring the distances between insects and the directions they moved in.
Overall, the researchers captured information on the position and orientation of around 65,000 individual locusts forming bands and another 9,500 locusts following in their wake.
A number of key points emerged.
Individual locusts, when isolated from each other, do not prefer to move in any one direction.
"If they are not in a group, they simply move in all directions, which tells us that group direction is a truly emergent phenomenon," Dr Buhl told the BBC.
During a locust swarm, most locusts move to the front of a group, forming dense bands.
Behind, the density of locusts quickly decreases, with many individual insects losing contact with the group altogether.
That is different to how other animals form large groups: fish schools tend to be much denser in the middle than at the edges, for example, while flocks of birds such as starlings are denser along their edges, all around the group, and less dense in the middle.
Locusts form groups characterised by an extremely dense front, but a loosely defined tail.
Locusts also only react to each other if they are 13.5cm apart or less.
So a dense group of locusts that are this close will align with each other, and move in the same direction.
High densities are critical to maintaining the structure of locust swarms
"That cohesive movement allows millions of locusts to migrate en masse," says Dr Buhl.
But those individuals following the dense band quickly lose focus, and start heading off in any direction.
"At the front of the locusts' bands, a massive number of locusts move in unison. Yet at the back, many of them straggle behind and seem to lose the group.
"That is probably happening because locusts' bands are always on the edge of losing their order. Everyday they seem barely just to make it as a group."
That may provide insights into why locusts swarm in the first place.
Locusts shift between two types.
Usually they are shy, rarely seen loners.
But juvenile locusts can suddenly come together into massive swarms; changing their appearance and behaviour.
These gregarious juveniles aggregate into these migratory "bands" that can contain billions of individuals that walk and hop through the habitat during the day and roost at night.
"It may well be something they have evolved to do, sit on the knife edge between two lifestyles," says Dr Buhl.
Only when they encounter enough fellow locusts in close proximity do they begin to align, and become gregarious.
The insights gleaned by Dr Buhl's team could help in the control of devastating locust swarms.
"There are actually very concrete and direct implications for control strategies," says Dr Buhl.
Working with mathematician colleagues, he has already ascertained that individual locusts move in a truly random and unpredictable way.
But the fact they then move in an ordered fashion when they reach a critical density may allow researchers to start to predict where swarms will appear.
By keeping track of a swarm's density, it may be possible to lay insecticide barriers in front of the locusts before the swarm's density decreases, and the insect's behaviour changes again, leading to new swarms heading in new directions.
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