Young solitary locusts, known as hoppers, are green, which camouflages them when they are feeding on plants. They are picky eaters, sticking to a limited range of plants, perhaps to better monitor their nutritional intake.
Eventually the hoppers develop wings and change colour from green to brown. They are now adults, but will still avoid each other except when they are attempting to mate.
The regular insect life cycle changes only after the rains come and vegetation blooms. Faced with a feast, the population swells, but the locusts remain fairly well isolated from each other â€“ until the land dries out again and the vegetation dies back. As the area available for feeding shrinks, the locusts are driven together.
If the population density reaches a critical level, trouble is afoot â€“ literally. The sight and smell of other locusts, and the sensation of other locusts’ legs rubbing up against them, trigger a host of behavioural and physiological changes. The frantic changes in behaviour can take as little as 2Â hours and are driven by a serotonin surge in part of the central nervous system.
Instead of milling around looking for food, the locusts begin to move in an orderly fashion â€“ they march. If they are adults, and have wings, they fly, covering 100 kilometres a day in their search for food. They abandon their fussy dietary habits and will eat almost any plant they find.
They also change colour to yellow and black, just like bees and wasps, and for the same reason: to warn off predators. It is not an empty threat: swarming locusts, with their whatever-we-find diets, eat a lot of toxic plants and accumulate the toxins in their bodies.
They may taste bad, but that doesn’t stop the locusts being eaten â€“ by each other. Locusts in swarms are desperate for salt and protein, and one of the easiest ways to get them is cannibalism. In fact, it may be that overcrowded locusts start marching partly to get away from their nearest neighbours.
All this makes locust swarms dangerous places, which locusts only brave because otherwise they would starve, and where the philosophy seems to be “every locust for itself”. It’s very different from the loner lifestyle, and calls for a different kind of brain.
Swidbert Ott and Steve Rogers of the University of Cambridge have discovered that swarming locusts have brains 30Â per cent larger than those of solitary locusts, even though their bodies are smaller. And the brains are not simply inflated.
Locust brains are quite simple: on each side of the head is an optic lobe taking in information from the eyes and performing basic processing, and these lobes feed into the central midbrain, which carries out higher-level processing.
In swarming locusts, the midbrain grew more than the optic lobes. This, and other subtle changes, suggest that because swarming locusts are constantly surrounded by wild activity, they do not need to worry about having particularly sensitive vision. However, they do need extra high-level processing power to cope with the extremely complex patterns of motion that they see.
In other words, locusts are another example of an animal developing a more sophisticated brain to deal with social living. The demands of living in groups, from remembering who is who in the hierarchy to lying to your neighbours, take a lot of brainpower. It seems that joining a ferociously cannibalistic swarm of crop-devouring grasshoppers is no exception.