A locust is a grasshopper that changes its body when the crowd arrives

The desert locust has two forms. Alone, it is a cryptic grasshopper. Crowded, it changes colour, shape, brain and behaviour, and it swarms. Same genes. Different life.

Schistocerca gregaria is born able to go either way. A nymph raised alone stays solitary: green or otherwise camouflaged, shy of other locusts, built to hop and hide. A nymph that is touched, jostled and crowded — the cues are mechanical, from the legs, as well as chemical — switches. It becomes brightly coloured, sociable, a stronger flyer, with different proportions and a different daily rhythm. The adults of a swarm travel in bands that eat a landscape. The transformation is called phase polyphenism. It was described by Boris Uvarov a century ago and has been taken apart since. Serotonin, in one well-known experiment, is enough to push the behavioural half of the switch in a few hours. The full change of body takes a moult, or two.

The genes do not change between a solitary locust and a gregarious one. The environment chooses which book the same genome reads. That is the fact to keep, because it is larger than the swarm. An animal can have two bodies, filed under one set of chromosomes, and the choice can be made by the press of neighbours. When the crowd thins, the line goes back. The plague and the harmless grasshopper are one species taking turns.