Locust plagues are one of the most infamous insect scourges, invading vast areas of Africa, Asia, Australia and the Americas. The reason that locusts form plagues is that they have an extraordinary capacity to change from shy, green, harmless grasshoppers into brightly coloured, swarming creatures when they experience crowding. This remarkable change can occur within the life of a single animal: the genome of the insect codes for both forms. I show that an important trigger for the change is bumping into other locusts. Stimulating touch-sensitive hairs on the back legs causes a rapid shift in behaviour, such that locusts become attracted to each other, rather than avoiding one another. Having identified the source of sensory stimulation that induces behavioural gregarization, we next analysed the associated neurochemical pathways involved and have recently shown that a pulse of serotonin causes the shift in behaviour upon crowding. Once a local aggregation reaches a critical number of insects, the locusts suddenly start to move as one. Using self-propelled particles models from statistical physics we have shown that this decision to start migrating does not involve leader locusts, but rather emerges collectively as a result of local interactions between individuals. Continuing to move as a group involves something very sinister, however. To illustrate, I next turn to another swarming animal, the Mormon cricket of North America. The reason these animals form vast marching bands is because they are seeking protein. The most abundant source of protein in a swarm of crickets is other crickets. The reason why they keep marching is that, if an insect stops, it gets cannibalized by the crickets coming from behind: they are on a forced march for protein. The same is true for locusts. The search for protein turns out to be a powerful force in shaping the biology not only of crickets and locusts, but of all animals - including humans. We have shown using experiments based on state-space geometric models for nutrition that many animals have a powerful appetite for protein. I show in humans that this protein appetite plays a key role in obesity. Protein comprising a minor part of our total energy budget, yet its intake is strongly regulated. I show how this combination leads to protein having the power both to drive the development of obesity - and to assuage it. Finally, I consider why it should be that many animals, humans included, should possess specific mechanisms that prevent overconsumption of protein. Using geometric models of nutrition I show that there are costs to over-consuming protein, and that the prevailing view that caloric restriction prolongs life is wrong - in insects at least.