behaviour*

As a basis for understanding the physics of behaviour in fruitfly larvae, we here develop an effective theory for the animals' motion. We define a set of fields which quantify deformations of the larva's anteroposterior axis, then search the space of possible theories that could govern the fields' low-energy physics. Guided by symmetry and stability requirements, we arrive at a unique free-field theory with few free parameters. Surprisingly, we can explain many features of larval behaviour by applying equilibrium statistical physics to this model. Our theory closely predicts the animals' postural modes (eigenmaggots), as well as distributions and trajectories in the mode space, across several behaviours. Our results show that real animal behaviour can be understood using relatively simple effective physics.