Insect swarms can be bound together by repulsive forces
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THE EUROPEAN PHYSICAL JOURNAL E
Regular Article
Insect swarms can be bound together by repulsive forces A.M. Reynoldsa Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, UK Received 9 January 2020 and Received in final form 1 June 2020 Published online: 19 June 2020 c The Author(s) 2020. This article is published with open access at Springerlink.com Abstract. The cohesion of insect swarms has been attributed to the fact that the resultant internal interactions of the swarming insects produce, on the average, a centrally attractive force that acts on each individual. Here it is shown how insect swarms can also be bound together by centrally forces that on the average are repulsive (outwardly directed from the swarm centres). This is predicted to arise when velocity statistics are heterogeneous (position-dependent). Evidence for repulsive forces is found in laboratory swarms of Chironomus riparius midges. In homogeneous swarms, the net inward acceleration balances the tendency of diffusion (stochastic noise) to transport individuals away from the centre of the swarm. In heterogenous swarms, turbophoresis —the tendency for individuals to migrate in the direction of decreasing kinetic energy— is operating. The new finding adds to the growing realization that insect swarms are analogous to self-gravitating systems. By acting in opposition to central attraction (gravity), the effects of heterogeneous velocities (energies) are analogous to the effects of dark energy. The emergence of resultant forces from collective behaviours would not be possible if individual flight patterns were themselves unstable. It is shown how individuals reduce the potential for the loose of flight control by minimizing the influence of jerks to which they are subjected.
Introduction In contrast with bird flocks, fish schools and migratory herds, sparse swarms of flying insects do not possess global order but are, nonetheless, a form of collective animal behaviour [1,2]. The collective behaviour is evident in their emergent macroscopic mechanical properties. Laboratory swarms of Chironomus riparius midges, for example, have macroscopic mechanical properties similar to solids, including a finite Young’s modulus and yield strength [3]. The collective behaviour of these swarms is also evident in their response to dynamic illumination perturbations. The swarm-level response can be described by making an analogy with classical thermodynamics, with the state of the swarm moving along an isotherm in a thermodynamic phase plane [4]. Applied oscillatory visual stimuli induce a viscoelastic response as the perturbations are strongly dampened, both viscously and inertially [5]. A distinctively different indicator of collective behavior lies in the composition of unperturbed swarms. Unperturbed laboratory swarms of Chironomus riparius midges consist of a core “condensed” phase surrounded by a dilute “vapour” phase [6]. Although these two phases have distinct macroscopic properties, individuals move freely between them, Supplementary material in the form of
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