title: Collective behaviour without collective order in wild swarms of midges.
abstract: Collective behaviour is a widespread phenomenon in biology, cutting through a huge span of scales, from cell colonies up to bird flocks and fish schools. The most prominent trait of collective behaviour is the emergence of global order: individuals synchronize their states, giving the stunning impression that the group behaves as one. In many biological systems, though, it is unclear whether global order is present. A paradigmatic case is that of insect swarms, whose erratic movements seem to suggest that group formation is a mere epiphenomenon of the independent interaction of each individual with an external landmark. In these cases, whether or not the group behaves truly collectively is debated. Here, we experimentally study swarms of midges in the field and measure how much the change of direction of one midge affects that of other individuals. We discover that, despite the lack of collective order, swarms display very strong correlations, totally incompatible with models of non-interacting particles. We find that correlation increases sharply with the swarm's density, indicating that the interaction between midges is based on a metric perception mechanism. By means of numerical simulations we demonstrate that such growing correlation is typical of a system close to an ordering transition. Our findings suggest that correlation, rather than order, is the true hallmark of collective behaviour in biological systems.
authors: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, Rome, Italy; Dipartimento di Fisica, Università Sapienza, Rome, Italy.
authors: Alessandro
authors: A
authors: Attanasi
authors: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, Rome, Italy; Dipartimento di Fisica, Università Sapienza, Rome, Italy; Initiative for the Theoretical Sciences, The Graduate Center, City University of New York, New York, New York, United States of America.
authors: Andrea
authors: A
authors: Cavagna
authors: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, Rome, Italy; Dipartimento di Fisica, Università Sapienza, Rome, Italy.
authors: Lorenzo
authors: L
authors: Del Castello
authors: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, Rome, Italy; Dipartimento di Fisica, Università Sapienza, Rome, Italy; Initiative for the Theoretical Sciences, The Graduate Center, City University of New York, New York, New York, United States of America.
authors: Irene
authors: I
authors: Giardina
authors: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, Rome, Italy; Dipartimento di Fisica, Università Sapienza, Rome, Italy.
authors: Stefania
authors: S
authors: Melillo
authors: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, Rome, Italy; Dipartimento di Informatica, Università Sapienza, Rome, Italy.
authors: Leonardo
authors: L
authors: Parisi
authors: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, Rome, Italy; Dipartimento di Fisica, Università Sapienza, Rome, Italy.
authors: Oliver
authors: O
authors: Pohl
authors: Dipartimento di Fisica, Università di Roma 3, Rome, Italy.
authors: Bruno
authors: B
authors: Rossaro
authors: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, Rome, Italy; Dipartimento di Fisica, Università Sapienza, Rome, Italy.
authors: Edward
authors: E
authors: Shen
authors: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, Rome, Italy; Dipartimento di Scienze per gli Alimenti la Nutrizione e l'Ambiente, Università degli Studi di Milano, Milano, Italy.
authors: Edmondo
authors: E
authors: Silvestri
authors: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, Rome, Italy; Dipartimento di Fisica, Università Sapienza, Rome, Italy.
authors: Massimiliano
authors: M
authors: Viale
mesh_terms: D000818:Animals; D001522:Behavior, Animal / Q000502:physiology*; D019295:Computational Biology; D004175:Diptera / Q000502:physiology*; D008297:Male; D008954:Models, Biological; D013037:Spatial Behavior / Q000502:physiology*