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. Our perception of collective behaviour in biological systems is closely associated to the emergence of order on a group scale. For example, birds within a flock align their directions of motion, giving the stunning impression that the group is just one organism. Large swarms of midges, mosquitoes and flies, however, look quite chaotic and do not exhibit any group ordering. It is therefore unclear whether these systems are true instances of collective behaviour. Here we perform the three dimensional tracking of large swarms of midges in the field and find that swarms display strong collective behaviour despite the absence of collective order. In fact, we discover that the capability of swarms to collectively respond to perturbations is surprisingly large, comparable to that of highly ordered groups of vertebrates.

The seemingly erratic motion of insects in a swarm exhibits the correlated behavior of particles near the critical point of a phase transition.

Response curves for the abundance value of chironomid species against water temperature were analyzed and optimum, tolerance, skewness and kurtosis calculated. The data base consisted of 127 taxa and 991 sites in running waters in Italy. Optimum temperature was calculated as weighted mean temperature, tolerance as weighted standard deviation, skewness and kurtosis as 3rd and 4th moment statistics respectively. Species were sorted according to optimum temperature. Most species are cold stenothermal and show tolerance for a narrow temperature range, very few species have a clearly plurimodal response. Cold stenothermal species have an optimum near their minimum tolerance value, warm water species have an optimum near their maximum tolerance value. The warm water species are more or less eurythermal. Lack of data prevents the demonstration of a relationship between thermal tolerance and the position of a species in the evolutionary tree, but the problem is worthy of further study.