Organismal aggregations exhibit fluidic behaviors: a review

Phase transitions in insect swarms

In contrast with laboratory insect swarms, wild insect swarms display significant coordinated behaviour. It has been hypothesised that the presence of a fluctuating environment drives the formation of transient, local order (synchronized subgroups), and that this local order pushes the swarm into a new state that is robust to environmental perturbations. The hypothesis is supported by observations of swarming mosquitoes. Here I provide numerical evidence that the formation of transient, local order is an accidental by-product of the strengthening of short-range repulsion which is expected in the presence of environmental fluctuations. The results of the numerical simulations reveal that this strengthening of the short-range can drive swarms into a crystalline phase containing subgroups that participate in cooperative ring exchanges-a new putative form of collective animal movement lacking velocity correlation. I thereby demonstrate that the swarm state and structure may be tuneable with environmental noise as a control parameter. Predicted properties of the collective modes are consistent with observations of transient synchronized subgroups in wild mosquito swarms that contend with environmental disturbances. When mutual repulsion becomes sufficiently strong, swarms are, in accordance with observations, predicted to form near stationary crystalline states. The analysis suggests that the many different forms of swarming motions observed across insect species are not distinctly different phenomena but are instead different phases of a single phenomenon.

Oxygenation-Controlled Collective Dynamics in Aquatic Worm Blobs

Many organisms utilize group aggregation as a method for survival. The freshwater oligochaete, Lumbriculus variegatus (California blackworms) form tightly entangled structures, or worm "blobs", that have adapted to survive in extremely low levels of dissolved oxygen (DO). Individual blackworms adapt to hypoxic environments through respiration via their mucous body wall and posterior ciliated hindgut, which they wave above them. However, the change in collective behavior at different levels of DO is not known. Using a closed-loop respirometer with flow, we discover that the relative tail reaching activity flux in low DO is ∼75x higher than in the high DO condition. Additionally, when flow rate is increased to suspend the worm blobs upward, we find that the average exposed surface area of a blob in low DO is ∼1.4x higher than in high DO. Furthermore, we observe emergent properties that arise when a worm blob is exposed to extreme DO levels. We demonstrate that internal mechanical stress is generated when worm blobs are exposed to high DO levels, allowing them to be physically lifted off from the bottom of a conical container using a serrated endpiece. Our results demonstrate how both collective behavior and the emergent generation of internal mechanical stress in worm blobs change to accommodate differing levels of oxygen. From an engineering perspective, this could be used to model and simulate swarm robots, self-assembly structures, or soft material entanglements.

Fast collective evasion in self-localized swarms of unmanned aerial vehicles

A novel approach for achieving fast evasion in self-localized swarms of unmanned aerial vehicles (UAVs) threatened by an intruding moving object is presented in this paper. Motivated by natural self-organizing systems, the presented approach of fast and collective evasion enables the UAV swarm to avoid dynamic objects (interferers) that are actively approaching the group. The main objective of the proposed technique is the fast and safe escape of the swarm from an interferer discovered in proximity. This method is inspired by the collective behavior of groups of certain animals, such as schools of fish or flocks of birds. These animals use the limited information of their sensing organs and decentralized control to achieve reliable and effective group motion. The system presented in this paper is intended to execute the safe coordination of UAV swarms with a large number of agents. Similar to natural swarms, this system propagates a fast shock of information about detected interferers throughout the group to achieve dynamic and collective evasion. The proposed system is fully decentralized using only onboard sensors to mutually localize swarm agents and interferers, similar to how animals accomplish this behavior. As a result, the communication structure between swarm agents is not overwhelmed by information about the state (position and velocity) of each individual and it is reliable to communication dropouts. The proposed system and theory were numerically evaluated and verified in real-world experiments.

Remote radio control of insect flight reveals why beetles lift their legs in flight while other insects tightly fold

In the research and development of micro air vehicles, understanding and imitating the flight mechanism of insects presents a viable way of progressing forward. While research is being conducted on the flight mechanism of insects such as flies and dragonflies, research on beetles that can carry larger loads is limited. Here, we clarified the beetle midlegs' role in the attenuation and cessation of the wingbeat. We anatomically confirmed the connection between the midlegs and the elytra. We also further clarified which pair of legs are involved in the wingbeat attenuation mechanism, and lastly demonstrated free-flight control via remote leg muscle stimulation. Observation of multiple landings using a high-speed camera revealed that the wingbeat stopped immediately after their midlegs were lowered. Moreover, the action of lowering the midleg attenuated and often stopped the wingbeat. A miniature remote stimulation device (backpack) mountable on beetles was designed and utilized for the free-flight demonstration. Beetles in free flight were remotely induced into lowering (swing down) each leg pair via electrical stimulation, and they were found to lose significant altitude only when the midlegs were stimulated. Thus, the results of this study revealed that swinging down of the midlegs played a significant role in beetle wingbeat cessation. In the future, our findings on the wingbeat attenuation and cessation mechanism are expected to be helpful in designing bioinspired micro air vehicles.

Bio-inspired compact swarms of unmanned aerial vehicles without communication and external localization

This article presents a unique framework for deploying decentralized and infrastructure-independent swarms of homogeneous aerial vehicles in the real world without explicit communication. This is a requirement in swarm research, which anticipates that global knowledge and communication will not scale well with the number of robots. The system architecture proposed in this article employs the UVDAR technique to directly perceive the local neighborhood for direct mutual localization of swarm members. The technique allows for decentralization and high scalability of swarm systems, such as can be observed in fish schools, bird flocks, or cattle herds. The bio-inspired swarming model that has been developed is suited for real-world deployment of large particle groups in outdoor and indoor environments with obstacles. The collective behavior of the model emerges from a set of local rules based on direct observation of the neighborhood using onboard sensors only. The model is scalable, requires only local perception of agents and the environment, and requires no communication among the agents. Apart from simulated scenarios, the performance and usability of the entire framework is analyzed in several real-world experiments with a fully-decentralized swarm of UAV deployed in outdoor conditions. To the best of our knowledge, these experiments are the first deployment of decentralized bio-inspired compact swarms of UAV without the use of a communication network or shared absolute localization. The entire system is available as open-source at https://github.com/ctu-mrs.

Insect swarms can be bound together by repulsive forces

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.

Random sequential addition simulations of animal aggregations provide null models of group structure

Apparent structure in animal aggregations such as fish and Antarctic krill schools may result from the tight packing of these elongated animals. This geometrical structure may be difficult to differentiate from behavior-induced structure resulting from individuals preferentially taking up certain positions relative to conspecifics to gain an adaptive advantage such as reduced locomotive cost. Here we use random sequential addition (RSA) simulations to quantify the effect of animal shape, aggregation organization, and aggregation density on 2D school structure. This technique allows for the generation of a null model for nearest neighbor distance and nearest neighbor position angle for a specific body shape and aggregation density, thus isolating the effect of geometry from that of behavior. We further identify a shape-specific aggregation density threshold above which the animal shape affects the spatial distribution of nearest neighbors. Nearest neighbor distance data of fish schools with densities above and below the threshold are found to agree well with nearest neighbor statistics found from RSA-generated schools.