  Aggregation is a common behavior by which groups of organisms arrange into
cohesive groups. Whether suspended in the air (like honey bee clusters), built
on the ground (such as army ant bridges), or immersed in water (such as sludge
worm blobs), these collectives serve a multitude of biological functions, from
protection against predation to the ability to maintain a relatively desirable
local environment despite a variable ambient environment. In this review, we
survey dense aggregations of a variety of insects, other arthropods, and worms
from a soft matter standpoint. An aggregation can be orders of magnitude larger
than its individual organisms, consisting of tens to hundreds of thousands of
individuals, and yet functions as a coherent entity. Understanding how
aggregating organisms coordinate with one another to form a superorganism
requires an interdisciplinary approach. We discuss how the physics of the
aggregation can yield additional insights to those gained from ecological and
physiological considerations, given that the aggregating individuals exchange
information, energy, and matter continually with the environment and one
another. While the connection between animal aggregations and the physics of
non-living materials has been proposed since the early 1900s, the recent advent
of physics of behavior studies provides new insights into social interactions
governed by physical principles. Current efforts focus on eusocial insects;
however, we show that these may just be the tip of an iceberg of superorganisms
that take advantage of physical interactions and simple behavioral rules to
adapt to changing environments. By bringing attention to a wide range of
invertebrate aggregations, we wish to inspire a new generation of scientists to
explore collective dynamics and bring a deeper understanding of the physics of
dense living aggregations.
