YSM Issue 87.4

The Science of
How Physicists, Engineers,
and Computer Scientists
are Learning from Midges
As evening falls at Mason Laboratory, after students have shuffled out and most
labs have locked up for the night, a single fly takes off. Buzzing around erratically,
almost confusedly, it looks lost at first. Soon, though, it is joined by another fly,
and then a third. As more and more take flight, something strange happens. With seven or
eight flies in the air, the scene is still chaotic—a loose-knit flurry of individual flies.
But as a few more join—a ninth, tenth, and eleventh—the turmoil transforms. The ball of
flies becomes uniform, and individuals get lost in the whirling, bustling mass.
The flies have formed a swarm.
The Power of Ten
How such order can emerge from chaos
is the subject of recent research by Nicholas
Ouellette, Associate Professor of Mechanical
Engineering at Yale. Ouellette’s lab studies
the science of swarms—or, more generally,
the dynamics of collective animal behavior.
Using a colony of midges, a type of small
fly, as a model system, Ouellette and his
colleagues are trying to figure out answers to
questions like how many individuals it takes
to make a swarm. Their surprising answer
was published last month in the Journal of the
Royal Society Interface: just ten. And this is only
one of many surprising and enlightening
results emerging from the nascent science of
swarms.
But what exactly is a swarm? “That is a much
deeper question than you might expect,” says
Ouellette. While most of us have an intuitive
notion of what it means to swarm—think
bees—defining the notion precisely is much
trickier. Ouellette distinguishes between
swarms and other animal groupings like
flocks or schools, which tend to move as
units. “A swarm is a collective behavior
that doesn’t show net ordering,” he says.
But Ouellette acknowledges that many
other researchers would disagree. In fact,
defining “swarm” fruitfully is an active area
of Ouellette’s research, and a focus for other
swarm scientists as well.
Unanswered Questions
The appeal of swarming for scientists is the
rise of complex patterns and behavior from
the collective action of many individuals.
This phenomenon, known as emergence, is
a key feature of swarms. For Ouellette and
other researchers, connecting the emergent
properties with the underlying individual
behaviors is a consuming problem. “What
kind of local interactions do you need
to make a swarm?” Ouellette asks in his
research. “And how would you model that?”
With the right local interactions and
enough individuals, a swarm results. At this
point, the collection of animals exhibits
macroscopic properties that truly belong
to the group as a whole, rather than its
members. Ouellette makes the analogy to
thermodynamics, where thinking in terms
of macroscopic properties is more familiar.
Temperature and pressure are natural
examples. Similar measures can be used to
characterize swarms, except the individuals
are flies, bees, or birds rather than molecules.
But this is a very big difference, and
perhaps the most interesting research
questions focus on these contrasts between
swarms and chemical or physical systems.
While chemists can accurately think of
their particles as bouncing about at random,
swarm scientists cannot. Midges fly under
their own power; they are active agents
rather than passive particles. And while the
large-scale behaviors of chemical aggregates
are generally well understood, animal
swarms remain more mysterious. Ouellette
uses empirical approaches, like filming and
tracking the midges, to shed light on how
they form and behave. His lab has worked
to evaluate which models of swarming
match up best with biological reality.
They also ask questions like the one about
the size of swarms. To determine the
minimum number of midges in a swarm,
Ouellette and postdoctoral researcher
James Puckett, now an Assistant Professor
12 Yale Scientific Magazine October 2014 www.yalescientific.org