When a squishy, lone baby millipede travels alone, it's a vulnerable target. But in a swarm surrounded by its siblings, it stands a better chance of surviving.
Millipedes are a diverse group of arthropods that can be found around the globe. Some species swarm together during mating season or to hunt for food, and some studies have theorised juveniles swarm together for protection from predators. (See how sawfly larvae deploy similar lines of defence.)
Noah Elhardt works at a farmer training centre in Senegal, where he recently stumbled upon one such swarm this past September. A nature photographer by hobby, Elhardt decided to film the swarms of millipedes he observed clustered on many of the country's roads.
"While migrating, the swarms often become two or three millipedes deep, so that the individuals on top are propelled along as if walking on a moving sidewalk at an airport," he said.
From the ground, it's easy to see the large clusters of 20 to 30 millipedes crawling over each other as they move forward. The ranks are so impenetrable that when a single ant tries to pass through, it stumbles until it's forced to crawl around the sides.
Millipedes are born in clusters that hatch at the same time. They stick together from day one. A similar cluster of millipedes can be seen in the video, only slightly larger than the juvenile patch, lingering only a few centimetres away.
According to Elhardt, the millipedes are likely feeding on the nutrient-rich patches of soil that spring to life during Senegal's rainy season. Juvenile millipedes can't subsist on dirt alone, so they seek out patches of soil that is covered in organic matter. Because this organic matter typically grows in patches that are exposed to sunlight, young millipedes are more exposed.
While a swarm of ants, for example, is barred by these millipede clusters, the group of young juveniles deploys a different mechanism if it fears the presence of a large predator.
When Elhardt startles the cluster by blowing in their direction, they immediately scatter, before quickly joining back together.
Lead Image: Swarms of millipedes. Photo from footage by Noah Elhardt.