These are pine processionary caterpillars (Thaumetopoea pityocampa). They form processions like this when they are in search of pupation sites. Laughing Squid reports that this particular procession, filmed by Sweden-based Mehmedsamirs is nearly 13 feet long. The procession may help the caterpillar defend itself from predators by fooling them into thinking it is a snake. These caterpillars also have another defensive feature: barbed hairs that can cause skin rashes. They are considered a pest in eastern Europe.