How does an informed minority of scouts guide a honeybee swarm as it flies to its new home?

When a honeybee swarm lifts off to fly to a new nest site, only the scouts know in what direction the swarm must fly, and they constitute only about 5% of the bees in a swarm. Nevertheless, a swarm will fly quickly and directly to its destination. How does the small minority of informed scouts indicate the swarm's flight direction to the large majority of uninformed bees? Two hypotheses have been suggested. The first proposes that the flying scouts streak through the swarm cloud in the direction of the goal, thereby indicating the travel direction visually (vision hypothesis). The second proposes that flying scouts release pheromones from their Nasanov glands at the front of the cloud of flying bees, thereby indicating the travel direction chemically (olfaction hypothesis). We tested both hypotheses by studying the flights of normal swarms and comparing them to the flights of swarms composed of bees whose Nasanov glands were sealed shut. Our results support the vision hypothesis and contradict the olfaction hypothesis. We identified fast-flying bees (‘streakers’) in swarms, as predicted by the vision hypothesis, but we found no effect of sealing the Nasanov glands of swarming bees. Sealed-bee swarms were perfectly capable of flying directly to a new nest site.