Balloon Bees, urinaria apis caeli Europa, are a type of honeybee native to Europe. Despite the name, balloon bees are not balloons, and in fact predate airships.
Originally a cave-dwelling species, balloon bees--like many species before them--found man-made structures that suited their evolutionary niche, and adapted. Balloon bees got their new names when they began to make nests in airship cells (the balloon of rigid airships is shaped by fabric stretched over a framework, rather than being inflated by the gas itself[1], which for a bee is near as makes no difference).
They are implied to travel as a swarm; however, when they select a cave (or airship) to settle in, the bees will spread out as far apart from each other as they can get, and proceed to build individual dens.
When Our Heroes are urgently attempting to escape from Big Rat Island, one of the many problems with Captain Pierre's airship is an infestation ✣ of these bees.