The "balloon-creatures" are an unnamed species of non-sapient, lighter-than-air predators native to the Earth's moon Luna, billions of years ago, back when Luna was a temperate Earth-like world with an oxygen atmosphere and covered in temperate forests.
These spider-like creatures have large inflated purple bodies filled with lighter-than-air gas (presumed to be hydrogen) produced by their metabolism. They float above the lunar forests resembling purple balloons, and weave dangling red web threads covered in an adherent substance to capture prey from the surface. These threads are strong enough to lift a Human being from the ground. From the base of the purple bladder protrude clusters of black spider-like limbs, a pair of black multi-faceted eyes, long salivating fanged mandibles and a thin pointed snout which perforates the prey's skin to suck out its body fluids.
- "The Moon Era", by Jack Williamson (1932)