Mark Knapp -- (sorry, the "quote" function doesn't work on my work computer) --
Belly hair is desirable for spinning and stacking on floating bass bugs because of its propensity to take dye well and achieve brighter colors, as Capt. Bob pointed out above, but I would say the reason most hair guys prefer it for floating bugs is because of its texture and composition. It has little pockets of air in the stem of each hair, making it partially "hollow". This obviously aids in floatation, but it also makes the hair highly compressible. This means that it flairs severely when compressed by thread. It also means that spun or stacked bunches of hair can be tightly compressed with a hair packer to form a dense body.