The Galileani are all related, descended from the same root stock - Callistans - and evolving after colonizing the other three moons long, long ago. Galileans are about the same level of civilization as Martians, having started at about the same time.  
Callistans, and thus all the Galilieani - evolved from something very like our Macrochiroptera, the great fruit bats, and are certainly of the same order. They are large and fragile, having evolved in the very light gravity of the Galilean moons, which is less than half that of Mars.
Galileani stand one and a half times as tall aa Martians, averaging 3 meters tall with a wingspread about the same. They are too weak to fly in Martian or Mercurial gravities, and too fragile to even stand in Earth or Venusian gravities for any meaningful length of time.
As in the Macrochiroptera, Galeliani arms have evolved into wings with thumb hooks. The feet, however, have become foot-hands, nearly as nimble as a human hand. There is a short tail, webbed to the inner legs, to aid in flight. The legs are longer than in Earth fruit bats, and more flexible, allowing the foot-hands to reach up over the head.
Like the Macrochiroptera, Galeliani have good hearing, but do not use echolocation. Their eyesight is excellent and binocular. They have a good sense of smell as well. They eat over-ripe fruit and flower nectar.
Ioans
Ioans are the smallest and least furry of Galileani. Io being the warmest and heaviest of the Galilean moons, they have lost most of their fur, retaining only a mane about their heads and a strip down their backs, all else bieng naked of fur.
Europans
Europans are lightly furred all over, and are specialized for drinking nectar. Their tongues are very long and tubular and their muzzles are longer and narrower than the other Galileani.
Ganymedians
Ganymedians are much like Europans, with a shorter, broader muzzle and shorter tongue than their kin. Their tails are longer than other Galileani, with most of its length being free of webbing.
Callistans
Callistans are bigger and furrier than other Galileani, with a very thick mane, and larger jaws for penetrating the thick-husked Callistan fruit.

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One of the most important things for any civilization is the ability to manipulate items to make things and the ability to carry things. Galileani can use their two thumb hooks over branches and bars, or rest their wings on the ground, freeing the two foot-hands for manipulation. In either mode, the tail acts as the third point of the tripod, and the tails are quite solid and strong.  In flight, items are carried in the feet and belly-packs. The thumb hooks are able close enough to grasp things, and are strong enough to support the whole body.

Galileani form temporary harems of about 1-6 females per male each breeding season, with the fittest males getting the largest harem, and many males - especially younger and undistinguished ones - not finding mates. Sexual selection plays a huge role in their society - with the definition of "fittest" changing as they became civilized. It isn't just physical fitness that matters now, as inventors, scientists, performers, and the like find mates with no problem.


Galileani are not at all territorial - they like getting up close and personal in huge breeding roosts - and are a naturally communal society. Tending and spreading their preferred fruit trees is an important part of their lives. As the Galilean moons get their light from the sun, but most of their heat from Jupiter, they migrated inward as Jupiter cooled. They are a highly technological civilization. Some of their preferred fruits grow on vinelike plants with gas-filled bladders that drift in the light gravity of the moons.

Their civilization first started with pieces of woven leaves tied between tree limbs to form enclosed volumes, heated by warm air rising from fires set in pits on the ground.  Drying fruit over the fires for preservation was been one of their first innovations.   Now they have cities with large open latticework buildings with jungle-gym like interiors.  There are no horizontal-surface floors, the lower surfaces of rooms funnel down into catch-basins for debris (their ancestral fruit bats are messy eaters, and traditional building layouts tend to persist). Food is now transported into the cities from the surrounding countryside.