Sucker-footed bats (Myzopoda aurita), resident to Madagascar were named for the suction cup like structures on their hands (see image). It has long been thought (ever since the little critters were named) that these structures no only look like suction cups but also work like them.
However, through recent research efforts we now know that these structures do not work like suction cups at all. Instead, these little pads function the way a fly climbs a windshield by a process called wet-adhesion. Specifically. a recent study by Riskin and Racey (2010) described the biomechanics of how sucker-footed bats attach to surfaces. By placing these bats on materials with even spaced holes (which would prevent suction) these researchers determined that sucker-footed bats don't! Instead like a fly, they use the surface tension of water (in this case sweat) to attach to surfaces (wet-adhesion). The water-based material used for wet-adhesion would be a sweat like substance.
Another interesting observation they made was that these bats cannot hang upside down. (Or they COULD but then they would plummet head first to the ground!)
Most bats hang upside down and this rare behavior of roosting head-side-up is likely because of how these bats detach themselves (and their wing/hand pads) from the surface of objects such as leaves that they roost upon. To let go of a surface and move forward these bats push their pads forward to break their attachment. This works well if they are say climbing a surface. However, upon roosting head down the force of gravity would be in the same direction (forward) and would thus passively deform their foot pad and start to detach them from the surface.
I will admit I am somewhat disappointed to learn these little bats are not in actuality sucker-footed. That said, the fact that they are more fly-like just might compensate for it... maybe they could be renamed fly-footed bats! Just a suggestion.
If you would like to read more the article can be found (HERE) and the citation is as follows:
Riskin, D. K., and P.A. Racey. 2010. How do sucker-footed bats hold on, and why do they roost head-up? Biological Journal of the Linnean Society 99:233-240.