or How bugs walk up walls.
All insects' legs have some variation of this organ, and in some species it enables them to adhere to surfaces which cannot be grasped with their legs' claws. When the claws are being used normally, the arolium stays stretched taut in between them and higher than the claws' grasp, like the skin between your thumb and index finger. If the claws are unable to catch a surface, however, they retract. In so doing, the arolium extends past them and becomes turgid with blood in about a tenth of a second, inflating like a balloon against the surface. The organ's surface is sticky with organic adhesive, and attaches to all of the area it was inflated against even after the arolium deflates a moment later. This near-instantaneous process leaves the insect with a secure hold on whatever glassy or polished surface was causing its problem. To detach, the insect merely closes its claws, and their outward force along with the arolium's retraction pull it free from the surface.
Bees, ants, the German cockroach, and I suspect -- though I couldn't find any supporting literature -- flies as well, use this method of grasping smooth surfaces, but even insects which can't walk up walls have arolia. The arolium developed, as far as science knows, as a sense of touch for the insect on a scale where articulated fingers wouldn't work out. Their quick tumescence and subsidence theoretically transmits enough information for the insect to decide whether it's holding food, a rock, or what-have-you. From that function, a few small evolutionary leaps covered the organ with stickiness and made it sensitive enough not to adhere too weakly or strongly to a surface.