Almost all insects have three pairs of legs, and each leg is segmented. The terminal segment of the leg is known as pre-tarsus. The pre-tarsus of insects bears a pair of claws, which are used to walk on rough surfaces.
However, the claws do not provide sufficient grip when the surface is too smooth, such as that of a leaf or a wall.
Hence, many insects, including ants, possess a median lobe between their claws, known as arolium which acts as an adhesive organ.
It is a flexible cuticular lobe, which can be folded and unfolded while walking. In general, the arolium is made up of dense tubular hairs whose tips are filled with glandular secretions.
These secretions contribute to the ant movement on smooth surfaces. Thus, the presence of the arolium on their legs enables ants to move on any smooth surface without falling. In some insects, the arolium is also believed to act as a vacuum cup or suction pad.