When they feel threatened, they lower their heads, raise their abdomens and squirt an unpleasant-smelling black liquid. This is a means of defence against potential predators. The liquid is said to smell much like leather to humans.
There is another reason why they sometimes raise their abdomens and hold that position for hours. In the desert, water is available in the form of fine mist in the morning. When a beetle leans down, microdroplets of water collect on its abdomen and then run down toward its head, so that it can drink the precious liquid.