Left: Sand under a rock overhang is pockmarked with antlion pits.
Right: Closer view of pit.
Antlions feast upon ants by digging pits with steep sides in sandy soil. An ant that ventures over the edge will slide down the side of the pit riding a wave of unstable sand. At the bottom of the pit, the ant is within the reach of the antlion mandibles. Attempts to climb the sides is rarely successful as the ant slides back down repeatedly. Antlions can block escape by tossing sand at the ant which…
Source: Living With Insects Blog