Antlion larvae, or 'doodlebugs', trap ants by digging inverted cones or pits in the soil. These pits are usually 1 1/2 to 2 inches in diameter and are usually found in the soil near or under the base of a house or other building which provides protection from direct rainfall. The antlion larvae prepare the sides of their pits with fine sand or soil particles so ants that fall in can't climb out, and land in the jaws of the 'doodlebug' waiting at the bottom. Predation in action can be observed by dropping an ant into the edge of a pit. The 'doodlebug' usually captures the ant but often flips soil and sand all around in the process. Antlion larvae eventually pupate in the soil, becoming adult insects that look somewhat like dragonflies, except that they are more fragile and are feeble fliers. Antlions are beneficial to man because of their ant diet and they cause no problems.
Adapted from the website of the Department of Entomology, Clemson University and from Mark Swanson's Antlion Pit: A Doodlebug Anthology