1. Antlions are opportunistic trap building predators that cannot control prey encounter. Their trap should ideally retain a great diversity of prey. However, building a single trap that captures many prey with varying characteristics can be challenging. 2. A series of five different ant species ranging from thin to large, of sizes ranging from 2.75 to 6.5 mm, and a mean weight ranging from 0.54 to 6.00 mg were offered in a random succession to antlions. The state of satiation of the antlions was controlled, and their mass and the depth of their pit were recorded. The reaction of antlion to the prey, the probability of capture as well as the time to escape were recorded. 3. The probability of an antlion reaction is an increasing function of the pit depth and a decreasing function of antlion mass. The probability of capture is highest for intermediate prey mass and is an increasing function of pit depth. The time to escape is a declining function of prey mass and an increasing function of pit depth. 4. There is an upper limit to prey mass given that large prey escape out of the pit. There is a lower limit to prey mass given the difficulty to apprehend the smallest, thin species. Consequently, there is a range of prey mass, corresponding to a medium-sized ant of 2 mg, for which the pit functions best. The physics of insect locomotion on sandy slopes was identified as the key to understanding the functioning of antlion pits.