Promachus giganteus, one of the largest species of North American robber flies, is quite common in New Mexico and West Texas throughout the month of June and sometimes as late as August. Sufficient rainfall is required to support a large population. Their prey includes large grasshoppers, cicadas, the largest Xylocopa, male Pepsis wasps and other large robber flies. Females frequently exceed 40mm in body length, with occasional giants exceeding 46mm. The wings of giganteus are proportionately longer than those of other Promachus. Though frequently mistaken, even by entomologists, for the large form of P. nigrialbus from central Arizona, giganteus can be separated by the mixture of gray pollen into the black abdominal bands, ovipositor beginning with segment six, sparse black spination dorsally on the femurs (appearing gray from a distance - those of nigrialbus appear nearly black) and larger size (though some nigrialbus individuals overlap small giganteus in length).