Mountain Stag Beetle encased in solid lucite. Large Size! Size: 4 1/4" length x 1 3/4" width x 1" depth. Stag Beetles are named for the large mandibles (jaws) that male stag beetles possess, which resemble the antlers of a stag (male deer). These jaws are used by the males to joust with each other. Stag beetles range in color from black to brown to orange and can range in size from 3.5 to 7.5 centimeters long. In some species of stag beetles the mandibles may be as long as the beetles body. Adult stag beetles are herbivores that feed on plants and sap while the larvae feed on decaying wood and are important in the decomposition of dead trees.