Stag beetles are omnivores, meaning they eat both plants and other insects. However, they prefer eating plants and only eat other insects if no other food source is available.
Some of the things stag beetles seek out include decaying wood, leaves, nectar, flowers and fruit that has fallen to the ground. Should there be a shortage of these things, the stag beetle hunts other, very small insects.
When a stag beetle eats, it does not actually use its large mandibles. Instead, it uses its pair of feathery galeae to dip in and out of a food source, absorbing the liquid and then drinking it.