Frogs may contain a variety of foods in their stomachs, including insects, mollusks, worms, arachnids, amphibians, reptiles, rodents, birds and fish. Frogs swallow their prey whole, so it is often possible to identify their recent prey items upon dissection. Most frogs used for dissections are medium-sized species that prey primarily on insects, arachnids and other frogs, but occasionally one is found to have swallowed a fish or snake.
Frogs primarily rely on their vision to find prey, and they often fail to notice prey that does not move. Scientists studying the prey-capturing habits of leopard frogs have found that while frogs can see insects that are 90 degrees to their right or left, they prefer to capture prey that is in front of them. However, frogs sometimes attempt to capture prey to either side, and some individual frogs demonstrate a preferred side.
While some frogs actively forage for food, the vast majority simply wait motionlessly for prey to come within range. Once the prey is close enough, frogs open their mouths and extend their tongues. Covered in sticky mucus, the tongue ensnares small prey. When attacking larger prey, the frog's arms are sometimes used to help force the animal into its mouth.