Carnivorous plants are among the most fascinating plant species. Mostly, they eat insects, although some occasionally devour small mammals and amphibians, according to the Botanical Society of America. Plants accomplish this feat with specialized structures, and many carnivorous plants have evolved their own unique structures to achieve the same goal.
Most plants produce carbohydrates in their leaves via photosynthesis and absorb the rest of their nutrients from the soil. In some environments, however, soil fertility is so depleted that plants can't absorb adequate nutrients. Some of these plants developed carnivory, the ability to capture and absorb their nutrients from the bodies of prey. Carnivory has allowed plants to colonize low-nutrient environments, and more than 600 species use it as a means of survival.
Carnivory evolved independently in multiple lineages of plants, resulting in carnivorous plants with highly diverse structures that accomplish the same purpose. Plants with pitfall traps form receptacles filled with digestive enzymes. Sticky traps act like flypaper, snagging insects that land on the plant. Snap traps, such as the well-known Venus flytrap, close upon their prey. Suction traps capture prey behind a hinged door, and lobster-pot traps lure prey into structures from which they cannot escape.
Because plants can't actively hunt their prey, they must attract them to their traps. Carnivorous plants have different structures that entice prey through scent, coloration, nectar and ultraviolet guides. For example, according to Flora of North America, the pitcher plant uses ultraviolet guides to not only lure prey but to direct the unsuspecting insects into the tube. Once inside the plant, slippery sides and downward-pointing hairs ensure that the prey drops into the pool of digestive enzymes waiting at the bottom of the plant.
Once the plant has captured its prey, it needs to break it down into the essential nutrients it needs, and to do this carnivorous plants produce digestive enzymes. On some plants, like the sundew, special glands contain these enzymes, and the plant brings the prey into contact with the gland to start digestion. Others, like the pitcher plant, fill with enzymes that the prey must fall into. Digestive enzymes used by carnivorous plants are similar to stomach acid, according to the Plant Biology site at the University of Illinois.
Perhaps the most fascinating structures belong to the active traps, like the Venus flytrap. These traps contain trigger hairs that sense movement when they are touched multiple times in close succession. Cells within the leaves rapidly lose water, causing the two halves of the trap to fall shut and close upon the prey. Fringes along the edges of the leaves interlock, trapping insects in a cage-like structure.