Plants are producers. They can provide energy for themselves through photosynthesis. Animals, on the other hand, have to feed on other plants and animals for survival. This is common knowledge. However, there are always exceptions in nature. Some plants actually “prey on” animals for food!
Plants that “prey on” animals are referred to as insectivorous plants because their targets are chiefly insects. There are about six hundred species of insectivorous plants, distributed mainly in the tropical and sub-tropical regions. Some of the plants make traps, and some even actively hunt for the small creatures. They can be broadly classified into four categories according to how they capture their prey.
There is only one species in the genus of Venus Flytrap (Dionaea muscipula). It originates from North America and favors moist habitats. Sweet nectar is released from glands on the leaf margins to attract insects. When the trigger hairs of the leaf are repeatedly stimulated, the shell-like lobes are rapidly closed and digestion of the prey begins. Venus Flytrap can be considered the most powerful hunter among insectivorous plants. They can snap shut their leaves in half a second—the speed is comparable to hunting animals!
Tropical Pitcher Plants
There are over 150 species in the Genus Nepenthes, the majority occurring in tropical Asian areas. Among them, Pitcher Plant (Nepenthes mirabilis), a native plant of Hong Kong, is commonly found in aquatic habitats in the northwest New Territories. Pitcher Plant is characterised by the special pitfall trap—a pitcher-shaped, modified leaf at the terminal, with a slippery surface and glands which secrete nectar or a fragrance to attract its target insects. When the small animals carelessly fall into the pitcher, the digestive fluid and digestive glands inside the pitcher will drown and consume the prey.
Sundew is a genus of approximately 170 species. Five species can be found in Hong Kong and the most common is Spoon-leaved Sundew (Drosera spathulata). Its leaf surface is covered with thick, red glandular hairs that secrete sticky fluid at the tips. The fluid attaches to the hairs like a myriad crystal balls. The hairs are extremely sensitive. When any insect comes near and is glued by the sticky fluid, the hairs will immediately entrap the insect by moving inwards and downwards and pressing the prey against the leaf surface. There is no escape for the poor creature, which can only wait to be digested!
The genus Utricularia, commonly known as bladderwort, has the most species and is the most widely distributed among insectivorous plants. There are over 233 species around the world, primarily small aquatic plants. Seven species have been recorded in Hong Kong. Bladderworts capture small animals with the underwater hollow sacs—the “bladders”. Outside the bladder valve are some bristles which, when triggered, open the valve and the prey is caught in a the rapid suction process. Enzymes are secreted to slowly digest the prey.
In the bladders, some algae and pollens are sometimes found. In the past it was believed that these are bycatches along with the animals. However, recent studies by a university found that bladderwort actually takes in the algae and pollens for nutrition. Hence, bladderworts may properly be called omnivorous plants. They carry out photosynthesis with their leaves too. Their diet is rather diverse!