Slippery Pitfalls

Among the largest of the carnivorous plants are the hanging pitcher plants. Some, such as those found in Southeast Asia, are vines that grow to the tops of trees. These contain traps that can hold up to two quarts [2 L] of liquid to catch creatures as large as frogs. Some are even reputed to have caught rats. But how do the traps work?

Each leaf of the pitcher plant resembles a jug, or pitcher, and has a lid to keep out the rain. An insect is lured by the bright color and abundant nectar of the pitcher, but the rim has a slippery surface. When the insect attempts to get the nectar, it loses its footing and slides into the pool of fluid at the bottom. Downward-pointing hairs inside the leaf prevent the insect from escaping. In addition, the nectar of some pitcher plants contains a narcotic that stupefies the prey.

The pitcher plant with the most striking appearance is surely the cobra plant, which grows in the mountains of California and Oregon in the United States. The pitcher closely resembles a cobra rearing up to strike. An insect finds its way into the mouth of the plant but then becomes confused by light that seems to be shining through small windows. The insect keeps flying toward the light in a futile attempt to escape. Exhausted, it falls into the liquid at the base of the plant and drowns.

North American pitcher plants are now gaining popularity with gardeners. Their beautiful flowers and elegant leaf shapes compare favorably with other plants, and they are easy to cultivate. They also consume flies in enormous numbers. Indeed, each clump of leaves can trap thousands of flies in a season. Bees are not at risk, as it appears that they are not attracted by the plants. But how are the flowers fertilized without trapping the insects that pollinate them? The flowers appear first, while the pitchers are still growing. By the time the pitchers come to maturity, the flowers have died and the pollinators have moved on.

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Hanging pitcher-plant leaf

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Flower and immature pitcher leaf of cobra plant

[Credit Line]

Copyright Chris Heath, Kentish Town City Farm, London

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North American pitcher plant

Its flower is about the size of an orange

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Leaves of the North American pitcher plant

Inset: A fly feeds on nectar that causes it to become drowsy

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◼ Insects Fight Back!

Not all insects succumb to the wiles of insectivorous plants. The blowfly, for example, has an appendage on each foot. Like a climber’s grappling iron, these enable the insect to clamber up the down-curved hairs of the pitcher plant. When the blowfly’s eggs hatch, the maggots feed on the decaying insects. Then, as soon as they are ready to pupate, they cut holes in the pitcher and escape. Caterpillars of a small moth, in their turn, cover offending pitcher-plant hairs with a web. Some spiders craftily spin their webs in the upper part of pitcher plants so as to have first call on any inquisitive insects, and at least one kind of spider has a special skin that allows it to seek shelter in the plant’s digestive juices if it is threatened.