We’re on a bit of a plant kick at MBHI this week. Maybe Western Montana’s first false spring has us ready to get our hands in the dirt. Or perhaps we’re all eagerly anticipating Rob’s Bugs and Brews talk later this month (tickets go live tomorrow morning at 9:00 AM MST). Either way, we’re running with it (or rooting for it? I’ll see myself out). On Wednesday, we introduced you to a few carnivorous plants on our weekly Online Bug Encounter. While we only showcased a couple of the more popular types (the Venus flytrap, Dionaea muscipula, and tropical pitcher plants, Nepenthes sp.), carnivorous plants are diverse and employ several mechanisms to capture and consume prey. True carnivory among plants evolved independently at least twelve times, meaning it wasn’t just a single bug-hungry plant that diversified into the several hundred species we see today.
When it comes to the different types of plant carnivory, the mechanisms behind prey capture and consumption are classified into five distinct categories: flypaper trap, snap trap, pitfall trap, suction trap, and eel trap.
Flypaper Traps
Flypaper traps utilize a gooey substance called mucilage to capture their prey. Nearly all plants produce mucilage and use it internally for water and nutrient storage, but carnivorous plants like sundew (Drosera sp.) and butterworts (Pinguicula sp.) secrete droplets of mucilage from glands on their leaves. Mucilage is easily visible on leaves of sundew plants: The glands are located on the tips of leaf outgrowths called trichomes, giving the plant a morning-dew-kissed look. On butterworts, the mucilage glands are located directly on the leaf surface. It’s not as readily visible as the mucilage droplets on a sundew, but if you gently run a finger across a butterwort leaf, you will feel the viscous substance. While it is mildly sticky, mucilage is not harmful to humans. In fact, the mucilage of sundews and butterworts is used to produce a Scandinavian yogurt-type product called filmjölk.
Once a prey item is caught in a plant’s flypaper trap, the plant releases digestive enzymes to break it down. The mushy, digested remains are then reabsorbed through cuticular pores in the plant’s leaves.
Snap Traps
Snap traps are the most well-known mechanism of prey capture and are spectacularly exemplified by the Venus flytrap (Dionaea muscipula). The inner surface of a Venus fly “trap” is covered with a series of sensitive hairs that act as triggers. Since it takes immense energy for the trap to snap shut, an insect must trigger at least two of these sensory hairs within a short timeframe. Once the trap closes, the ensnared prey must trigger five more sensory hairs for the plant to initiate digestion. The Venus flytrap’s ability to count and remember how many sensors have been triggered is a remarkable example of plant arithmetic and memory.
Venus flytraps are also exceptionally fire resistant. The traps close at the onset of rapid temperature increases and rely on seasonal fires to eliminate competing plants in their ecosystem.
Pitfall Traps
Pitfall traps are another well-known capture method, as seen in the tropical pitcher plants, Nepenthes. However, while diverse, Nepenthes is not the only type of pitcher plant. The family Sarraceniaceae, native to the Americas, contains three genera of pitcher plants, two of which occur in North America. Cephalotus follicularis, the only member of the family Cephalotaceae, is a small Australian species unrelated to other pitcher plants.
The pitfall method involves prey involuntarily slipping into the plant’s “pitcher,” a hollow, pitcher-shaped cavity filled with digestive enzymes. Some insects have evolved a mutualistic relationship with certain pitcher plants, feeding on the doomed prey while providing nutrients to the plant through their waste. Likewise, the mountain treeshrew (Tupaia montana) uses the mature pitchers of Nepenthes lowii as “toilets,” defecating into the pitchers when it visits the plant for a sweet, nectary treat.
Suction Traps
Many plant experts consider bladderworts to have the most sophisticated traps, which consist of a vacuum-like “bladder” that rapidly sucks in unsuspecting prey. Many bladderwort varieties are aquatic, and terrestrial species require damp substrates like moss or heavily saturated soil. Bladderworts do not possess root systems; the long, branching stems grow beneath the substrate or water and produce the bladder (and photosynthetic leaves in the case of terrestrial species, which typically erupt from the substrate). In aquatic species, the bladder traps are large enough to capture small vertebrates like tadpoles and fish fry, while terrestrial species feed on microscopic organisms that swim through the saturated substrate. Bladderwort traps are “set” by the constant pumping of water: As the bladder pumps water out, it creates negative pressure inside the trap. Once triggered, the bladder releases a “trapdoor,” rapidly “vacuuming” the nearby prey and water.
Eel Traps
Also known as lobster-pot traps, eel traps act similarly to household fruit fly traps. Like bladderworts, plants in the Genlisea genus lack roots but possess subterranean traps that feed on microscopic soil protozoa. The hollow, corkscrew-shaped traps contain inward-facing hairs, allowing the organisms to climb in but not out. Some pitcher plant species utilize a similar method to prevent prey from climbing back up the walls of the pitcher. Think of it as the plant world’s version of parking lot spike strips; once you drive one direction over the spikes, backing up becomes a useless (or very, very stupid) option.
Since 2000, roughly three new species of carnivorous plants have been discovered every year. Unfortunately, because of the sensitive nature of their ecosystems and their specific growing requirements, they risk rapid extinction in the face of climate change and habitat loss (the rare plant trade is another story entirely). A 2020 assessment found that roughly one-quarter of known carnivorous plant species are currently at risk of disappearing forever. Like their insect prey, these plants are vanishing from the wild, and unless we humans take decisive action, we risk losing these utterly fascinating organisms altogether.