Carnivorous plants have long fascinated plant enthusiasts, well before Charles Darwin published the first truly popular and widespread book ‘Insectivorous Plants’ discussing the known carnivorous species and their adaptations to growing in low-nutrient environments. Today, botanists are continuing to expand on our understanding of carnivory in plants, finding species as common as teasel exhibiting carnivorous behavior. Darwin himself noted multiple species that may be trapping and digesting insects in ways similar to sundews and butterworts, to include the familiar tobacco and petunia. With the ever-expanding definition of carnivory in plants and sub-categories like proto-carnivorous, near-carnivorous, and even ‘murderous’ plants, it’s best to focus on the truly carnivorous species – those that trap, kill, digest, and absorb nutrients from their prey. These include – but are not limited to – American pitcher plants (Sarracenia spp.), sundews (Drosera spp.), butterworts (Pinguicula spp.), cobra lily (Darlingtonia californica), Venus flytrap (Dionaea muscipula), and the extensive bladderworts (Utricularia spp.).
The aforementioned carnivorous plant species each have unique adaptations to trap and digest prey, ranging from pitfall traps to sticky leaves, spring traps and vacuum suction traps. However, like so many other flowering plants, these species depend on insects for pollination. This begs the obvious question of how a carnivorous plant might go about attracting insects for pollination while avoiding trapping, killing, and digesting said pollinator. There are three mechanisms by which carnivorous plants avoid killing those insects that are vital to their reproduction: spatially, temporally, and through specific attractants/deterrents. Spatial separation is seen in butterworts and Venus flytraps, among others, where the flower is held high above the trapping portion of the plant. For instance, Venus flytrap leaves and traps form in a rosette closely hugging the ground, while the flowers are sent high atop a scape, allowing flying insects to safely gather pollen. Perhaps the best adapted spatial separation is seen in bladderworts, the largest carnivorous plant genus. Bladderworts lack roots, instead having modified stems and leaves that grow submerged in water or boggy soils. The small traps have hairpin triggers at their mouths, utilizing vacuum suction to capture nearly-microscopic prey like mosquito larvae. Flowers are born on long, tall stalks held well above ground or the water’s surface, clearly accessed by flying pollinators but kept well away from the dangerous traps.
Temporal separation is seen in multiple genera of carnivorous plants, including the American pitcher plants (Sarracenia spp.) and the California native cobra lily (Darlingtonia californica). Flowers appear before truly carnivorous leaves appear, allowing pollinators to visit flowers with little-to-no danger of falling into the pitcher-like traps. Little is known about the pollination of Darlingtonia, though some generalist Adrenid bees have been observed visiting the flowers. Sarracenia flowers are known to be frequently visited by bumblebees, sweat bees, and flies, and their flowers are specifically adapted to avoid self-pollination. Pollinating insects enter the flower in one direction, tumble around in search of pollen, and exit in another direction that keeps them from contacting the female portion of the flower (stigma). This ensures greater genetic diversity among populations, resulting in a more robust and resilient gene pool.
The third mechanism of attracting and/or deterring insects is a common method for avoiding killing those insects the plants depend upon for pollination. Simply put, some carnivorous plants exude sweet smells and attractive colors on their flowers, while their traps emit unpleasant smells and/or colors and patterns. For example, butterworts – whose leaves lay flat on the ground and are most often visited by crawling insects – have foliage that smells of fungus, thus attracting beetles and other insects attracted to fungi. The stinky leaves are unattractive to most pollinators that are attracted to sweet floral scents. Interestingly, some Sarracenia will begin to emit sweet smells from their pitfall trap leaves, enticing and trapping the very insects that may have previously been pollinating their flowers (so sinister!). Carnivorous plants truly are evolutionary marvels, and the ever-expanding definition of carnivory in plants keeps this group increasingly more interesting!
Sarracenia alata flower
Dionaea muscipula flower