If you thought pitcher plants were cool, enter the amazing world of sticky or flypaper traps. These traps are dominated by the genus Drosera, commonly known as the sundews, representing more than 180 species. They are the only genus of active sticky traps, which are most interesting because they often feature moving leaves, and are found in all continents except, of course, Antarctica.

Unlike pitcher plants that were centered in Southeast Asia, almost fifty percent of sundew species are found in Australia. Consisting mostly of rosettes that stand straight upwards, the plants can be as short as a centimeter in length to as long as a meter. Some species are climbing and can reach a couple of meters in length. Rosettes are suited to capture walking prey whereas climbing species tend to target flying prey.

Other genera of sticky traps are passive. One species called Drosophyllum lustanicum, known as the Portuguese dewy pine, which grows in the Mediterranean. It is unique in that it grows in relatively dry and alkaline soils.

A liana found in West African rainforests, namely, Sierra Leone, Liberia, and Ivory Coast called Triphyophyllum peltatum is apparently the largest carnivorous plant. Of its three stages in its life cycle, it is only carnivorous in the second stage when it produces long leaves bearing sticky tentacles, as seen in Drosera, which is why it is called the part-time carnivorous plant. Unfortunately, excessive deforestation spurred by palm oil plantations has already destroyed much of its habitat.

Butterworts, also known as Pinguicula, are represented by more than 90 species found in wet, humid, and montane areas of the Northern Hemisphere such as Europe, Asia, North America, and West Africa. The largest diversity is in Central America. They are composed mostly of thick fleshy leaves bearing sticky tentacles that trap prey.

Byblis, sometimes also known as the rainbow plants, are a group of species native to Australia.

Roridula species, R. dentate and R. gorgonias, are thought to be borderline carnivores; they trap insects but lack the enzymes to digest them. One study, however, reported to have detected enzyme activity in the leaf surface of R. gorgonias. 

Deadly Glue

Pitcher plants rely on capturing insects by making their rims or peristomes slippery and through the viscoelastic fluid found at the bottom of the pitchers. Drosera plants use a strategy similar to the latter. They are armed with mucilage, a thick, sticky fluid composed of water and polysaccharides that act like glue (hence the name sticky traps). This mucilage has properties similar to the viscoelastic fluid found inside pitchers: it is sticky and elastic. In some cases it is so elastic that it can be stretched into threads up to a meter long! Some species have mucilage that is more elastic than others. The elasticity and adhesive properties of the mucilage is determined mainly by the ratio of polysaccharides to water.

The secretions of Roridula plants are not water-based like those of Drosera. Instead, they are resinous containing uncured rubber. Therefore, they do not dry up like the mucilage of Drosera.  The adhesive droplets of the R. gorgonias, reported to be the strongest among all flypaper traps, are efficient at trapping relatively large insects.

Protruding from the leaves are short stalked glands called tentacles, which have colored glands at the tips oozing mucilage with nectars to attract prey. Guised as nectar, these beautiful glistening dew drops attract insects. But these droplets are deadly. Unaware of the misfortune about to befall them, insects—flying or crawling—stop by to have a quick sweet drink, but as they settle on the tentacles, they become ensnared by the mucilage.

Upon contact with insects, the glands ramp up mucilage production to coat the insects further, making it difficult for them to escape. Movements of the insect trigger the tentacles to bend toward them to ensure they cannot escape. If that wasn’t enough, nearby glue-tentacles also wrap themselves around the insect, maximizing contact with them. Sensing the location of prey, the tentacles bend in that direction to enclose prey. Tentacle movement can take only a few minutes to a couple of hours. Soon the hapless insect finds itself smothered in mucilage and dies of exhaustion to free itself.

Some interesting species exhibit another type of tentacle that are longer and devoid of mucilage. These can move extremely rapidly and capture prey in the plant’s vicinity.

Many species have another tactic that precludes any chance of escape: The entire leaf from which the tentacles emerge slowly curls tightly around prey to enclose them (which reminds me almost of how a constrictor strangulates its victims). This movement is slower and usually takes a few hours.

This time-lapse video below, filmed by Tim Shepherd for the BBC show LIFE, shows Drosera capensis capturing a midge. First, the tentacles bend toward the midge and then the entire leaf rolls around it, encapsulating it.

When stimulated, leaves usually take a few hours to roll around prey, but in some cases, movement can be very extremely fast; for D. burmanni it takes only a few seconds. Daubed in mucilage and entangled in the tentacles, insects try hard to extricate themselves. But often the plant succeeds in capturing their meal. The glands at the tentacle tips then secrete a cocktail of digestive enzymes that break down the carcass and later absorb the nutrients from them.

Against the odds

Amazingly, one bug is able to evade the extremely gluey secretions of Roridula gorgonias, the borderline carnivore mentioned earlier, which reportedly has the strongest glue among the fly paper traps. In fact, the bug, Pameridea roridulae, actually lives on the plants and is able to walk freely on its surface, frequently touching the sticky droplets on the tips of its tentacle-shaped hairs without being trapped. Insects glued onto the leaves of the plant serve as food for the bug. But the plant is still happy as it gets its nitrogen from the bug in form of poop. Just like some pitcher plants, this fly paper trap has evolved to make use of poop to obtain nitrogen. The bug defecates on the leaves and the plant absorbs the droppings through the leaf cuticles of the plant.

How is it able to break free from the adhesives coated on the tentacles? The outermost skin layer of the bug, known as the epicuticle, secretes a thin layer of protective grease-like fluid that adheres to the glue on the plant tentacles. When the bug lifts itself, the grease layer detaches from the bug, preventing the plant’s sticky droplets from trapping the bug down. When compared with the fly Calliphora vicin, the bug requires less work to break free from the adhesive glue. Unlike the bug, flies have a non-continuous layer of grease, which makes them easy targets of the plant’s glue.

The excised tentacles can even bend by 360 degrees. How do they move so fast? Scientists think that it may be due to the fracturing of the cells where the tentacles are bent. These snap-tentacles are only good for a single use, but this shouldn’t be much of a problem because the leaves can grow back within a few days. Scientists surmise that snap-tentacles help the plant capture larger prey from the periphery.

To bend or not to bend?

Tentacles and leaves usually respond specifically to prey only. Mechanical stimulation of the glue-tentacles in D. capensis did not elicit enzymatic activity. The leaves of D. capensis only bend in response to live flies, and are unaffected by dead flies or small fruit fly-sized stones placed on them. Even mechanical stimulation or touching of leaves with a brush could not trigger the leaves to bend. But when freshly crushed flies were placed, they responded by strong bending, which suggests that the plant responds selectively to certain compounds found in insects.

What triggers the bending movement? The accumulation of jasmonates in the curling part of the leaves plays a part. Jasmonates are plant hormones that play a crucial role in regulating plant defenses against herbivores as well as their responses to unfavorable environmental conditions. In fact, researchers found that the application of jasmonates alone could trigger the bending of the leaves.

Scientists hypothesized that the red colored tentacles of D. capensis may play a role in luring fruit flies. Experiments, however, showed that this is not the case as similar numbers of prey were caught by another version of the plants with white tentacles.

Feeding of D. capensis plants with fruit flies resulted in increased uptake of phosphorus and nitrogen by the plants after 10 weeks, which in turn led to higher above-ground biomass and more leaves compared with unfed plants. Uptake of phosphorus was higher than that of nitrogen implying that the plants were more deficient in phosphorus, and once that limitation was overcome through prey, their photosynthetic efficiency increased as well.

Pollinator or prey? 

Like other carnivorous plants, the flowers of Drosera are displayed high above on a long stalk separating them from the leaves. Two reasons have been proposed to explain this arrangement: either it prevents potential pollinators from becoming prey or it makes the flowers more visible to potential pollinators so that they receive more pollination visits.

An experiment was carried out to find out which explanation is more likely using two Drosera plants, one with flowers on short stalks, and the other with flowers on a long stalk, occupying the same area. The results showed that no pollinators were actually captured by either of the species. But the plants bearing the shorter stalks received fewer insect pollination visits compared with the plant bearing flowers on longer stalks, giving support to the latter explanation that flowers on long stalks are more prominent and evolved to attract pollinators.

Competition with Spiders

Do carnivorous plants compete with other animals for arthropod prey? Botanists and ecologists have been pondering this possibility for years. Indeed, the first study to investigate the question found evidence that they do compete. The wolf spiders, Rabidosa rabida, and Sosippus floridanus, were found to share the same habitat and eat the same insects as the pink sundew, Drosera capillaris, located in southeastern USA.

In the field, wolf spiders constructed their webs far away from sundews, probably to reduce to competition. And in the presence of sundews, spiders would build larger webs. The fitness of D. capillaris was adversely affected in the presence of spiders; they produced fewer stalks, flowers, and seeds. But when the sundews were fed crickets, which were also consumed by the spiders, they produced more leaves and seeds. So the spiders seemed to be depriving the plant of bugs, and thus nutrients.

Deadly for bugs, Useful for us

Sticky traps aren’t just a natural wonder for us to marvel at. Like pitcher plants, they have the potential to improve our lives. While the adhesives produced by the plant may be deadly for insects, they can be useful for us. Researchers found that the nanostructure of adhesives extracted from three sundew species consisted of a network of polysaccharide nanofibers with tiny pores in between. This material was used as a coating to allow nerve cells to attach onto. What’s more, the adhesive also has antibacterial properties. These characteristics make the material well suited for use in growing regenerative tissue in the lab. In fact, bone cells and soft tissue cells were shown to attach to the material.

References

Anderson, B. (2010) Did Drosera evolve long scapes to stop their pollinators from being eaten? Annals of Botany, 106: 653–657. doi:10.1093/aob/mcq155

Jennings, D. E., J. J. Krupa, T. R. Raffel, and J. R. Rohr (2010) Evidence for competition between carnivorous plants and spiders. Proceedings of the Royal Society B – Biological Sciences, 277: 3001-3008.

Lenaghan, S.C., Serpersu K., Xia L., He, W., and Zhang, M. (2011) A naturally occurring nanomaterial from the Sundew (Drosera) for tissue engineering, Bioinspir. Biomim. 6 046009. doi:10.1088/1748-3182/6/4/046009.

Nakamura, Y., Reichelt, M., Mayer, V.E., Mithofer, A. (2013) Jasmonates trigger prey-induced formation of ‘outer stomach’ in carnivorous sundew plants, Proc R Soc B, 280: 0130228. http://dx.doi.org/10.1098/rspb.2013.0228

Pavlovič, A^., Krausko, M., Libiaková, M., Adamec, L. (2010) Feeding on prey increases photosynthetic efficiency in the carnivorous sundew Drosera capensis, Annals of Botany, 106: 653–657, doi:10.1093/aob/mcq155

Poppinga S, Hartmeyer SRH, Seidel R, Masselter T, Hartmeyer I, et al. (2012) Catapulting Tentacles in a Sticky Carnivorous Plant. PLoS ONE 7(9): e45735. doi:10.1371/journal.pone.0045735

Voigt, D. and Gorb, S. (2008) An insect trap as habitat: cohesion-failure mechanism prevents adhesion of Pameridea roridulae bugs to the sticky surface of the plant Roridula gorgonias, The Journal of Experimental Biology 211, 2647-2657.

The links to all the articles are the full-text, except the one on tissue engineering.