American Society of Naturalists

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“Testing Darwin’s hypothesis about the wonderful Venus flytrap: marginal spikes form a ‘horrid prison’ for moderate-sized insect prey”

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Alexander L. Davis, Matthew H. Babb, Matthew C. Lowe, Adam T. Yeh, Brandon T. Lee, and Christopher H. Martin (Feb 2019)

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Venus flytrap marginal spikes, Darwin’s ‘horrid prison’, have a surprising nonlinear effect on prey capture success

A representative prey capture arena containing one plant, one cricket, a ramp, and a petri dish of water.<br />(Credit: Alexander L. Davis)
A representative prey capture arena containing one plant, one cricket, a ramp, and a petri dish of water.
(Credit: Alexander L. Davis)

Venus flytraps (Dionaea muscipula) are threatened carnivorous plants native only to the Southeastern United States near the border of North and South Carolina. While Charles Darwin was enamored with all carnivorous plants, he was particularly struck by the Venus flytrap which he dubbed “the most wonderful plant in the world”. He performed a number of careful experiments in his personal garden and described several hypotheses about the function of different structures that are unique to flytraps. One of these structures was the marginal spikes, found on the edge of the traps. He predicted the marginal spikes would form a “horrid prison” for medium and large insects that triggered the trap to close, but small insects would escape through the space between the spikes. Over the 130 years following Darwin’s prediction, no studies had directly investigated the role of the marginal spikes in prey capture.

Researchers at the University of North Carolina, Chapel Hill combined laboratory experiments, a semi-natural experiment, and data from wild Venus flytraps to determine the effect of the marginal spikes on prey capture ability. They found that the marginal spikes drastically improve the plant’s ability to capture small and medium-sized insects, but have no effect for larger prey. The study also confirmed that Venus flytraps only capture a minority of the insects that trigger the traps to close. Since flytraps are one of only two carnivorous plants with mechanical traps instead of the more common sticky or pitfall traps, these findings bolster our understanding of the selective forces underlying the diversity of carnivorous plants, and offer insight into the origins of one of the most marvelous innovations among plants.


Abstract

Botanical carnivory is a novel feeding strategy associated with numerous physiological and morphological adaptations. However, the benefits of these novel carnivorous traits are rarely tested. We used field observations, lab experiments, and a semi-natural experiment to test prey capture function of the marginal spikes on snap traps of the Venus flytrap (Dionaea muscipula). Our field and laboratory results suggested inefficient capture success: fewer than 1 in 4 prey encounters led to prey capture. Removing the marginal spikes decreased the rate of prey capture success for moderate-sized cricket prey by 90%, but this effect disappeared for larger prey. The nonlinear benefit of spikes suggests that they provide a better cage for capturing more abundant insects of moderate and small sizes, but may also provide a foothold for rare large prey to escape. Our observations support Darwin’s hypothesis that the marginal spikes form a ‘horrid prison’ that increases prey capture success for moderate-sized prey, but the decreasing benefit for larger prey is unexpected and previously undocumented. Thus, we find surprising complexity in the adaptive landscape for one of the most wonderful evolutionary innovations among all plants. These findings enrich understanding of the evolution and diversification of novel trap morphology in carnivorous plants.