How does the Venus flytrap move without muscles? Sense without nerves?

Natalie Rossington 

No plant has captured the attention and inspired the imagination of people as much as the Venus flytrap, or Dionaea muscipula. With its alien-like death trap leaves and its carnivorous eating habits (that disturbingly break the food chain rules we all learned in elementary school), it has inspired Hollywood horror filmmakers for years. For the filmmakers of The  Little Shop of Horrors (1986), the venus flytrap conjured up images of giant blood thirsty plants, actively coaxing its human prey closer and closer until WHAM!

Yummy. (tvbomb.com)

Dinner. 

Although this depiction of a venus flytrap is clearly inaccurate and, albeit, just a little terrifying too, it does parallel the mechanism in which the real plant acquires food. The plant attracts an all-too trusting victim (generally some kind of insect) with a sticky smörgåsbord of tasty nectar. While the victim greedily laps up as much ooey-gooey nectar as possible, it unsuspectingly triggers a timer. This silent physiological timer ticks away inside the leaf until WHAM!

Help meeeeeee! (redorbit.com)

Dinner.  The insect is now clenched inside of a botanical jail oozing deadly enzymes that slowly dissolve the victim's insides into fly soup. Yummy.

The plant’s leaf jaws clamp shut at an alarming speed: as little as 100 milliseconds (Forterre et al, 2005). This kind of rapid plant movement is rarely seen in the muscle-free plant world. How can the Venus flytrap achieve this kind of  snapping speed without muscles? How is it able to sense an insect without nerves?

How does that trap snap so fast? Discovering the answer to this question required an interdisciplinary team of botanists, engineers, and theoretical physicists. Using ultraviolet paint (image below), high speed video, and triangulation software, the team determined precisely how the leaf deforms during the snap.

Venus flytraps with ultraviolet paint look extra alien-like (Forterre et al, 2005).

They determined the quick snap is caused by leaf geometry. During the snap, the leaf changes rapidly from a convex to a concave shape. This causes a rapid release in the elastic energy stored in the leaf structure used to maintain this shape. The diagram below shows how just one half, or one lobe, of the trap changes shape during the snap (Forterre et al, 2005).

The change in Venus flytrap leaf geometry from opened to closed (adapted from Forterre et al, 2005).

This change in shape reminds me of Luigi’s Pizza and Pasta in Goleta, California (hear me out - I swear this association has an important point). I have fond memories of the restaurant/arcade I visited as a kid. I always begged my mom for quarters, but alas I was not very good at any of the games (my best was whack-a-mole) and never seemed to win many tickets. Generally, the best prize I could afford with my measly ticket salary was a little plastic half-circle rubber pop-up toy. 

Eight year-old Natalie wishes the pop-up toys she won were as awesomely large as these. The toys I won were no larger than a nickel (and probably worth about the same too). (windycitynovelties.com)

These little toys use the same shape-change mechanism as the Venus flytrap. The toy is inverted to form a convex shape and placed on a flat surface such as a table. After a few seconds, the toy rapidly changes from the inverted convex shape to its more natural concave shape, converting the elastic energy into movement and pops up off the table. A little confused? Watch the YouTube video below (my favorite part is when the popper hits the unsuspecting camera man).

This is how the plant moves so quickly without any muscles - by physically changing the leaf shape. But how does the plant "know" when to change the shape of the leaves? How can it sense an insect without nerves?

Each plant trap has six small trigger hairs - three hairs on each lobe of the leaf. While the distracted fly greedily laps up nectar, a stray leg may strike a hair. When the hair is hit, the stimulation generates an action potential, or a kind of electrical signal (Volkov et al, 2009). A single hit does not cause the leaf to close but sets a silent timer. Two hair strikes within 20-30 seconds and SNAP (Benolken and Jacobson, 1970)! A second strike generates an additional electrical signal which is sent to the leaf midrib and activates the leaf closure. The leaf can even be closed without striking hairs but by inserting electrical probes into the leaf (Volkov et al, 2007). Sounds like some evil plant shock therapy. 

Watch out for these pesky hairs or you will get eaten.

Evil plant shock therapy in action (Volkov et al, 2007).

I will leave you with the most artistically beautiful and scientifically accurate video ever produced about the Venus flytrap, complete with squishing and squealing fly sound effects. It is narrated by none other than David Attenborough, because really, he can make anything interesting with that awesome accent (not that Venus flytraps need the extra help).

Citations:

Benolken, R.M., and S.L. Jacobson. 1970. Response properties of a sensory hair excised from Venus’s Flytrap. Journal of General Physiology 65: 64-82.

Forterre, Y., J.M. Skotheim, J. Dumais and L. Mahadevan. 2005. How the Venus Flytrap snaps. Nature 433: 421-425.

Volkov, A. G., T. Adesina, and E. Jovanov. 2007. Closing of Venus Flytrap by electrical stimulation motor cells. Plant Signaling & Behavoir 2: 139-145.

Volkov, A.G., H. Carrell, and V.S. Markin. 2009. Biologically closed electrical circuits in Venus Flytrap. Plant Physiology 149:1661-1667.