A tiny worm that leaps high into the air — up to 25 times its body length — to attach to flying insects uses static electricity to perform this astounding feat, scientists have found. The journal PNAS published the work on the nematode Steinernema carpocapsae, a parasitic roundworm, led by researchers at Emory University and the University of California, Berkeley.
"We've identified the electrostatic mechanism this worm uses to hit its target, and we've shown the importance of this mechanism for the worm's survival," says co-author Justin Burton, an Emory professor of physics whose lab led the mathematical analyses of laboratory experiments. "Higher voltage, combined with a tiny breath of wind, greatly boosts the odds of a jumping worm connecting to a flying insect."
"You might expect to find big discoveries in big animals, but the tiny ones also hold a lot of interesting secrets," adds Victor Ortega-Jiménez, co-lead author and assistant professor of biomechanics at the University of California, Berkeley. He conducted the experiments, including the use of highspeed microscopy techniques to film the parasitic worm — whose length is about the diameter of a needle point — as it leaped onto electrically charged fruit flies.
The researchers showed how a charge of a few hundred volts, similar to that generated by an insect's wings beating the air, initiates an opposite charge in the worm, creating an attractive force. They identified electrostatic induction as the charging mechanism driving this process.
"Using physics we learned something new and interesting about an adaptive strategy in an organism," says Ranjiangshang Ran, co-lead author of the paper and a postdoctoral fellow in Burton's lab. "We're helping to pioneer the emerging field of electrostatic ecology."
Co-authors include Saad Bhamla and Sunny Kumar, who study biomechanics across species at Georgia Institute of Technology, where preliminary experiments were performed; and Adler Dillman, a nematode biologist at the University of California, Riverside.
The shocking lives of tiny organisms
Static electricity, that tiny zap you sometimes feel when your hand touches a metal doorknob or you pull a sweater over your head, occurs when a buildup of electrons discharges quickly upon contact with a conductor.
While the phenomenon is little more than an annoying shock at the human scale, emerging evidence shows that static electricity plays a crucial role in the lives of some small organisms.
In 2013, for example, Ortega-Jiménez discovered that spider webs take advantage of the charge of flying insects to electrostatically ensnare them as they pass by.
Other research has shown how electrostatics help bees to collect pollen, flower mites to hitch rides on hummingbirds and balloon spiders to drift on silk strands over large distances.
Burton and Ortega-Jiménez recently co-authored a commentary piece for the journal Trends in Parasitology highlighting research on electrostatic forces and ticks.
"Ticks can get sucked up from the ground by fluffy animals, purely through the static electricity in the animal's fur," Burton explains.
While conducting experiments to validate this electrostatic effect on the attraction of ticks to charged hosts, Ortega-Jiménez developed a new technique to control the electrical potential of a tethered tick. This breakthrough turned out to be the missing piece that allowed the researchers to continue with new experiments on nematodes.
As the jumping worm turns
For the current paper, the researchers wanted to investigate how electrostatic forces, in combination with aerodynamics, affects the success rate of S. carpocapsae to connect with a flying insect.
S. carpocapsae is an unsegmented roundworm, or nematode, that kills insects through a symbiotic relationship with bacteria. The worm thrives in soils nearly everywhere on Earth except the Poles. It is increasingly used for biological pest control in agriculture, with researchers around the world studying how to further drive its effectiveness as a natural pesticide.
When the worm senses an insect overhead, it curls into a loop and then launches itself in the air as high as 25 times its body length. That's the equivalent of a human being jumping higher than a 10-story building.
"I believe these nematodes are some of the smallest, best jumpers in the world," Ortega-Jiménez says. During their dizzying, acrobatic leaps, he notes, they rotate at 1,000 times per second.
If the worm hits its target, it enters the insect's body through a natural opening. It then deposits its symbiotic bacteria, which kills the insect within 48 hours. After the death of the host, the worm feeds on the multiplying bacteria, as well as on the insect tissue, and lays eggs. Several generations may occur in the insect's cadaver until the juvenile worms emerge into the environment to infect other insects with bacteria.
Painstaking experiments
The researchers designed experiments to investigate the physics involved in the worm's prowess at connecting with a flying insect.
In nature, the wings of a flying insect rubbing against ions in the air can generate hundreds of volts. The physicists needed to know the exact charge of the fruit flies used in the experimental model. That required Ortega-Jiménez to attach a tiny wire connected to a high-voltage power supply to the back of each fruit fly to control its voltage.
"It's very difficult to glue a wire to a fruit fly," he says. "Usually, it took me half an hour, or sometimes an hour."
Another challenge was identifying the right conditions to induce worms in the experimental setup to jump. Ortega-Jiménez used a substrate of moistened paper. The paper had to be just wet enough, but not too wet. Finally, a worm needed the encouragement of a gentle puff of air or a slight mechanical disturbance before making the leap toward a suspended fruit fly.
Ortega-Jiménez conducted dozens of experiments, recording them with a special high-speed camera capable of capturing the midair trajectories of the submillimeter worms, which are essentially invisible to the human eye, at 10,000 frames per second.
He also created a tiny wind tunnel for some of the experiments, so that the physicists could analyze the role of ambient breeze in the worm's target success rate.
Digitizing the data
Using computer software, Ran digitized the trajectories of the worms, drawing from about 60 videos of experiments. The process was time consuming in instances when a worm left the focal plane of the camera, blurring the image, in which case Ran needed to click by hand to record its position.
Ran used a computer algorithm known as Markov chain Monte Carlo (MCMC) to analyze the digitized data. ("Markov" stands for the mathematician who developed the algorithm, while "Monte Carlo" refers to the area of Monaco famous for its casinos.)
"MCMC allows you to do random explorations, using different sets of parameters, to determine a mathematical probability for an outcome," Ran explains.
Ran identified a set of 50,000 plausible values of fitting parameters for a single worm's trajectory — such as the voltage of the insect, the physical dimensions and the launching velocity of the worm — to test the probability of a particular charge in a worm allowing it to hit its target.
With no electrostatics, only one out of 19 worm trajectories successfully reached the target.
The model showed that a charge of a few hundred volts — a magnitude commonly found in flying insects — generates an opposite charge in a jumping worm and significantly increases the odds of it connecting to a midair insect. A charge of just 100 volts resulted in a probability for hitting the target of less than 10%, while 800 volts boosted the probability of success to 80%.
A worm expends a vast amount of energy to jump and faces risks of predation or drying out while suspended in the air.
"Our findings suggest that, without electrostatics, it would make no sense for this jumping predatory behavior to have evolved in these worms," Ran says.
Science past and future
The researchers had theorized that electrostatic induction was the mechanism driving the interplay between the worm and its target. Sifting through research papers eventually led them to a law of induction posited by Scottish physicist James Clerk Maxwell.
"Maxwell, one of the most prolific physicists of all time, had a wild imagination, similar to Einstein," Ran says. "It turns out that our model for the worm-charging mechanism agreed with a prediction for electrostatic induction that Maxwell made in 1870. There are many buried treasures in scientific history. Sometimes being a scientist is like being an archeologist."
Drag force was another key part of the equation, due to the tiny size of the worm. The researchers use the comparison of a bowling ball flying through the air, which is not much affected by drag force, and a floating feather, which is highly dependent on it.
Ran drew from the experimental data to simulate the effects of electrostatic charge combined with various wind speeds. The results revealed how the faintest breeze, just 0.2 meters per second, combined with higher voltage further increased the likelihood of a worm hitting its target.
The work serves as a new framework for further investigations into the role of electrostatics in ecology.
"We live in an electrical world, electricity is all around us, but the electrostatics of small organisms remains mostly an enigma," Ortega-Jiménez says. "We are developing the tools to investigate many more valuable questions surrounding this mystery."
The work was supported by a grant from the W.M. Keck Foundation and the Tarbutton Postdoctoral Fellowship of Emory College of Arts and Sciences.
