Some parasitic nematodes are known to propel themselves explosively into the air to land on their insect prey.
A new study by UC Berkeley and Emory University biologists and physicists suggests that they're not just blindly leaping into the unknown, hoping against hope to hit their mark. They actually rely on an electrical attraction to home in on their host.
Victor Ortega-Jiménez, a UC Berkeley assistant professor of integrative biology, and his colleagues tested a hypothesis that the electrical attraction between tiny animals is important for finding and capturing food. In 2013, he discovered that spider webs take advantage of the charge of flying insects to electrostatically ensnare them as they pass by. And he recently wrote a commentary about the discovery that ticks are electrically attracted to furry animals thanks to the static charge they generate, similar to that produced by rubbing a balloon through your hair. From those observations, Ortega-Jiménez began to wonder if nematodes, which are as slender as strands of spider silk, might be pulled toward charged flying insects.
"We live in an electrical world, electricity is all around us, but the electrostatics of small organisms remains mostly an enigma," Ortega-Jiménez said. "We are developing the tools to investigate many more valuable questions surrounding this mystery."
Working with tethered fruit flies, he recorded dozens of leap attempts by the nematode Steinernema carpocapsae, a parasitic roundworm common in soil worldwide. He wired a battery to the tethered fly to give it an electrical charge that would attract the worms and captured the leaps with a high-speed camera shooting at 10,000 frames per second. The worms, less than a millimeter long, can jump up to 25 times their body length.
"I believe these nematodes are some of the smallest, best jumpers in the world," Ortega-Jiménez said.
Justin Burton, a physicist at Emory, developed a mathematical model that postdoctoral fellow Ranjiangshang Ran used to analyze the data. The model showed that a few hundred volts - a static charge easily produced by an insect's wings beating in air - 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 slight wind actually improved chances of success.
"Our findings suggest that, without electrostatics, it would make no sense for this jumping predatory behavior to have evolved in these worms," Ran said.
If a worm fails to land, it risks being eaten or drying out while suspended in the air.
Ortega-Jiménez explained that when the worm senses a mechanical perturbation, which can be an insect overhead, it curls into a loop and launches itself in the air. If the worm hits its target, it enters the insect's body through a natural opening and deposits bacteria that live amicably in the nematode. These bacteria are lethal to insects, however, killing them within 48 hours. After the host dies, the worm feeds on the multiplying bacteria and 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.
"Using physics, we learned something new and interesting about an adaptive strategy in an organism," Ran said. "We're helping to pioneer the emerging field of electrostatic ecology."
The study was published Oct. 14 in the journal Proceedings of the National Academy of Sciences.
