Spiders don’t have wings, but that doesn’t mean they’re stuck on land. Carried on a parachute of silk strands, spiders have been known to drift kilometers above Earth’s surface. These so-called ballooning spiders may even soar across oceans to reach new habitats. Electrical charges in the air might provide a cue on when to fly, new research suggests.
This invisible signal might help explain why spiders’ takeoff timing is so unpredictable. Some days, large numbers of spiders balloon together. On other days, they stay firmly grounded despite similar weather conditions.
When conditions are right, some spider species climb to a high point. There, they release silken strands as they await the breeze needed to help them soar away.
To take off, spiders need gentle air currents. Past studies had shown that wind speeds under 11 kilometers per hour (7 miles per hour) were best. But such a light breeze shouldn’t be strong enough to get some of the larger ballooning-spider species off the ground, points out Erica Morley. She’s a sensory biologist at the University of Bristol in England.
As a result, she notes, scientists have long wondered if some other force might be involved.
For example, electrical charges in Earth’s atmosphere form a field that attracts or repels other electrically charged objects or particles. This electric field varies in strength. It grows around objects such as leaves and branches on trees. It also fluctuates with the weather. Under the right conditions, could these electrical charges push against the silk threads of a spider’s parachute — fanning them out to keep a spider aloft?
Morley and Daniel Robert, a sensory biologist at the University of Bristol, decided to find out if spiders can sense these electric charges. First they blocked natural electric fields in a lab. They then artificially created an electrical field. It mimicked what spiders in the wild could experience. Linyphiidae is the second largest family of spiders. The scientists placed teeny-tiny spiders from that family into that electric field.
Even with no breeze, when spiders sensed this field, they perched on the tips of their legs. Spiders use this ballerina-like pose before they balloon. Scientists call it the “tiptoe stance.” When the researchers turned off the artificial electric field, the spiders got off their tiptoes.
The researchers shared their findings July 5 in Current Biology.
The effect is hairy
Tiny hairs on the spiders’ bodies react to moving air and to electric fields, the researchers found. But they don’t react to both in the same way. As long as air was blowing on them, the hairs stood on end. With an electric field, the hairs stood up most dramatically when the field switched on. Over the next 30 seconds or so, those hairs gradually fell back to their resting position.
These new findings link the pre-flight tiptoe pose to the presence of an electric field. But spiders might need something more to actually take off, adds Moonsung Cho. He studies aerodynamics in Germany at the Technical University of Berlin. Explains Cho, who wasn’t involved in the study, some spiders in the study did float away. The researchers did not, however, measure that liftoff behavior.
Cho has studied ballooning in a different family of spider. The ground crab spiders seem to sense wind speed with their legs before going aloft. They wiggle one spindly leg around to sense moving air and decide whether wind conditions are right for flight. Cho’s team reported this finding June 14 in PLOS Biology.
So responding to electric fields probably isn’t the full story when a spider decides it’s time for takeoff.
(for more about Power Words, click here)
aerodynamics A field of research that focuses on reducing the resistance (or drag) from air flowing past some solid object.
atmosphere The envelope of gases surrounding Earth or another planet.
behavior The way something, often a person or other organism, acts towards others, or conducts itself.
biology The study of living things. The scientists who study them are known as biologists.
current A fluid — such as of water or air — that moves in a recognizable direction. (in electricity) The flow of electricity or the amount of charge moving through some material over a particular period of time.
electric charge The physical property responsible for electric force; it can be negative or positive.
electric field A region around a charged particle or object within which a force would be exerted on other charged particles or objects.
family A taxonomic group consisting of at least one genus of organisms.
field (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).
fluctuate (n. fluctuation) To vary at irregular intervals and often by amounts that are hard to predict.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
habitat The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.
link A connection between two people or things.
particle A minute amount of something.
silk A fine, strong, soft fiber spun by a range of animals, such as silkworms and many other caterpillars, weaver ants, caddis flies and spiders.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
spider A type of arthropod with four pairs of legs that usually spin threads of silk that they can use to create webs or other structures.
weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.
Journal: E. Morley and D. Robert. Electric fields elicit ballooning in spiders. Current Biology. Vol. 28, July 23, 2018, p. p2324.e2. doi: 10.1016/j.cub.2018/05.057.
Journal: M. Cho. An observational study of ballooning in large spiders: Nanoscale multifibers enable large spiders’ soaring flight. PLOS Biology. Vol. 16, June 14, 2018, p. e2004405. doi:10.1371/journal.pbio.2004405.