Re: God I miss the swifts. They don't come here any more because of the bloody spraying. One of my parti Archived Message
Posted by Ken Waldron on June 7, 2020, 3:44 pm, in reply to "Re: God I miss the swifts. They don't come here any more because of the bloody spraying. One of my parti"
-Seconded. Like everyone else I always thought that swifts fed on flies until I was disabused a number of years ago by someone actually doing a bit of research. Turns out their main source of food are aerial spiders: the ones that go "ballooning" i.e. spin a thread that lifts them aloft. Like everyone else too I thought that spiders ballooned or flew by the gossamer thread being lifted by a breeze...but no. Someone doing a bit of research likewise discovered its a bit more complicated: apparently the spiders detect electric fields and use these to fly by means of electrostatic repulsion: "Every day, around 40,000 thunderstorms crackle around the world, collectively turning Earth’s atmosphere into a giant electrical circuit. The upper reaches of the atmosphere have a positive charge, and the planet’s surface has a negative one. Even on sunny days with cloudless skies, the air carries a voltage of around 100 volts for every meter above the ground. In foggy or stormy conditions, that gradient might increase to tens of thousands of volts per meter. Ballooning spiders operate within this planetary electric field. When their silk leaves their bodies, it typically picks up a negative charge. This repels the similar negative charges on the surfaces on which the spiders sit, creating enough force to lift them into the air. And spiders can increase those forces by climbing onto twigs, leaves, or blades of grass. Plants, being earthed, have the same negative charge as the ground that they grow upon, but they protrude into the positively charged air. This creates substantial electric fields between the air around them and the tips of their leaves and branches—and the spiders ballooning from those tips. This idea—flight by electrostatic repulsion—was first proposed in the early 1800s, around the time of Darwin’s voyage. Peter Gorham, a physicist, resurrected the idea in 2013, and showed that it was mathematically plausible. And now, Morley and Robert have tested it with actual spiders. First, they showed that spiders can detect electric fields. They put the arachnids on vertical strips of cardboard in the center of a plastic box, and then generated electric fields between the floor and ceiling of similar strengths to what the spiders would experience outdoors. These fields ruffled tiny sensory hairs on the spiders’ feet, known as trichobothria. “It’s like when you rub a balloon and hold it up to your hairs,” Morley says. In response, the spiders performed a set of movements called tiptoeing—they stood on the ends of their legs and stuck their abdomens in the air. “That behavior is only ever seen before ballooning,” says Morley. Many of the spiders actually managed to take off, despite being in closed boxes with no airflow within them. And when Morley turned off the electric fields inside the boxes, the ballooning spiders dropped..." https://www.theatlantic.com/science/archive/2018/07/the-electric-flight-of-spiders/564437/ -It's truly an amazing world and we know so little...
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