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Why Don't Spiders Get Stuck in Their Webs?

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When a bug flies into a spider web, the game is over. It’s almost instantly stuck, and a sitting duck for the web’s owner. When you or I walk into a web, we’re a little better off than the bug because we won’t be dinner, but the sticky strands of web are still a pain in the butt to pick off of clothes and skin.

The spider itself, which spends much more time in contact with the web than you or any bug, doesn’t seem to have any issues getting stuck as it moves around. What gives?

For a long time, people thought spiders didn’t get stuck because their legs were coated in an oil made inside their bodies. With their legs lubed up like this, there was nothing for the silk web strands to stick to. Early 20th century naturalists proposed this idea — that the spider “varnishes herself with a special sweat,” as one elegantly put it — after observing spiders in the wild. The hitch is that, for all the research on spiders scientists have done in the meantime, no one had bothered to test the idea until recently.

A study published last year by two biologists in Costa Rica, Daniel Briceño and William Eberhard, suggests that spiders stay unstuck thanks to a combination of behavior, anatomy and, yes, even an oily non-stick coating.

What a Web They Weave

The first thing that helps spiders from getting trapped is that not every part of every web is sticky. In many orb weaver spider webs, for example, only the spiral threads are made with sticky silk. The “spokes” that support the structure of the web and the center part of the web where the spider rests are made with “dry” silk.

Using the center area and the spokes, a spider can move all around the web, and even off of it, without any concern for getting stuck.

Neat Feet

The spiders that Briceño and Eberhard studied used the dry threads for moving around most of the time, but when prey landed on the webs and the spiders went to retrieve their dinner, they inevitably had to charge across a sticky section. Unlike their prey, though, the spiders didn’t just whack into the sticky threads willy-nilly. The scientists found that the spiders walk very carefully when on the sticky sections, holding their body clear of the web and making minimal contact with the threads with only the tips of their legs.

Under a microscope, Briceño and Eberhard saw that the sticky threads do indeed make contact with the spider and stick to the setae, or short bristly hairs, on their legs. As a spider pulls its leg of the web, though, the droplets of adhesives that sit on the thread slide toward the edge of the bristle, where they have contact with only the thin tip and easily pull away. All these bristles are also in irregular rows and break free from the sticky droplets one by one, not all at once, which keeps the adhesive force of multiple droplets from combining.

Smooth Like That

What is it about the setae that lets them shed the web’s adhesives so easily? When Briceño and Eberhard washed a detached spider leg and applied it to a sticky thread, the leg stuck and wasn’t as easily removed. They figured that the bristles must have either a chemical coating of anti-adhesive substances or a structural surface layer with anti-adhesive properties. After analyzing several compounds washed off the the spiders’ legs, they found several several oily substances — including n-dodecane, n-tridecane, and n-tetradecane — that could act as a non-stick coating.

The researchers couldn’t tell where the chemicals had come from, but scientists’ descriptions from the last century suggested that they were applied by the spider’s mouth. Sure enough, when Briceño and Eberhard washed a live spider’s legs, it passed each of the legs through its mouthparts, but they didn’t test whether or not any anti-adhesive material was being applied.

To see if the spiders were coating their own legs would require a pretty simple experiment, Eberhard told me via email, but the spider they were working with, Nephila clavipes, is only seasonally abundant. The study would have to wait until the population climbed again, so the source of the non-stick chemicals is still a mystery for now. In the meantime, he said, he’s looking into how spiders deal with a different type of silk, called cribellum silk, which can be sticky without being wet.

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Big Questions
How Does Autopilot Work on an Airplane?
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How does autopilot work on an airplane?

Joe Shelton:

David Micklewhyte’s answer is a good one. There are essentially a few types of features that different autopilots have. Some autopilots only have some of these features, while the more powerful autopilots do it all.

  • Heading Hold: There’s a small indicator that the pilot can set on the desired heading and the airplane will fly that heading. This feature doesn’t take the need for wind correction to desired routing into account; that’s left to the pilot.
  • Heading and Navigation: In addition to holding a heading, this version will take an electronic navigation input (e.g. GPS or VOR) and will follow (fly) that navigation reference. It’s sort of like an automated car in that it follows the navigator’s input and the pilot monitors.
  • Altitude Hold: Again, in addition to the above, a desired altitude can be set and the aircraft will fly at that altitude. Some autopilots have the capability for the pilot to select a desired altitude and a climb or descent rate and the aircraft will automatically climb or descend to that altitude and then hold the altitude.
  • Instrument Approaches: Autopilots with this capability will fly preprogrammed instrument approaches to the point where the pilot either takes control and lands or has the autopilot execute a missed approach.

The autopilot is a powerful computer that takes input from either the pilot or a navigation device and essentially does what it is told to do. GPS navigators, for example, can have a full flight plan entered from departure to destination, and the autopilot will follow the navigator’s guidance.

These are the majority of the controls on the autopilot installed in my airplane:

HDG Knob = Heading knob (Used to set the desired heading)

AP = Autopilot (Pressing this turns the autopilot on)

FD = Flight Director (A form of navigational display that the pilot uses)

HDG = Heading (Tells the autopilot to fly the heading set by the Heading Knob)

NAV = Tells the autopilot to follow the input from the selected navigator

APR = Tells the autopilot to fly the chosen approach

ALT = Tells the autopilot to manage the altitude, controlled by the following:

VS = Vertical Speed (Tells the autopilot to climb or descend at the chosen rate)

Nose UP / Nose DN = Sets the climb/descent rate in feet per minute

FLC = Flight Level Change (An easy manual way to set the autopilot)

ALT Knob = Used to enter the desired altitude

This post originally appeared on Quora. Click here to view.

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Big Questions
What's the Difference Between Vanilla and French Vanilla Ice Cream?
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While you’re browsing the ice cream aisle, you may find yourself wondering, “What’s so French about French vanilla?” The name may sound a little fancier than just plain ol’ “vanilla,” but it has nothing to do with the origin of the vanilla itself. (Vanilla is a tropical plant that grows near the equator.)

The difference comes down to eggs, as The Kitchn explains. You may have already noticed that French vanilla ice cream tends to have a slightly yellow coloring, while plain vanilla ice cream is more white. That’s because the base of French vanilla ice cream has egg yolks added to it.

The eggs give French vanilla ice cream both a smoother consistency and that subtle yellow color. The taste is a little richer and a little more complex than a regular vanilla, which is made with just milk and cream and is sometimes called “Philadelphia-style vanilla” ice cream.

In an interview with NPR’s All Things Considered in 2010—when Baskin-Robbins decided to eliminate French Vanilla from its ice cream lineup—ice cream industry consultant Bruce Tharp noted that French vanilla ice cream may date back to at least colonial times, when Thomas Jefferson and George Washington both used ice cream recipes that included egg yolks.

Jefferson likely acquired his taste for ice cream during the time he spent in France, and served it to his White House guests several times. His family’s ice cream recipe—which calls for six egg yolks per quart of cream—seems to have originated with his French butler.

But everyone already knew to trust the French with their dairy products, right?

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