Posts Tagged ‘spider’

By Priyanka Runwal

From spooky abandoned houses to dark forest corners, spider webs have an aura of eternal existence. In reality, the silk threads can last hours to weeks without rotting. That’s because bacteria that would aid decomposition are unable to access the silk’s nitrogen, a nutrient the microbes need for growth and reproduction, a new study suggests.

Previous research had hinted that spider webs might have antimicrobial properties that outright kill bacteria. But subjecting the webs of three spider species to four types of bacteria revealed that the spiders use a resist strategy instead, researchers report October 23 in the Journal of Experimental Biology.

The scientists “challenge something that has gone significantly overlooked,” says Jeffery Yarger, a biochemist at Arizona State University in Tempe, who wasn’t involved in the research. “We just assumed [the silk] has some kind of standard antimicrobial property.”

Spiders spin strings of silk to trap food, wrap their eggs and rappel. Their silk webs can sport leaf debris for camouflage amidst tree canopies or leftover dead insects for a meal later. These bits and bobs lure bacteria and fungi involved in decomposition to the web, exposing the protein-rich web silks to the microbes.

“But [the microbes] don’t seem to affect spider silk,” says Dakota Piorkowski, a biologist at Tunghai University in Taichung, Taiwan.

To check if the silk was lethal to bacteria, Piorkowski’s team placed threads from three tropical spider species — giant golden orb weaver (Nephila pilipes), lawn wolf spider(Hippasa holmerae) anddome tent spider (Cyrtophora moluccensis) — in petri dishes and grew four types of bacteria, including E. coli, in perpendicular lines across the silk. “The idea is that if the silk has antibacterial properties, you should see no growth between the piece of silk and … bacteria,” Piorkowski says.

There was no evidence of this “clear zone” of dead bacteria in spots where the bacteria came in direct contact with the silk, the researchers found. So the team then tested if the silk kept hungry bacteria at bay by blocking them from its nitrogen reserves. Wetting the silk threads with an assortment of nutrient solutions showed that the bacteria readily grew on all three types of spider silk when extra nitrogen was available. That indicated that the bacteria are capable of growing on and possibly decomposing the silk, as long as the threads themselves aren’t the only source of nitrogen.

The researchers hypothesize that an outer coating of fat or complex protein on the silk may block bacteria’s access to nitrogen.

Randy Lewis, a spider silk biologist at Utah State University in Logan, cautions against ruling out antibacterial features in all spider silks, though. Underground webs of tarantulas (SN: 5/23/11), for example, can face environments rife in microorganisms compared with that experienced by aerial web-spinning spiders, he says, and may need the extra protection.

Spider webs don’t rot easily and scientists may have figured out why

by Ed Yong

On October 31, 1832, a young naturalist named Charles Darwin walked onto the deck of the HMS Beagle and realized that the ship had been boarded by thousands of intruders. Tiny red spiders, each a millimeter wide, were everywhere. The ship was 60 miles offshore, so the creatures must have floated over from the Argentinian mainland. “All the ropes were coated and fringed with gossamer web,” Darwin wrote.

Spiders have no wings, but they can take to the air nonetheless. They’ll climb to an exposed point, raise their abdomens to the sky, extrude strands of silk, and float away. This behavior is called ballooning. It might carry spiders away from predators and competitors, or toward new lands with abundant resources. But whatever the reason for it, it’s clearly an effective means of travel. Spiders have been found two-and-a-half miles up in the air, and 1,000 miles out to sea.

It is commonly believed that ballooning works because the silk catches on the wind, dragging the spider with it. But that doesn’t entirely make sense, especially since spiders only balloon during light winds. Spiders don’t shoot silk from their abdomens, and it seems unlikely that such gentle breezes could be strong enough to yank the threads out—let alone to carry the largest species aloft, or to generate the high accelerations of arachnid takeoff. Darwin himself found the rapidity of the spiders’ flight to be “quite unaccountable” and its cause to be “inexplicable.”

But Erica Morley and Daniel Robert have an explanation. The duo, who work at the University of Bristol, has shown that spiders can sense the Earth’s electric field, and use it to launch themselves into the air.

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.

It’s especially important, says Angela Chuang, from the University of Tennessee, to know that spiders can physically detect electrostatic changes in their surroundings. “[That’s] the foundation for lots of interesting research questions,” she says. “How do various electric-field strengths affect the physics of takeoff, flight, and landing? Do spiders use information on atmospheric conditions to make decisions about when to break down their webs, or create new ones?”

Air currents might still play some role in ballooning. After all, the same hairs that allow spiders to sense electric fields can also help them to gauge wind speed or direction. And Moonsung Cho from the Technical University of Berlin recently showed that spiders prepare for flight by raising their front legs into the wind, presumably to test how strong it is.

Still, Morley and Robert’s study shows that electrostatic forces are, on their own, enough to propel spiders into the air. “This is really top-notch science,” says Gorham. “As a physicist, it seemed very clear to me that electric fields played a central role, but I could only speculate on how the biology might support this. Morley and Robert have taken this to a level of certainty that far exceeds any expectations I had.”

“I think Charles Darwin would be as thrilled to read it as I was,” he adds.

The silk spiders produce is tougher than Kevlar and more flexible than nylon, and Air Force researchers think it could it could be key to creating new materials that take the load and heat off troops in the field.

Scientists at the Air Force Research Lab and Purdue University have been examining natural silk to get a sense of its ability to regulate temperature — silk can drop 10 to 15 degrees Fahrenheit through passive radiative cooling, which means radiating more heat than it absorbs, according to an Air Force news release.

Those researchers want to apply that property to synthetics, like artificial spider silk, which is stronger than Kevlar, the polymer typically used in body armor, and more flexible than nylon.

Enhancing body armor and adding comfort for troops is one of many improvements hoped for by a team led by Dr. Augustine Urbas, a researcher in the Functional Materials Division of the Materials and Manufacturing Directorate.

“Understanding natural silk will enable us to engineer multifunctional fibers with exponential possibilities. The ultra-strong fibers outperform the mechanical characteristics of many synthetic materials as well as steel,” Urbas said in the release. “These materials could be the future in comfort and strength in body armor and parachute material for the warfighter.”

In addition to making flexible, cooler body armor, the material could also be used to make tents that keep occupants cooler as well as parachutes that can carry heavier loads.

Artificial spider silk may initially cost double what Kevlar does, but its light weight, strength, flexibility, and potential for other uses make it more appealing, according to the release.

Air Force researchers are also looking at Fibroin, a silk protein produced by silkworms, to create materials that can reflect, absorb, focus, or split light under different circumstances.

It’s not the military’s first attempt to shake up its body armor with natural or synthetic substances.

Two years ago, the Army said it was looking into using genetically modified silkworms to create a tough, elastic fiber known as Dragon Silk.

Dr. James Zheng, chief scientist for project manager Soldier Protection and Individual Equipment, told Army Times at the time that while the Army is developing and testing material solutions all the time, “Mother Nature has created and optimized many extraordinary materials.”

At the end of 2016, then-Air Force Academy cadet Hayley Weir and her adviser, professor Ryan Burke, successfully tested a kind of viscous substance that could be used to enhance existing body armor. Weir did not reveal the formula for the substance, but she used plastic utensils and a KitchenAid mixer to whip up the gravy-like goo, placing it in vacuum-sealed bags and flattened into quarter-inch layers.

The material was designed to be lighter than standard Kevlar and offer more flexibility for the wearer. During tests, when struck by bullets, the gooey material absorbed the impact and stopped the bullets.

By Deepa Padmanaban

A striking new species of crab has been found living in tree-holes high above the ground. The animal, which fits in the palm of a human hand, has a deep bluish black body that stands out against the tree bark that it prowls for worms and seeds to eat.

Scientists discovered the crab—named Kani maranjandu—in the lush forests of the Western Ghats in south India. It’s an entirely new genus and species named after the Kani, the tribal community that noticed the crabs, and maranjandu, the local colloquial term for tree crab.

The forest-dwelling Kani first reported sightings of “long-legged crabs” on trees in 2014. A. Biju Kumar, a professor of aquatic biology at the University of Kerala, was at that time leading a project to survey the Western Ghats of Kerala for freshwater crabs. After months of tracking the tree crabs with the help of the tribesmen, Kumar and his student Smrithy Raj recently managed to catch a couple of these elusive crabs.

In the Journal of Crustacean Biology, the scientists describe Kani maranjandu as having a distinct hard outer shell or carapace that is broad, swollen, and convex. Most conspicuously, the legs are extremely long, with slender, curved, sharp ends that help them get a good grip on the tree, making them effective climbers.

The crabs live in water-filled hollows of tall evergreen and deciduous trees. The Kani tribesmen detect their presence by looking for air bubbles coming out of the hollows. Outside the hollows, the crabs move rapidly on tree trunks, using their pincer-bearing thick front legs to propel themselves.

The crabs are shy creatures, retreating deep inside the hollows when approached. The younger ones take shelter in the canopy of the trees, up to about 30 feet. That’s unusual for crabs, which don’t normally climb more than a few feet into trees.

“This lifestyle of tree living indicates that, since they cannot disperse widely through the sea, their range tends to be limited to a very narrow area,” says Tohru Naruse, an expert on crab biodiversity at Japan’s University of the Ryukyus. He not involved in the discovery.

This geographical restriction could mean that any impact on their habitat could put the species at greater risk.

Biju Kumar also stresses the importance of the crab’s habitat: the large trees and forest ecosystem of the Western Ghats. The crabs’ existence hinges on rainwater collected in tree hollows, and the crabs have been observed to change trees if the hollows dry up. The broad, swollen carapace is an adaptation that helps them hold water in their gill chambers.

“It also suggests that the tree-climbing behavior and morphology of Kani maranjandu, and possibly other related, undiscovered species, has evolved where they are distributed,” adds Naruse.

For Peter K.L. Ng, a National University of Singapore biologist who helped classify Kani maranjandu, the species’ most alluring feature is how it illustrates crab evolution. “The exciting thing for me is that these crabs, regardless of where they have been found, and how they are related (or unrelated) to each other, they have nevertheless evolved to use specialized habitats to enhance their survival—in this case, tree-holes and climbing,” he says.

Dwarf spiders don’t need to take paternity tests to know who the father is—for the most part. Right after copulation, males plug up the genital tract of females (red box in picture) to ensure that competitors can’t deposit sperm. Researchers studying the technique found that the larger the plug, the more difficult it is for subsequent males to remove. Described this month in Behavioral Ecology and Sociobiology, the “stoppers” effectively prevent 67.5% of males who show up later from breeding.

Thanks to Dr. Rajadhyaksha for bringing this to the attention of the It’s Interesting community.