Fishing spider. Photo by D. Sillman
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(I want to thank Robert Steffes who passed along a recent Atlantic Monthly article about the electrical nature of spider’s webs. That article led to the development of this post).
Deborah and I love spiders. We have a significant community of them living in our house, and we take care to see that as little harm comes to them as possible. Our basement, in particular, is a rich habitat for a variety of types of spiders.
Pholcid spider> Photo by D. Sillman
Pholcid spiders (also called “cellar spiders”) are numerically the most abundant in our house. We have clusters of them in every dark corner of the basement and in most of the ceiling corners of the upstairs rooms. Pholcid spiders are thin, delicate looking, long-legged spiders that make webs that look like jumbled messes of silky threads. Typically, they hang upside down under their web and wait for some insect (house flies, gnats, mosquitoes, stink bugs, etc.) to get entangled in the chaos of the silk. Their webs are not sticky but are so complex in structure that an arriving insect has difficulty extracting itself before the spider gets over to it to deliver a bite of paralyzing venom. The spider then pumps enzymes into the dead or dying prey and sucks out its digesting body fluids. When the spider is finished, it disentangles the dead insect from its web and lets it fall to the ground. Under a pholcid spider’s web, then, is a dry pile of insect carcasses upon which it has fed.
If you blow on a pholcid spiders web you can trigger an energetic threat response. The spider will start vigorously shaking the web in an attempt to get whatever might be disturbing it to back off. This behavior has led to one of the many common names for the pholcid spider: the vibrating spider.
Pholcids are not terribly particular about their types of prey. They often eat each other and will attempt to entangle and disable almost anything that gets caught in its web. I have seen pholcids go after large beetles and even bumblebees. Sometimes the large prey individual gets away, sometimes it does not.
I could go on about my basement spiders, but I have spent some time doing that already (see Signs of Spring 12 May 14, 2015). Lets talk about some recent research concerning the spider silk fibers of their webs.
Photo by VA State Park Staff, Wikimedia Commons
Spider silk, of course, is made of up proteins. Proteins tend to have very significant negative charge because of the predominance of acidic amino acid residues in their polypeptide chains. Five years ago, in a research paper published in Scientific Reports, a group at the University of California-Berkeley determined that these negative charges inherent in spider silk were an important factor in the ability of a spider’s web to capture insects. When an insect flies, its wings not only generate lift and linear direction, but they also generate significant electrical charges. To be specific, the beating wings make the flying insect quite positively charged. When a positively charged insect flies near a negatively charged spider web the attraction between these opposite charges pulls the web strands toward the insect. This interaction also, then, pulls the insect toward the web. Upon contact with the web, even if, like in the pholcid webs described above, there is no sticky mucopolysaccharides coating the silk, the insect is at least slightly stuck on the spider’s webbing due to this electrostatic attraction. For many insect-spider web interactions this electrostatic adherence gives the spider sufficient time to reach the captured insect, wrap it in more silk and deliver its venomous bite. So a spider web, because of its electrical nature, has an extended active space around it that draws flying prey into its entangling threads.
Spider attempting to balloon. Photo by Sarefo, Wikimedia Commons
There is another behavior in spiders that has had a simple, but unsatisfying explanation for many years: the phenomenon called “ballooning.” When a spider balloons it typically climbs up on some surrounding vegetation and then raises itself up on its extended legs (a behavior called “tip-toeing”). It then lifts its abdomen into the air and releases a long strand of spider silk. The silk rises in the wind and then drags the spider off of its perch and up into the air. Ballooning is an excellent way for densely crowded spiders to disperse in order to find potentially more conducive habitats. It is also an excellent way for a spider to escape from a predator. It is an amazingly efficient transport system over short and also over long distances. Spiders have been found far out at sea (Charles Darwin famously found hundreds of ballooning spiders covering the lines of his ship the Beagle sixty miles off of the shore of Argentina). Ballooning spiders can travel over a thousand miles and have been collected two and a half miles up into the atmosphere!
The accepted explanation for this ballooning behavior was that the spiders simply sensed an adequately robust wind and then released their silk sails and took off. This ignored, however, the observation that spiders could balloon even when there was no wind at all! Some new research adds some continuity and also some complexity to the spider flying system.
Researchers at the University of Bristol have determined that spiders can sense the Earth’s electrical fields. Tiny hair-like structures on their feet (called “trichobothria”) bend and deform in response to changes in electrical fields surrounding them. Further, the Earth’s atmosphere is positively charged in contrast to the surface of the Earth (and all of the plants and animals touching the surface) which tends to be negatively charged. On stormy days this charge separation can generate extremely powerful electrical currents (and bursts of electrical discharges (lightning)) but even on clear, calm days there is a substantial voltage generated as one rises up from the Earth’s surface. When a spider releases its ballooning silk, its negative charge is attracted to the positively charged atmosphere, and it is this electrostatic attraction that generates the lift for the spider’s flight.
In the experiments at Bristol spiders were placed in closed containers. When a electric field was generated around the containers the spiders “tip-toed,” ejected ballooning silk, and even lifted off of the floor of their containers even though there was no wind at all in the laboratory. Their ballooning stopped when the electrical fields were shut off.
Electric spiders! Pulling in positively charged, flying insects to their webs! Pulling themselves up into the charged atmosphere to catch a breeze to take them almost anywhere on Earth! I need to tell my basement spiders about all of this!
On 9-14-2019, between the time of 12 noon and 3 pm, I was watching netflix on a plasma type smart tv. The window AC unit (same wall as stove) was running next to the propane gas fire place logs, under this was a silky spider web attached between the air gap between the metal stove and the brick hearth. A thin beam of sunlight hit the spider web and a plasma corona discharge illuminated from the silk to the top of the bricks, a dancing display of white plasma light, I will forever clean the propane logs of all spider webs in case it could discharge a spark on a propane gas valve near it on the back of the stove. So consider this a safety warning to all users of such stoves and their manufacturers, I believe spider webs and the right condition can cause sparks enough to blow up a gas leak, as an engineer I make this incredible observation. What I witnessed was cold plasma discharge and suspect the negative and positive affinities were bleeding cold air ions from the AC unit and shunting them to ground on the brick hearth, whereby the bricks contain silicon, which conducts between the metal stove and that spider web connected between as the air gap became perhaps a spark plug gap, the dust on the spider web silk also suspect. I didn’t placed my finger on it and just like a Tesla plasma ball effect I could move the discharge by blowing air on it. I started researching tonight on the internet and found this link so I comment on what I saw. I hope it scares people into cleaning their gas fire logs just in case in theory a charge could cause an explosion. This is a new stove installation, one year old.
So interesting! Thanks for the great discussion and explanations, Bill. My guess is that those spiders in the basement already know all about their spider cousins’ electrical and ballooning talents. 😉 Must have been exciting to see the pileated woodpecker fledgling! Jennifer
Wonderfully entertaining and educational, Bill. I had completely forgotten about that remarkable discovery of the electrical process.
Meanwhile…I wish I knew how to write such a grabber as this last sentence to conclude what I try to write.