It sounds like a scene in a horror movie. Imagine you are walking along a trail. As you come around the bend, you see a Cobwebby Thistle (Cirsium occidentale) standing at least 6 feet tall. But you’re not worried – the thistle is at least two feet in from the trail. You approach…the music swells ominously…suddenly…the plant lurches towards you and gouges its spiny thistles deep into your shirt, a hold that will…not…let…go. Shriek!
Is this scene real or is it a product of my overactive imagination? Well – cobwebby thistles are native to California and they can grow 6 feet tall. But the “cobwebs” you see in the photo of the thistle are not really cobwebs, they’re simply white fibers that are produced by the plant. So I admit it – this scene is a product of my overactive imagination.
But not entirely. The idea of cobwebs reaching out to capture insects has a basis in reality. According to a recent story on “The Atlantic” website, “We know, or think we know, that spider webs shift their shapes in order to capture…prey.” In other words, there are grasping webs.
Displacement “of spiral silk threads from their resting positions ranged from about 1–2 mm, depending on the size of the charged test object”. That distance may not seem like much to us, but to a honey bee, perhaps 12-16 mm long, it’s about 10-20% of its body size. This displacement occurs because of an electrostatic attraction between the insect and the web.
So what’s the source of this electrostatic attraction? A spiderweb normally carries a negative or neutral charge, while a free-flying insect, such as a bee, can acquire a positive electrostatic charge simply by flying through the air. Scary thought…isn’t it? Just by going about your normal everyday business, you generate the very force that allows you to be captured.
But I wonder if this is true for all insects? In an SCV Outdoor Report that I wrote back in April, scientists in the UK reported that “bumblebees are able to find and distinguish electric signals given out by flowers”. Since both flowers and in spiderwebs carry a negative electric charge, perhaps a bumblee can sense when a spiderweb is nearby. Perhaps not all insects are helpless victims of grasping spider webs.
Hmmm…the plot thickens…doesn’t it? After all, not every character in a horror movie is a victim, right?
P.S. Anybody got an electrostatic magic wand that I can borrow for a day? I’d like to see if I can deform a cobweb on my front porch.
Upcoming Outdoor Events:
Trail Maintenance Schedule. Come join our volunteers as they help maintain our trails. Contact Steve at firstname.lastname@example.org for time and place.
Wednesday mornings, July 3, 10, 17, 24 & 31.
Saturday mornings, July 6 & 20.
New trail maps available. If you’d like to explore a bit on your own, the City of Santa Clarita has a website with trail maps of our local open spaces.
There’s also a new website for bicycle riders.
Ask Dr. Norm. Do you have questions about the flora, fauna, animals, rocks, etc. in our Santa Clarita Valley? Here’s a place for you to ask your questions.
Dr. Norman Herr, Ph.D., is a professor of science and computer education at California State University, Northridge.
Tell Us About Your Hike: Here’s a new website where you can post pictures, provide feedback and make suggestions about the City of Santa Clarita’s trails and open spaces.
You can listen to stories like this every Friday morning at 7:10 a.m. on “The SCV Outdoor Report”, brought to you by your hometown radio station KHTS (AM1220) and by the Mountains Recreation and Conservation Authority.
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Source: Santa Clarita News