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| Jumping spiders are solitary hunters that don't have webs. Basile Morin, CC BY-SA 4.0. |
Insects have ears all over, though rarely on their heads, perhaps because their eyes are so large. Crickets have something like exposed eardrums on their legs, while other insects have hearing apparatuses on their abdomens, necks, antennae, or mouth parts. Praying mantises have one ear in their chest. Most of the insects that can hear are deaf to everything but a particular frequency, such as the wing-beats of a female or the sounds of a predator. Some, though, like cockroaches and spiders, hear a broader range of sounds. Both roaches and spiders hear through their feet, detecting surface vibrations, as do scorpions. Roaches are around 100,000 times more sensitive to these sounds than we are.
Somewhat creepily, spiders can listen to our conversations. Jumping spiders have a range from 150 to 10,000 Hz, so we can hear a bit more at each end than they do. And, for most spiders, their vision is about as good as that of a small dog and they’re color blind, like dogs, but a few may have full color vision, which makes their vision more like that of pigeons. All of them can see ultraviolet and polarized light, which we can’t. Overall, our vision is probably better, considering the amount of brainpower we devote to enhancing what we see. (Check out my earlier series of posts on how our brain creates and modifies what we see, beginning with Misperceiving Reality.)
This creates an interesting image. Imagine you wander into your living room at night and you turn on a light. On the arm of your couch, unseen by you, is a spider out on her nightly hunt. You start talking to someone on the phone or to someone in the next room and she hunkers down to listen. Then she looks up at you with several of her eyes, before scurrying off on her eight legs down the side of your sofa’s arm, making a quick escape before you spot her and try to kill her with a shoe or some bug spray, or if you’re really afraid of spiders, with your .45. If you do notice and chase her, she’ll hear you scream and be able to keep an eye on you as she runs away since she has nearly 360-degree vision. And how many other creatures are lurking throughout your home, anxiously watching and listening to you. It’s enough to make you call the fumigator.
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| Remey takes aim. © Remey, 2006. |
Researchers have found that even though they have a brain the size of a poppy seed, one type of jumping spider is particularly curious and intelligent, carefully evaluating and planning attack routes and quickly switching between tactics.
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| She has jumping-spider eyes. Behind the Lens, CC BY-SA 4.0. |
Spiders have eight eyes. Two are for looking forward, two see polarized light that they use something like a compass, two keep track of their direction, and two point down to keep track of distances.[1] The two largest are their forward viewing eyes. Like ours, they can move these around to look at things. They provide sharp vision and, unlike those of most spiders, they can change focus, but they have a narrow field of view. They’re a bit like our foveae. On either side of this central pair are two wide-angle eyes that pick up the slightest bit of motion. Without these eyes they’re unable to follow moving objects. One of their other pair track movement behind them. And like cats and dogs, spiders have a reflective layer behind their retinas, so if you shine a LED light around your garden at night, you’ll see many pairs of eyes reflected back at you.
Spiders that do have webs can use their webs for communication, with a spider in one part of the web signaling to another in a different part by strumming the web’s silk strands. The web can also pick up vibrations in the air—similar to an antenna and amplifier—and acting as the spider’s eardrum can enable the spiders to hear what’s going on around them. While they don’t have ears, they sense the vibrations through the claws that they use for holding onto their webs. And it appears that by stretching and crouching, spiders can tune their webs to pick up certain frequencies and listen for particular sounds.
Another spider trick is that their webs are electrified with static electricity. The glue on the web is charged, so that when an insect gets too close, the web swings out and sticks to it.[2]
And additional nifty little trick is that some spiders use electricity to fly. This is called ballooning and it’s where the spider extends a free strand of thread into the air that then catches on electrical charges in the sky and whisks the spider away on the wind, occasionally carrying it hundreds of miles or kilometers away.
A not-so-nifty trick is that spiders inject their victim with venom which paralyzes it and starts liquefying its tissues while it’s still alive. They then tear the victim apart and vomit enzymes from their gut onto the tissues. After several minutes of softening, they suck up the juices.
You can tell whether some species of spiders have eaten recently by whether its abdomen is swollen or thin. I’ve see very full and starving cane spiders around our house. They’re easy to spot since they’re so huge, fast, and are horrifying enough to make you jump back. But it’s harder to tell with the similarly large, sedate, bright-yellow, and not-so-shocking garden spiders. Both have about a four inch leg span, but cane spiders actively hunt, while garden spiders just sit in their web.
Spiders will eat up to four times a day, but since they’re not very active they can go weeks without eating. Some spiders catch and eat fish, birds, lizards, snakes, bats, mice, and rats. In Australia a huntsman spider was even caught eating a pygmy possum. But it’s nice to know that at least one species of spider is mostly vegetarian.
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[1] FECYT - Spanish Foundation for Science and Technology, “Tarantula wolf spiders use their lateral eyes to calculate distance”, ScienceDaily, April 20, 2017, https://www.sciencedaily.com/releases/2017/04/170420093729.htm, citing Joaquin Ortega-Escobar and Miguel A. Ruiz, “Role of the different eyes in the visual odometry in the wolf spider Lycosa tarantula (Araneae, Lycosidae)”, The Journal of Experimental Biology, vol. 220, no 2, 2017, pp. 259-65, https://journals.biologists.com/jeb/article/220/2/259/18622/Role-of-the-different-eyes-in-the-visual-odometry, https://doi.org/10.1242/jeb.145763.
[2] University of Oxford, “How electricity helps spider webs snatch prey and pollutants”, ScienceDaily, January 14, 2014, http://www.sciencedaily.com/releases/2014/01/140114113339.htm, citing Fritz Vollrath and Donald Edmonds, “Consequences of electrical conductivity in an orb spider's capture web”, Naturwissenschaften, 2013, https://doi.org/10.1007/s00114-013-1120-8.
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