April is spider behavior month*, folks! Are you ready? Are you excited?
I know many people aren’t super fond of spiders, and hey, there’s no shame in that. Deep within the heart of many a human is an impulse to get the jitters when we see something small, black, and crawly. I’m even going to discuss some theories for why that is in a moment.
But to simply fear and loathe spiders is to do them a disservice. They are an extraordinarily successful group of arthropods that live on every continent besides Antarctica (so, if you really want to avoid them, go there) and are the seventh most diverse order on the planet, with over 45,000 different species known to man.
In terms of evolution, spiders are a Big Deal. And I guarantee that the more you learn about them, the less frightening they’ll be!
Throughout April I’m going to write three different articles on spider behavior, because yes- there’s too much behavior to fit into just one. Today’s article will pull double-duty as both an introduction to the physical diversity in spiders as well as a more in-depth look of some of their amazing hunting techniques.
But first, before we discuss spider behavior, let’s go off on a little tangent about human behavior. Why are so many people afraid of spiders?
The Psychology of Arachnophobia
Arachnophobia, being a phobia, is rather worse than just a general nervousness around spiders. Those with the phobia may be affected so badly that they have panic attacks even at the mere thought of a spider being in a room. It’s estimated that roughly 5% of the population could be diagnosed with this phobia, which is a huge amount.
But even if your discomfort with spiders isn’t severe enough to be classified as a phobia, spiders seem to make most people just that: uncomfortable. Why is this?
One of the more popular theories is that humans are programmed to be wary of spiders because there are several species with venom potent enough to be deadly to humans. In this case, the fear would be hardwired into our genes for our own safety. However, the number of spider species that actually pose a threat to humans could be counted on your fingers; the vast majority of spiders are completely harmless. It seems rather extreme to have evolved a hardwired anxiety for spiders to combat a few, widely-scattered species. (Especially considering how exaggerated the danger of even these species is, as I expanded on here.)
Skeptics of the evolutionary theory point out that fear of spiders is actually less common outside of Western European societies; consider, for instance, the West African god/mythic figure Anansi, who usually takes the form of a spider. Anansi, while often depicted as a trickster, is also a wise and benevolent fellow who gave humans the gift of agriculture, among other things. In parts of India and Pakistan the spider is actually considered a sign of good fortune, in Egypt there’s a tradition of putting a spider into the bed of a newly-married couple, and in many South American countries spiders often find their way onto the menu. Not what you’d expect to see if humans have a hardwired fear of spiders!
Most completely negative perceptions of spiders, in fact, stem from Western culture, and most start appearing in the Middle Ages. (Even in the Greek story of Arachne, spiders are depicted as skilled weavers and not villains.) In 11th-century southern Italy, reports of a literal dancing fever began to spread: this was of course “tarantism,” attributed to the bite of of Lycosa tarantula, which confusingly enough is a type of wolf spider. It is also completely harmless to humans. However, the medieval Italians believed quite strongly that the bite of this spider caused fever, excitability, vomiting, and sweating, and that the only way to survive was to begin dancing the tarantella until you dropped.
Also prevalent during the Middle Ages was the concept that spiders carried disease, and that any food or water that came in contact with the spider was poisoned. Spiders were even seen as harbingers of the black plague.
Supporting the cultural theory of arachnophobia is the fact that many studies have found that humans tend to associate spiders more strongly with disgust, along with harmless creatures such as slugs and maggots, rather than fear along with creatures such as bees and wasps. There is also no consensus among arachnophobes as to what the most frightening features of spiders are (the legs? the eyes? the fuzziness?), which one would expect if we had a hardwired genetic anxiety.
Despite this compelling evidence for a Western European cultural cause for spider disgust, there has yet to be a cross-cultural study examining attitudes towards spiders to my knowledge. And there is some evidence that humans are predisposed to fear spiders. Arachnophobia tends to run in families, though the predictor is level of disgust rather than fear of spiders. And children as young as three tend to show fearful reactions to spiders.
The answer may actually lie in between a cultural and genetic source for this phobia. Studies with another commonly-feared animal, the snake, have shown that while primates may not be born with a fear of snakes, they learn it much more easily than other fears and take a lot of convincing to give it up. The experiment in question showed several rhesus macaques who had never been exposed to snakes images of snakes and flowers: the monkeys were afraid of neither. Then the researchers paired the images with footage of a monkey reacting fearfully. The macaques immediately picked up the fear of snakes, but not the fear of flowers. So we may pick up fear for animals that are potentially dangerous much more easily thanks to evolution.
A study with humans pairing images of spiders with electric shocks found that the fear of spiders is much harder to extinguish once learned than fear of things such as flowers and houses. (Why flowers again? Also, what a horrifying experiment that must have been.)
Luckily for those of you who are arachnophobic, the phobia is very treatable regardless of the cause. Exposure therapy is often the easiest way to get over it (seeing other people interact calmly with spiders is known to help), though for people with extreme arachnophobia, drugs such as beta blockers and D-cycloserone can also help when used along with therapy. There is actually even an iOS app out now called “Phobia Free” designed to desensitize arachnophobes to spiders.
So, in summary: very few spiders are actually dangerous, fear of them is probably mostly cultural (but may be a type of fear we are predisposed to learn), and the fear, while hard to extinguish, can be extinguished with the right treatment.
All right, that’s enough about human behavior. Let’s get on to the spiders!
Know Your Spiders
Spiders are, as most of us know, arachnids, along with scorpions, psuedoscorpions, whip scorpions, ticks, mites, and harvestmen, among others. Arachnids are most commonly identified by the fact that they have eight legs rather than six as in insects, though not all of them actually have eight legs. The actual feature that all arachnids have in common is the presence of chelicerae and pedipalps, two pairs of appendanges that resemble fangs and a shortened pair of legs, respectively. The chelicerae are used in feeding, while the pedipalps are used like arms.
As I mentioned before, the order of spiders in particular (Araneae) is among the most diverse of any in the animal kingdom, with over 43,678 species recorded to date. The order is divided into three suborders: Mesothelae, Mygalomorphae, and Araneomorphae. Mesothelae is the most basal of the suborders and contains only about 87 species. The most famous of these are the trapdoor spiders- we’ll get to them.
Mygalomorphae is more or less the “big scary spider clade” as it includes both tarantulas and Australian funnel web spiders. Unlike other spider species, which live for about a year, these ones can actually live for up to 25 years! Rad.
The final and largest suborder, however, is Araneomorphae. This clade includes such characters as lampshade spiders, crab spiders, cobweb spiders, jumping spiders, wolf spiders, orb weaver spiders, huntsman spiders… look, there are a lot of spiders out there, and most of them are Araneomorphs.
Spiders as a group first emerged on the scene about 300 million years ago during the Carboniferous period, predating such creatures as mammals and dinosaurs. It was a great time for arthropods in general- the climate was highly moist and oxygenated, and arthropods, which breathe via diffusion, were loving it.
The first spiders were within the Mesothelae suborder, and included species such as Eocteniza silvicola and Protocteniza britannica. These spiders were very small (P. britannica was about 15 mm long), slow-moving, and had their spinnerets located in the center of their abdomen like belly buttons rather than at the end like modern spiders. They were most likely ground-dwellers that built rudimentary nests or trapdoors with their silk.
Quick note on some prehistoric not-spiders: Attercopus (Tolkien fans, recognize the name?) was once believed to be the earliest genus of spiders, as they lived during the Devonian era 380 million years ago, but has since been reclassified as belonging to a sister taxon. The chunky Megarachne servini, which lived 298 million years ago, was once thought to be the largest spider ever: 20 inches long from end to end. This was even promoted in an episode of the documentary series Walking With Beasts. However, the fossil used for the classification was actually that of a sea scorpion! The foot-long goliath birdeater tarantula retains its title as the largest spider ever, and happily for us it is still alive today.
Ok, anyhow: by the Triassic, Mygalomorphae and Araneoporphae showed up, and it was basically spider time, all the time from there.
What Spiders Do
Ok, this has been a really long introduction to spider behavior month without any actual spider behavior. So let us get down to brass tacks, as they say. Spider tacks.
Most species of spider are strict carnivores that mostly prey on insects and other small arthropods. Some do prey on larger beasts such as mice, lizards, and birds, though accounts of this are often exaggerated. Spiders evolved in tandem with their prey insects and are best equipped to deal with things that have an exoskeleton.
Spiders possess specialized chelicerae that are pointed and hollow for injecting venom into their prey, which immobilizes it for pre-digestion. This is because spiders evolved to have a totally liquid diet. This enables them to minimize the size of their digestive tract and maximize their ability to store food for long periods of time, which is useful if your main hunting tactic is hoping an insect will bumble into your web. I went into the digestion of spiders in a lot more detail here, by the way, and even discussed whether or not they can fart. (They theoretically could.)
There is at least one species of spider that feeds almost exclusively on plant matter: the recently-discovered Bagheera kiplingi. The vast majority of their diet consists of Beltian bodies, which are basically little nutrient-rich nubs that plants put out to attract ants. The ants then protect the plant. Ironically, B. kiplingi does no such thing in exchange for the meal, and the ants and it do not get along. Upon occasion this spider does eat meat, though this mostly consists of snatching ant larvae or others of its own species.
B. kiplingi is not the only species known to be omnivorous, though: many spiders will partake in nectar, fruit, or pollen upon occasion, particularly juveniles who’ve still got some growing to do.
What about spider speed and strength? Well, spiders are not the cheetahs of the arthropods; they have primitive respiratory systems and prefer to ambush prey rather than chase it down. They draw in air through their spiracles, which are located on their rear abdomen close to their spinnerets. So yes, spiders do breathe through their butts.
Being poor respirators is why spiders do that little “jerk and move a few steps away” thing when you poke them. They don’t want to waste energy by running further. And if you poke them enough, they may perform the “catapult myself into space” escape strategy, which can, unfortunately, land them on your face. So please let the poor things be.
An interesting tidbit on spider locomotion: while spiders use muscles to retract their legs, they use hydraulics to extend them. That’s right: the inside of a spider is pressurized. This means that if you were to puncture their cephalothorax, they essentially “pop.” Aside from the likelihood of spider stuff squirting out, the loss of pressure means that the spider will be unable to extend its legs. This is also why the legs of dead spiders curl up.
All spiders have four pairs of eyes, and the vision of certain species is among the best within Arthropoda. Their two main eyes, located at the front of the head, are capable of forming images, while their secondary eyes (usually located at the sides and/or top of the head) expand their field of vision and have varying degrees of complexity. They also come in lots of different spatial arrangements depending on the spider’s hunting preferences. Web-spinners and lurkers like crab spiders tend to have small eyes clustered all together at the front of their face, while more active hunters have large eyes circling their head.
If you ever go out late at night during the spring with a headlamp on, you will see lots of tiny glittery things down in the grass. These are the reflections from the eyes of nocturnal spiders that have tapetum lucidum, much like cats do.
Sight is not the primary sense of the web spinner, as evidenced by their tiny lil’ eyes. Instead, they depend on sensitive hairs and bristles known as setae, which can do basically anything: They can touch! Detect vibrations! Smell! Taste! Sense changes in the air pressure! This means that a spider walking over your face is essentially smelling and licking you. Suddenly, all your spider encounters just got a whole lot more intimate.
Aside from their eight legs, spiders also have pedipalps, which resemble a shortened pair of legs covered in setae right by the face. While they look like legs, they are probably actually modified antennae from an arthropod ancestor. Spiders use these palps like little hands, rotating and examining food, cleaning their faces, and for males, even transferring sperm to the female during mating. (We’ll get to the harrowing experience of spider sex in a later post.)
Here is a rather enchanting video of a jumping spider wiggling its palps and cleaning itself.
But aside from all that other anatomy (cute as it may be), the part of the spider I am most fascinated by is the brain. Yes, spiders have brains, and pretty large ones too by arthropod standards. Also unlike other arthropods, their ganglion are centralized in their cephalothorax and not spread throughout their body. The interior of the cephalothorax is actually mostly composed of nervous tissue, pushing some organs like coxae down into the legs.
Jumping spiders (Salticidae) are acknowledged as some of the brainiest of spiders. In some experiments, jumping spiders were able to learn and remember color cues in order to locate a food item. Recently, some scientists even managed to insert a really tiny electrode into a jumping spider brain and found that the spider used single neurons to do tasks (combining information from eight different eyes) that would take vertebrates many more.
The real intelligence of spiders is in their hunting strategies, however, and some spiders use surprisingly clever tactics. But let’s begin with the web-spinners.
Which came first, the spider or the silk? Well, the silk, actually. I mentioned Attercopus earlier as an arachnid once thought to be a Devonian-era spider; this was thought because Attercopus possessed spinnerets. So the ancestors of spiders made silk before they were spiders.
The earliest known spider web was found fossilized in amber from 100 million years ago- this leaves a gap of 200 million years from initial spider origins. So what were spiders doing with their silk all that time?
Well, silk has many, many uses aside from web-spinning. In fact, spinning webs is only a fraction of what modern spiders use their silk for- of those that even make webs! Spiders use their silk for things such as protecting their eggs, climbing vertical surfaces, and even mating.
The earliest spiders probably hunted in similar ways to modern trapdoor spiders: they dug a burrow, lined it with silk to keep it clean and dry, and constructed a trapdoor with dirt and detritus that they could affix to their hole with a silk hinge. Then they sat, and they waited.
The speed of the strike comes from the fact that the spider rests its legs on slender lines of silk it placed all around the burrow. As soon as one twitches, the spider lunges with rather alarming speed and accuracy.
From these early home-hunters rose spiders that used their silk in increasingly complex ways. The earliest webs were probably sheet webs, basic sticky silk covering a small plant or landmark on the ground to catch crawling insects. Later innovations included funnel webs, tubular webs, tangle webs, and finally the famous spiral orb web.
Of course, that is about as generalized overview of web types as you can get. The variations are endless, and the evidence suggest that many lines of spiders have lost the ability to spin webs and then regained them several times.
Spinning silk isn’t one-size-fits-all, either. While some spiders can only produce one or two types of silk, some species can produce up to eight different types for different purposes. Here are just a few examples:
- Tubiliform: a stiff, non-sticky silk used for protecting eggs
- Apullae (major and minor): Ampullae major is a strong, non-sticky silk used for securing draglines. Ampullae minor is used as stand-in scaffolding during web construction (and is eaten when the gaps are filled by other silk types).
- Pyriform: a strong, sticky silk used for gluing different lines together.
- Flagelliform: stretchy and sticky. Used in the capture spiral of orb webs.
- Aciniform: the toughest silk of all, used for wrapping up prey. Also used to gift-wrap spider sperm.
- Aggregate: lines with large sticky droplets, used by bolas spiders.
Producing silk has a high energy cost, so many spiders will take care to eat any excess when repairing or rebuilding their webs. Some spiders actually steal web material from other spiders to eat.
The sit, wait, and wrap-it-up method of prey capture used by web-spinning spiders is hardly the only one, though it is certainly the most familiar one.
But many spiders use their webs in a much more… active way. Take the net-casting spider: it does exactly what its name implies.
By the way, uhh… take a look at the eyes on this fellow.
I also mentioned the bolas spiders earlier, and they too hunt exactly as you would expect.
To finish up our trio of spiders-that-do-exactly-what-their-name-implies, there are the spitting spider. Unlucky prey will encounter a faceful of silk imbibed with paralyzing venom. I didn’t include a video of this one because the action happens so fast that it looks like the prey item just stops moving on its own.
But of course, there are other even weirder ways to use webs. My favorite is the diving bell spider. As previously mentioned, spiders breathe through their butts. The diving bell spider traps air using the hairs around its abdomen and proceeds to hunt prey underwater like an eight legged scuba diver. You thought you were safe in the water, bugs??
Not only does the diving bell spider trap air for use when hunting, it builds large air bubbles underwater, which it anchors with silk. In these pockets it can eat prey at its leisure without running the risk of getting snatched by any landlocked predators.
Now, as cool as all these web-spinning and prey-catching techniques are, they are not based on intelligence. The spiders have the behavior encoded into their genomes and never have to learn it. But there is one group of spiders that do learn their hunting techniques, and use them to outwit their insect prey. I am talking, of course, about the jumping spiders of the genus Portia.
Aside from a line to help secure themselves during their jumps, most jumping spiders do not use silk to aid in their hunting. Instead, they stalk, chase, and lunge at their prey like tiny springy lions. Within Portia, however, members do spin webs to catch small insects- though they still stalk their preferred prey: other spiders.
But how do you trap a spider that is already trying to trap other creatures (and potentially, you)? The answer is carefully and cleverly. Portia spiders will find their way to the edges of the webs of their prey and delicately pluck, drum, massage, wiggle, and even slap parts of the web. The web owner, fooled into thinking there is a delicious morsel waiting for it, runs right into Portia’s jaws.
Sounds simple enough- but it really isn’t. There are a myriad of different web-spinning species that Portia might prey on, and they each have their own rhythm. If the vibrations don’t sound just right, Portia might not encounter an unsuspecting victim but an enraged homeowner. This can be dangerous- many of Portia’s prey items are much larger than it is!
Good vibrations aren’t the only thing Portia spiders have to memorize. They also stalk several different spider species, including other jumping spiders. To attack from the front or the rear, which behavior patterns to look out for, and many other tricks must be stored in Portia’s repertoire. How does the little spider manage it?
Well, some of it is based on instinct. Portia occasionally prey on spitting spiders, which I mentioned before- they’re the faceful-of-silk-and-venom guys, and they’re about as pleasant as you’d expect. To complicate things, they also like to eat other spiders- so it can get into a real spider-eat-spider situation. Portia, even those that have never encountered a spitting spider, prefer to attack them from behind to avoid the whole… you know. So this seems to be a genetically encoded behavior.
But other behaviors are not. After all, other spiders don’t always react in exactly the same way: many are able to learn from experience themselves, at least when it comes to interpreting the vibrations in their webs. In these cases, Portia have to mix it up. They may alter the signals they send out through their prey’s webs using trial-and-error to figure out what elicits the best response. For example: a rapid approach might be fine for a small prey spider, and Portia can mimic a small, rapidly-struggling insect. But if the prey in question is larger than Portia, this is not a good idea. For these spiders, it might be better to imitate something very large, or just send confusing signals, so that the prey item approaches more slowly. They can manipulate the speed, predatory behavior, and even the angle of approach from these spiders through their vigorous drumming.
A major sign of intelligence is flexibility in behavior, and Portia have it in spades: when their pre-programmed methods fail, they switch over to trial-and-error, and learn what words and what doesn’t. They may even be able to plan ahead! In one experiment, Portia were placed in a maze where they could clearly see but not reach a prey item in front of them. The spiders quickly learned to take a detour even if it meant that the item went out of sight for a while. This may not sound that smart, but it’s a task many vertebrates will fail.
The brain of a Portia spider is about the size of a poppy seed. (Portia spiders are about the size of a thumbnail themselves.) Compared to vertebrate brains, the number of neurons it possesses are minimal. The drawback to this is that these spiders often have rather slow processing speeds, especially for visual criteria. But can you blame them? They’re using single neurons for tasks that would take humans twenty!
Wrapping Up The Spiders
I hope you lot enjoyed this very long, very generalized introduction to the world of spiders and their behavior. They are fascinating, diverse creatures and I would have to write several books to truly do them justice. But since I don’t have time for that right now, I hope even you spider experts out there learned something from this article!
But don’t fret. In a week or two, I’ll be posting the second of the three-article series on spider behavior. I hope you guys enjoy reading the word “stridulations,” because it’s all about hot, sticky spider sex.
The next spider article is up- read about erotic spider activity here! Or, if you’d prefer to take a break from the sticky fellows, you can read about other things like why animals play and bird yaoi here on my site. (Full list of articles here!)
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Very interesting article. The silk uses are amazing!
Quick fact-check: Jumping spiders do have 8 eye pairs – its just that the Poster Median Eyes are reduced in size and dont really form an image. They do monitor light levels, though. You can actually see them in the image you used. Small black dots in between the Anterior and Posterior Lateral Eyes.
Thanks for the correction, Spiderbro!
Okay, another quick correction. Not *all* spiders do have 4 pairs (8 total) eyes. For instance the Recluse and Spitting spiders are 6 eyed spiders.
Some spiders actually have no eyes at all like the cave spiders.
Now that I have that out of my system, I will read the rest, but great effort here on this. Looking forward to reading more (when I can spare more time).