I'm worldbuilding an alternate earth which has less gravity and the high oxygen levels of the late carboniferous. Thus, resulting in very large invertebrate life. In one scene, the main character is pursued by a giant spider. But, as I was writing the scene, I realized that I don't know what sounds a spider of that size would produce. Let's assume that the spider in this world are almost identical to the American Loxosceles reclusa in every way save for size. Realistically, what sounds would such a creature produce that would be audible to humans?
$\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$– L.Dutch ♦Jul 13, 2021 at 17:01
There would be a mechanical clicking or clacking sound as it moves. (I'm extrapolating that it becomes a loud clacking sound based on size.)
The largest spider in the world makes a clicking sound as it moves through the jungle undergrowth. Link
Further, this species makes a strange noise as a warning to other organisms to stay away called stridulation. Youtube
The mechanics of the sound are sort of like how a cricket performs its chirp. Presumably, your giant spider might make any sort of sound that uses a similar mechanism.
1$\begingroup$ Imagining a huge cricket using the same mechanism of rubbing "teeth" against other body parts would sound crazy ... imagine being alone in the wood and suddenly there is a loud "tack tack tack tack tack tack tack" ... pause ... "tack tack tack tack tack tack tack" ... coming closer :-) $\endgroup$– fhoJul 12, 2021 at 14:16
2$\begingroup$ The dude in the article doesnt need to extrapolate: “Its feet have hardened tips and claws that produce a very distinct, clicking sound, not unlike that of a horse’s hoofs hitting the ground,” [The one dude in the first link] $\endgroup$– HobbamokJul 12, 2021 at 15:03
5$\begingroup$ @fho I prefer to imagine that spiders are never larger than 1cm across and that they are inherently terrified of humans. I refuse to imagine a "tack tack tack" sound moving closer to myself. $\endgroup$– John OJul 12, 2021 at 17:11
$\begingroup$ It should be noted that some stridulating spiders produce a sound only when rubbing against a surface, unlikely to apply in the OP scenario of a giant spider chasing prey. $\endgroup$– rekJul 12, 2021 at 18:36
$\begingroup$ @JohnO, I've had what I think was a wolf spider in my bathtub, waiting for me. With the legs, it was larger than my "man-hands". I had to take the strainer off the drain for it to be washed down, after unloading 1/8th a can of spider spray on it. healthline.com/health/wolf-spider-bite#identification Also, youtu.be/u68RFrusWOw?t=111 $\endgroup$ Jul 12, 2021 at 21:55
No sound at all
The majority of spiders alive today are some form of ambush or trap predators. Making sound kind of spoils the surprise that the spider was planning.
For a giant spider that is actively pursuing a tasty main character, I would expect the spider to be eerily silent. I would not expect any vocalizations from the spider at all while it is hunting. The goal of the spider isn't to scare, or warn its prey, the goal is to capture the prey with the least effort required.
It is rare for spiders to work in groups or packs (yes, social spiders exist, but it is rare). It is not likely that spiders will want to warn other spiders of nearby threats. There probably aren't too many predators that try to take down a horse-sized spider which a spider would want to scare off using its voice. Overall, there are few reasons for a spider to want to vocalize ever.
Any time the delicious main character is not looking at the spider, it is difficult to tell exactly where the spider even is. And looking at a pursuer while trying to run away means you're not looking where you are running towards... making it all the more likely for the juicy main character to stumble or crash into a tree.
The reason you don't know what sounds a spider would produce are mostly because it is in the interest of spiders to not produce any sounds.
$\begingroup$ Given the Loxosceles reclusa is not known to make any sounds and simply scaling it up isn't going to make it start, this is the obvious and correct answer. $\endgroup$– rekJul 15, 2021 at 15:48
$\begingroup$ You are assuming the only sound it makes would be vocal and during hunting, yet the question is about what sounds it would make. If it's chitin would create sound as it moves that would be noticeable. We also see other ambush predators like Tigers to be very capable of emitting very very loud sounds. This specifically to scare away other predators or others of their own species in a bid for territory. So the reasoning does not hold up. $\endgroup$– DemiganJul 19, 2021 at 13:11
$\begingroup$ @Demigan from the OP in the chat regarding this post: "I'm referring to the sound of any internal vocalization. Not the sound of the spider interacting within its environment." This clarification is the basis for my answer. $\endgroup$ Jul 19, 2021 at 13:18
Whatever you want
One of the limiting factors on the size of spiders and other arachnids is their relatively simple, primitive respiratory system. However, increasing the atmospheric percentage of oxygen from today's 21% to the late Carboniferous 35% will only allow a modest increase in the size of a spider. So a bird eating spider might be able to chow down on sparrows rather than hummingbirds, but it's not going to be eating emus or humans.
The bigger limiting factor on land-based arachnid size is the size constraint on exoskeletal creatures in Earth's gravity. (Except where the weight is supported by water, which is why very large crustaceans are possible.) The effects of the square cube law literally do not support large arachnids. Increasing the percentage of oxygen in the atmosphere will not change this constraint at all.
So you can make a giant spider's vocalisations sound any way you want them to, because they need a completely reworked, more efficient respiratory system (which can incorporate any sound-producing characteristics you like) and magic to overcome physical size limits. With a rebuild that complete the sounds produced could be anything from ultrasonic bat-like sounds used for sonar to pig-like squeals to Celene Dion, whatever you think is most frightening.
7$\begingroup$ The question explicitly states lower gravity so I guess it might be possible. $\endgroup$– AntJul 12, 2021 at 13:04
36$\begingroup$ I have a new phobia. Spiders Singing Celine Dion. $\endgroup$– IT AlexJul 12, 2021 at 13:28
9$\begingroup$ Spiders and insects don't have the same kind of respiratory system. Not at all. Spiders have lungs, and their blood carries oxygen. (That's in general. Some spiders don't have lungs, but that is a secondary adaptation.) $\endgroup$– AlexPJul 12, 2021 at 16:06
2$\begingroup$ @Ant: Any practical difference in gravity isn't going to make much difference. Your planet's gravity has to be high enough to hold an atmosphere, allow running, &c, so maybe about Mars' 1/3 g. You need a total redesign: e.g. an 8-legged creature with a vertebrate-like endoskeleton, efficient lungs & circulatory system, &c. Once you've done this, having it make any sort of sound, from a simple roar to a lyrebird's vast vocal range, is trivial. $\endgroup$– jamesqfJul 12, 2021 at 16:49
2$\begingroup$ @jamesqf Holding an atmosphere is not governed by the gravitational acceleration $g$ at the surface; it is governed by the depth of the gravitational potential well (roughly, $g$ times the planet's radius -- the square root of this gives the escape velocity). A planet with low surface gravity could hold oxygen at ~300 K (like the Earth) if the planet is big. Such a planet would need to have a low density (hollow?). $\endgroup$– nanomanJul 13, 2021 at 7:38
Expanding on this answer:
If Spider Were the Size of Horses, What Sound Would They Make?
Arachnids have at least some variability in breathing apparatus. Most arachnid species use book lungs:
However harvestmen use trachea (not the same as human trachea, but holes in the side of the body that extend inwards creating surface area) that are thought to derive from book lungs. This is likely an adaptation for their small size, as mites also have the same structure.
It's also thought that lower oxygen levels were not the cause of the extinction of large land invertebrates, but the evolution of vertebrate predators which could out-compete and prey on even with invertebrates larger than these vertebrates. Which is to say that higher levels of oxygen don't lead to significantly larger invertebrates, but a lack of vertebrate predators might. We can even see this with the massive coconut crab, whose large adult form spends the vast majority of time on land, can weigh up to 10 pounds, and be about 1 meter in diameter. The coconut crab likely is larger than the largest spider ever in history despite lower oxygen levels today, and said spider did not look like modern spindly arachnids, and would have been incapable of chasing a rat down, let alone a full grown human.
Such a planet would also not be able to have that much less gravity than earth, as at a certain point the escape velocity of essential molecules is so low that your atmosphere disappears (mars is such a planet, so no going below 1/3 gravity) so unless you're being chased inside a pressurized habitat, these spiders won't have a chance to live on the planet at all, though even on mars it would take millions of years for the atmosphere to disappear.
$\begingroup$ It is also entirely possible book lungs evolved directly form gills not trachea, and trachea may have evolved from book lungs. zobodat.at/pdf/Arthropod-Systematics-Phylogeny_77_0267-0284.pdf $\endgroup$– JohnJul 13, 2021 at 13:33
Spiders are known for making clicking noises. Nesting spiders often lie in wait for hours and days at a time waiting for unfortunate victims. They are tuned to the vibrations of the environment and any slight disturbance to their web will provoke a reaction. This attunement to vibrations is based on their hair - which form "elaborate arrays of sensors".
Spiders have no need for complex vocal cords and air breathing, although they do require oxygen. Combined with a reworked breathing system, I imagine spider breathing and communication would sound like a series of fast rhythmic clicks and hisses.