Spider Limbs

Don't tell him the trope name isn't Octopus Arms.

Spiders are creepy. People with spider legs growing out of their body much more so. It does have a few perks, though. Characters who have an extra set of spidery appendages can often use them to scuttle about on walls and ceilings freely, jump great distances, and even reach the jar of cookies hidden on the top shelf!

These spider limbs can also be used instead of walking on human legs, giving the character an extra meter or two of height, and almost seeming to float. These limbs can be of any make. Maybe they're Artificial Limbs, Combat Tentacles with Mundane Utility, or even inky tendrils born of the deepest darkness. Though for some reason they're always attached/growing out of the small of the back.

The limbs aren't usually prehensile, but some of the stranger variants are equipped with claws, hands, or cartoon strength static cling that allows the limbs to manipulate objects, as the picture of Doctor Octopus arms show. Likewise, tentacle rather than spider based limbs can also be used as Tentacle Ropes.

Usually, this is exclusively a case of Bad Powers, Bad People due to the spider/octopus/tentacle factor. Still, there are cases of Dark Is Not Evil.

Not to be confused with Multi-Armed and Dangerous, but may overlap if the extra combat arms can also be used to scuttle around.

Examples of Spider Limbs include:

Anime and Manga

The Diclonii's 'Vectors' in Elfen Lied.
Medusa in Soul Eater utilises this as well, as part of her snake/arrow themed powers. She can balance on her tail and create many "snake" arrows that sprout out of her back, among other uses.
In Busou Renkin, Tokiko's busou renkin, the Valkyrie Skirt, takes on the form of four scythe-like blades that latch onto her legs and are controlled by nerve impulses through her skin. She can use them to move over difficult terrain, as well as climbing up walls and ceiling.
Inuyasha Big Bad Naraku and his Combat Tentacles fall into this category. Though there is a certain element of Body Horror for sure, the underlying theme is definitely arachnid in nature as he was once a human named Onigumo (demon spider) and all of his incarnations manifest a spider on their backs.

Comic Books

Spider-Man in all his incarnations has a lot of instances of this trope. Firstly, there's arch enemy Doc Ock, then there's the Powered Armour Tony Stark gave Spider-Man in the Civil War, and there's also Midnight Sons rogue Spider-X, who had boney spider-limbs.
- - Pre-dating the Civil War Iron Spider armor, a possible future Spider-Man was shown to be a genius with Powered Armor using a similar system to Doc Ock's. Interestingly, the future Goblin serving as his nemesis had equivalent technology on her armor as well.
- The third and fourth Spider-Woman both possessed these at different points. Originally they were a power of Charlotte Witter (Spider-Woman IV), as a result of genetic manipulation by Doc Ock. After a lot of back-and-forth power-stealing, the limbs - along with the other powers of all three other "Spider-Women" - ended up with Mattie Franklin (Spider-Woman III).
- This has happened to Spidey before, but he managed to cure his condition thanks to the help of Dr. Curt Connors (aka the Lizard).
The X-Force villain Forearm was so-named because... well, you can figure it out.
The villain Mojo from the X-Men series moved around on robotic spider legs as his species has no spine.

Literature

In Quest for the Fallen Star", the "Tenebrite" variant of Ill-creature is usually humanoid, but can morph its lower body into a set of spider legs.
While he doesn't possess any extra limbs, titular character The Arm from Nancy Farmer's The Ear, the Eye, and the Arm looks decidedly creepy thanks to his extra-long limbs. When he decides to ramp that effect up a bit, he unhinges them a little and crooks them in odd ways; he even uses this to scare some primitive tribesmen into thinking he's a boogie man.
Sarpedon of the Soul Drinkers, although he uses them only for movement and occasionally stabbing people.
Mordred Deschain from The Dark Tower. Look at the picture of him from The Other Wiki.

Film

John Carpenter's The Thing features a scene with a disembodied head growing a set of Spider Limbs after its original infected corpse becomes compromised.

Live Action Television

The Sci Fi Science episode "How to Build a Super Suit" had Dr. Michio Kaku's design, which included a pair of extensible robotic arms attached to the back and controlled by the user's brain. Dr. Kaku even mentioned Dr. Octopus by name. The arm control system is fully within the realm of current, or at most, Twenty Minutes Into the Future science. A early version's been done with chimpanzees.
The Spider Dalek emerged while developing a proposed revival of Doctor Who in the mid-1990s.

Tabletop Games

The Fiends (Neberu) in Demon: The Fallen are associated with spiders (since they were the ones who "webbed" the entire universe together on the Fourth Day of Creation) and therefore prone to growing an extra pair of hands in their High-Torment Apocalyptic Forms.
In Exalted, the Terrestrial Circle Sorcery spell Lightning Spider gives the caster eight extra spiderlike limbs made of lightning!
The Book of Vile Darkness sourcebook for Dungeons & Dragons has a spell that gives the caster a set of four giant spider legs. This lets them climb, move quicker, and raise their body out of the way of enemy attacks.
- Around the same time, the Song and Silence sourcebook also featured the Fang of Lolth prestige class, which culminated with fully functional Half-Human(oid) Hybrid/Spider limbs.
In the Infernum D20 setting, this is the effect of the first mutation for the Chain of the Spider. The demon (or fallen angel, or corrupted human) sprouts a number of secondary appendages to give them eight limbs in total—of themselves, however, they simply make it easier for the character to climb; it takes the Chain of Multiple Arms mutations to make them strong enough to be used in combat or for tasks.
Occasionally found on tech-priests of the Warhammer40000 Imperium.

Video Games

Andariel, the first boss from Diablo II features four spider legs growing from her back, but they're only used for attacking.
- Ditto for Kerrigan, the Queen of Blades in StarCraft.
The Jedi Kazdan Paratus in Star Wars: The Force Unleashed is a rare heroic example...except he's gone senile and insane, making him all too creepy.
Laughing Octopus from Metal Gear Solid 4 is a rare exception to the small-of-the-back rule; her Combat Tentacles come from the top of her head, leaving her body dangling down when she uses them to move around.
Mimi's Nightmare Fuel form from Super Paper Mario.
Ares from God of War uses this in the climactic duel against Kratos.
Lord Recluse, the Big Bad in City of Heroes, and quite a few of his minions.
- Villain players get their own set if they use the appropiate archetype.
Alice Moonlight, a later boss from the sequel to No More Heroes has these tipped with beam katanas. One of her attacks even ends with her in a position that makes her look very much like a spider.
The first really messed-up boss in Survival Crisis Z has spider limbs growing from the stump of its neck (its head having exploded within a minute of its being introduced). The legs hoist it in midair and allow it to rapidly chase you around the room, even as it fountains gallons of blood from its neck. It's that sort of game.
Yagrum Bagarn, the last of the Dwemer race in Elder Scrolls 3: Morrowind became infected with the corprus disease. Unable to walk under his own power due to its ravages, he constructed a series of mechanized spider legs to carry his bloated form.
Urgot the Headman's pride, from League of Legends, was rebuilt like that.
A few of the Beetleworx have these.
The Fiend Spider Ninjas in Ninja Gaiden II have four large spiked Spider Limbs protruding from their backs, which are used to great effect.

Web Original

The Beldrobbaen clan in Drowtales 'verse contains several members who carry giant spiders on their backs, most notably the leader Waes'soloth and her daughter Naal'suul.
- And the driders too. Much like centaurs, only with drow for top and giant spider for bottom. They can even make clothes with the silk.
In some of its incarnations, the Slender Man is capable of growing extra, amorphous limbs from his back.
Doctor Octogonapus (who is, after all, The Lazer Collection's resident Doctor Octopus Expy). Also, Randall, after discovering that Doctor Octogonapus is his father.)
Arachne of the Whateley Universe can manifest Spider Limbs along with her normal human limbs. She can also grow more spider-y in other ways when she chooses to, but she routinely is stuck with spider fangs that leak poison when she gets excited.

Web Comics

Gynette from Stubble Trouble is a shape-shifting, centauroid, half-spider jackal.

Western Animation

Irken's PAKs in Invader Zim can extrude these on command. He has a strikingly matter-of-fact handling of these (to-him) familiar tools. They are likely purely instinctual. There's one episode when Zim's right next to something blowing up, and when the smoke clears you can see his spider arms in the same position as his actual arms, trying to shield himself from the blast.
The Teen Titans cartoon often has Gizmo zipping around on four mechanical legs. Kinda like Doc Ock, but they only have about three joints each.
- Does Fang count? He uses four giant spider legs to carry himself rapidly across any sort of terrain and to hoist himself out of reach of his enemies. Of course, they don't sprout from his back, per se, but from the abdomen of the mutant giant spider that has replaced his head. The humanoid body just sorta hangs there, waving around limply, as it's pulled around by the spider legs.
In Spider-Man: The Animated Series Spider-Man once grew four extra arms.
- This is based in a story from the comics.
One of the villains in The Secret Saturdays can transform into a humanoid spider with legs growing out of his back.
Darkwing Duck once had a Monster Clown toy that features these, and they end in rather large axes to boot. It was probably the creepiest thing Quackerjack ever made. The kicker? It was made by an Alternate Universe counterpart who was actually a good guy.
- Darkwing Duck himself, in the episode "Aduckyphobia," received a radioactive spider bite that granted him two extra pairs of arms, though they seemed to have minds of their own (he got the hang of it when he learned they responded to verbal commands). The Spider-duck costume was pretty sweet.
Ulimate Spidermonkey in Ben 10: Ultimate Alien.
Having the alternate forms of spiders, Tarantulus and Black Arachnia from Beast Wars naturally have these. Theirs come with machineguns.
Nidhiki in Bionicle 2: Legends of Metru Nui has been mutated to have four arachnid legs.
Airachnid in Transformers Prime has six legs sticking out her back, and she switches from bipedial to... uh, hexapedal at will. It's theorized should she adopt an altmode, it'll be a helicopter.
- Theory confirmed, its a helicopter.
In the two-part fourth season finale of Star Wars: The Clone Wars, Darth Maul returns after being presumed dead for many years with his lower half replaced by mechanical spider legs. He has these briefly before they are replaced by more human-like legs.