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Arachnus (マルカラ Marukara?) is a species native to SR388. At the near-top of the food chain, they are very fearsome predators. The biped is distinguished by its three-fanged maw, segmented eyes, elongated neck, a hard shell on its back, and the large claws on its forearms. It is known to curl into an armored ball when threatened in order to roll towards its target in an attempt to ram it, and will uncurl periodically to swipe with its claws or spit fireballs from its mouth.

Samus Aran encounters two Arachnus specimens in the Metroid timeline, one of which is an imitation created by the X Parasites.


Metroid II: Return of Samus[]

In Metroid II: Return of Samus, Samus Aran can fight an Arachnus, which is found posing as an Item Sphere in the claws of a Chozo Statue in Arachnus' room. It is the only boss in the game not belonging to the Metroid species, and thus the sole optional boss as well. The creature possesses two attacks: spitting a small projectile from its mouth and jumping towards Samus in its invulnerable ball form. The Arachnus will immediately do the latter attack whenever Samus fires her Arm Cannon or deploys Morph Ball Bombs. It will also bounce around for an extended period of time if she herself is in her ball form.

Bombs are its weakness, though they only inflict damage when the boss is not curled up. The Spring Ball item appears out of the boss' remains. At the time of Metroid II's release, it had yet to be stated if the creature's ability to jump while curled up was a result of the item falling in its possession.

Metroid: Samus Returns[]

Metroid Samus Returns Arachnus Chozo Statue Item Sphere Trap (Area 2)

Arachnus in the hands of the Chozo Statue pretending to be an Item Sphere in Samus Returns

In the remake Metroid: Samus Returns, Arachnus' attacks and behavior are greatly expanded to resemble those seen in Fusion, including swiping projectiles, lobbing fireballs and ramming into Samus in ball form in a manner similar to the Boost Ball. It can jump while in ball form, as well as creating a shockwave-producing ground pound. Like in Metroid II, it poses as an Item Sphere in the hands of a Chozo Statue, though its large size and different appearance make its disguise less effective. It will reveal itself if Samus approaches or attacks it, destroying the Chozo Statue in the process. The Arachnus can also be damaged using Beam shots, Missiles and Bombs, the latter of which will cause it to slip, knocking the creature out of ball form. When killed, it will leave the Spring Ball behind once again; the game's official strategy guide confirms that the item allows the boss to jump in its ball form.

Like in the original game, the creature is the first boss Samus encounters that is not a member of the Metroid species, however it now belongs to a trio of non-Metroid bosses in Samus Returns, which includes Diggernaut and Proteus Ridley.

Metroid Fusion[]


Arachnus mimicked by X Parasites in Metroid Fusion.

In Metroid Fusion, the first boss Samus encounters is the imitation of an Arachnus, created when six regular X Parasites and a Core-X merged together. Through gene manipulation, the X have not only changed the creature's pigmentation and increased its size, but have also enhanced its strength and abilities.

Compared to the natural counterpart's simplistic fireball, the X-mimicked Arachnus's fireball triggers multiple spirals of fire to continuously emerge from the ground until they reach the other side of the room. The blue projectiles from its claws cover more vertical height that is proportionate to the creature's new size, which now dwarfs over Samus. The Arachnus causes a small earthquake whenever it rams against a wall in ball form, causing Samus to lose her grip if she is holding onto a ledge and wound her if she falls directly on top of the organism. The imitation has lost much of the quick mobility the uninfected variant on SR388 displayed. Additionally, the lack of the Spring Ball item in its possession makes the Arachnus unable to jump while curled up.

It is a relatively easy fight, and though Samus can damage Arachnus with beam shots, her newly recovered Missiles are more effective. If shot with the latter weapon when the imitation is merely walking, it will flail its head back and forth with a distorted visage while roaring in pain. The boss has 150 health, and the Core-X has 30. On Easy mode in the Japanese version of the game, Arachnus has half health and only uses its rolling attack. Once Samus defeats Arachnus and absorbs the Core-X out of its shell, she regains her Morph Ball ability.

The Arachnus briefly appears in the prologue of Metroid Dread, recapping the events of Metroid Fusion.

Official data[]

Metroid II: Return of Samus manual[]

M2 art enemy Arachnus
Enemies (p. 27)
"It will roll up its body like an armadillo, then rush at Samus."

Metroid II: Return of Samus Virtual Console manual[]

Enemies (1) (p. 14)
"This creature attacks by rolling up into a ball like an armadillo."

Metroid Fusion manual[]

"Native life-form of SR388. They will roll themselves into balls and attack. Dodge and attack with missiles."

Metroid: Samus Returns Official Guide[]

"A native life-form that swallowed the Spring Ball Item Sphere and inadvertently absorbed its power, Arachnus is an armadillo-like beast that spins and spits projectiles to attack its enemies. The Spring Ball gives it the ability to jump in the air while curled into its shell. Defeating it shakes the Item Sphere loose, giving you the opportunity to take it for yourself."
WALKTHROUGH - AREA 2 (p. 66-67)
"That wasn't an Item Sphere! That was the armadillo-like native of SR388, Arachnus! Disregarding the Metroids you've fought up to this point, this marks the game's first full-fledged boss battle, and it's quite the challenge. Arachnus can't be damaged if shot from behind or while it's rolled up into a ball; only shots to its red-and-yellow flesh have any hope of harming it. Missiles should be your go-to for this fight, so use them wisely, if you've followed our Walkthrough so far, you should have plenty of missiles to work with. Use your Lightning Armor at the start. It doesn't last more than a few hits, but it gives you a bit of time to get accustomed to Arachnus' attack patterns.

One of Arachnus' more common moves is its energy wave attack. Its hands begin to sparkle, then it throws out a wave about the same height as Samus. It often performs this attack up to three times, but it may throw fewer waves. The first wave is easy to dodge, but subsequent waves are trickier. The flashing hands are emphasized on the first wave but largely absent on follow-ups, which can catch even the most self-respecting bounty hunter unaware. The timing between waves is always the same, so focus on getting a feel for it and jumping when your gut tells you the time is right.

Arachnus rolls into a ball to perform a variety of attacks. The first is a speedy dash across the chamber. This attack is easy to dodge. Wait for it to spin up for a few seconds, then jump into the air as high as your Power Suit will carry you. You get enough hang time that even a slightly off jump puts you out of harm's way. Like the energy wave attack, Arachnus performs this attack three times in a row before stopping. Until it finishes spinning, you don't have an opportunity to damage it.

That's not where the ball-based attacks end. Arachnus also loves to roll into a ball and bounce around the chamber. Your best response is to roll up into the Morph Ball and wait for Arachnus to approach you, then roll under it when it jumps into the air. It continues to follow you around the chamber for several jumps, so race to the other side and wait for it to approach, roll under, then repeat. Don't bother trying to wait this attack out by using the Spider Ball to climb walls. Every time Arachnus hits the ground, you're shaken loose and dropped. As with the spinning, Arachnus is invulnerable while bouncing around the chamber.

Its third and final ball attack involves Arachnus jumping into the air, pausing for a moment, then slamming down on the ground. This slam creates a shockwave that damages you if you're caught in it. It's an easy attack to dodge by running away from Arachnus and jumping when it slams to the ground. It only performs this attack once before transitioning into another attack, but you can't damage it until it finishes.

Arachnus occasionally launches fireballs from its mouth in an arcing pattern towards your current location. The ground ignites for a second wherever the fireball lands, so expert to jump away from the fireball, rather than above it. Arachnus shoots three fireballs every time it performs this attack. Dodging these can be tricky, but once you have it down, use the opportunity to shoot some missiles at Arachnus' unarmored regions."


Arachnus in Return of Samus does not have its own battle music, with SR388 Underground continuing to play during the battle. It first received its own theme in Fusion with VS. Marukara, and then received another theme in Samus Returns with Boss Battle 3. The Berserker Lord's theme is an arrangement of VS. Marukara. Fittingly, the Berserker Lord and Arachnus both appear as the first bosses of their games.

Curiously, while Arachnus has a new theme in Samus Returns, VS. Marukara was remixed as Boss Battle 6 in the same game, which plays during the Diggernaut's chase and battle sequence. Because of this, when Boss Battle 6 was first heard in the E3 trailer, fans had mistakenly assumed that the theme was meant for the battle against Arachnus.

Development notes[]

In trailers for Fusion, Samus is seen fighting Arachnus in a much larger area than the final game, with the scenery implied to be in Sector 1 (SRX) instead.[1]

There are unused and incomplete animations of the Arachnus morphing and unmorphing in midair in Fusion, as well as its head in an idle pose.[2]


  • Despite the name, Arachnus does not have any apparent relations with actual arachnids (spiders and scorpions) and instead has more in common with armadillos.
    • Its Japanese name, Marukara (マルカラ?), may have hinted at Arachnus's association with the Morph Ball, also known as Maru Mari (丸まり?) in early media. Both names begin with "Maru", a Japanese word for "circle".
  • The phenomenon of power-ups being used by individuals other than Samus can also be seen in the Ing Sub-Guardians from Metroid Prime 2: Echoes, as they use the items stolen from Samus prior to releasing them upon their demise. Remarkably, both Arachnus and Ing are able to activate power-ups without the need of a suit or any other piece of equipment.
    • Prior to the release of Metroid: Samus Returns and its official strategy guide, Metroid Fusion was the only game to truly suggest the Arachnus' ball jumping trait from Return of Samus was not a natural aspect of the species, due to the X-mimic's inability to display this power without the Spring Ball in its possession. The evidence from Fusion was, however, questionable; the X are known to remove abilities from their hosts and imitations in exchange for new ones, as seen with Dessgeega.
  • Samus acquires the Morph Ball ability from defeating the X-mimicked Arachnus. Based on the fact that the Arachnus species is naturally capable of curling up into a ball, it can be assumed that Samus' recently acquired Metroid genes robbed the natural ability of the creature and implemented it into her Fusion Suit as the Morph Ball item. The power to rob abilities from prey has been displayed by several Metroid strains in the past.
  • All versions of Arachnus encountered in the Metroid series have rewarded Samus with Morph Ball-related items.
  • The Zebesian Oums resemble the ball form of an Arachnus. The Griptian on the BOTTLE SHIP acts similar to the Arachnus as well.
  • The Arachnus in Metroid Fusion also bears similarities to the Omega Metroid in the same game. Both are larger forms of bosses previously seen in Metroid II, both have clawed mouths and claws on their hands, they both have a partly hunched posture and slowly walk towards Samus, they're the only bosses Samus can damage with uncharged beam shots, and their death sequences involve them thrusting their heads back repeatedly while roaring and exploding.
  • In Metroid Fusion, the Screw Attack and Shinespark can kill Arachnus in one hit, via hacking. This is likely due to its vulnerabilities being the same as a regular enemy, as shown by Samus being able to damage it with the basic Power Beam.
  • The Arachnus' artwork for Metroid: Samus Returns has the exact same pose and "camera angle" as its Metroid II: Return of Samus counterpart. However, the former has darker colors and is more detailed than the latter.
  • The walkthrough of the Arachnus boss fight in Metroid: Samus Returns Official Guide mistakenly states several times that Arachnus is invulnerable while in its ball form. This is not entirely true as it can be knocked out of its ball form using Bombs. Doing this makes the fight slightly easier as it allows Samus to interrupt its ball form attacks and create an opening for her to attack it. Interestingly, the guide itself never points this out, thus it is likely that the authors were unaware of it.
  • Prior to the release of Samus Returns, the X-mimicked Arachnus's claw projectile was believed to be an unnatural addition caused by the X Parasite instead of being an ability present in the natural Arachnus species.


References and Footnotes[]

  1. ^ E3 Metroid Fusion trailer on
  2. ^ biospark. Metroid Fusion - Unused Sprite Animations. YouTube. June 21, 2021. Retrieved December 7, 2022. (starts at 7:41)