Arachnotron

The arachnotron is a smaller counterpart to the spiderdemon, retaining its brain-and-machine construction while using a plasma weapon instead of the spiderdemon’s super chaingun.

Origin and first appearance

Players first encounter the arachnotron in MAP07: Dead Simple of Doom II, a map whose entire structure is built around monster encounters. The level’s second phase reveals arachnotrons from behind walls after the mancubi have been eliminated. The arachnotrons are pre-placed in the map rather than spawned during the encounter; depending on skill level, two, four, or seven mancubi must be killed before the walls lower and reveal them. This framing is reinforced by the Doom II instruction manual’s official description:

“Maybe cybernetics wasn’t such a great idea after all. Look what the demons have done with it. It seems unfair somehow you’re not the only guy in Hell with a plasma gun.”

That description doubles as design intent: the arachnotron exists to mirror and threaten the player’s own plasma capability.

Anatomy and design

The arachnotron’s body is a large exposed brain with two small arms and color-reactive eyes seated inside a four-legged metal chassis with articulated hydraulic limbs. Despite the “arachnid” etymology — roughly translatable as “spider instrument” — the creature possesses only six limbs total (two arms, four mechanical legs), falling short of the eight required to be a true arachnid. The Doom source code labels it MT_BABY, and its sprite root is BSPI, suggesting the working title during Doom II’s development was simply “baby spider.” Early pre-release screenshots confirm that prototype versions were more direct miniaturizations of the spiderdemon, sharing its palette and chaingun attack. A slightly earlier face iteration was subsequently used as the foundation for the mindweaver enemy featured in the Legacy of Rust expansion.

Sandy Petersen proposed the arachnotron as a smaller version of the spiderdemon because he wanted a monster that could be scattered around levels more frequently.

Combat behavior

Once alerted — announced by a revving mechanical shriek — the arachnotron locks onto its target and sustains plasma fire until the target breaks line of sight, it is staggered into a pain state, or it is destroyed. This sustained-fire behavior mirrors the spiderdemon, the Wolfenstein SS, and the heavy weapon dude. It fires at a pace exactly one-third as fast as the player’s own plasma gun: each shot cycles every nine tics, whereas the player weapon recycles in just three. Each plasma burst travels at 25 map units per tic (875 map units per second), making these among the fastest projectiles in the game. Per-shot damage ranges from 5 to 40, with a rate of approximately 233.3 shots per minute. Aside from the color, the projectiles fired by an arachnotron are identical to those of the plasma gun, and a successful hit can obscure the player’s vision.

Despite the slow look of its animation frames, the arachnotron moves at 12 map units per frame (140.0 map units per second). Its pain chance is exactly 50%, and it can be interrupted when it enters its pain state. In infighting scenarios, the arachnotron’s sustained plasma output makes it dangerous to many monsters, while tougher opponents can survive its fire and continue attacking.

Its death animation culminates in its head exploding into gore as the mechanical chassis collapses, accompanied by a sound reminiscent of malfunctioning machinery.

Tactics: how to defeat an arachnotron

• Use cover and break line of sight whenever possible. This stops its plasma attack and gives you a safer opportunity to reposition or attack.
• Keep moving when you are exposed. Its fast plasma projectiles are difficult to dodge if you remain predictable.
• Use rapid-fire weapons to exploit its 50% pain chance. Repeated hits can interrupt its attacks by forcing it into a pain state.
• Use high-damage weapons when you have a safe firing window. Ending the fight quickly reduces the time you remain exposed to its plasma fire.
• Avoid the chainsaw unless you have a safe opening. Its large size makes sustained melee attacks difficult without leaving yourself exposed.
• Do not rely on partial invisibility for protection. The arachnotron’s plasma becomes harder to predict against a partially invisible target.
• Use infighting to your advantage when appropriate. Letting other monsters engage it can drain its health and create an opportunity to finish it afterward.
• Prioritize arachnotrons controlling important firing lanes. Removing the one covering your route can make repositioning and movement much safer.

Data at a glance

The canonical data for the Doom II / Final Doom version: 500 hit points, thing type 68 (decimal) / 44 (hex), mass 600, radius and height both 64 units, and a reaction time of 8 tics. It spawns plasma balls (MT_ARACHPLAZ) and uses the sprite name BSPI across its full animation sequence.

Doom 64 variant

The Doom 64 arachnotron is substantially redesigned, both visually and mechanically. The mechanical body is enlarged, with the brain head lowered inside the chassis rather than perched on top, and the legs stretch out at upward-pointing acute angles, giving the creature a more insectoid silhouette. Instead of a single plasma gun, the Doom 64 version carries twin cannons that fire paired blue plasma bolts in bursts of five — a different cadence than the original’s continuous stream. Its height increases to 80 units (versus 64 in the original), and per-shot damage drops to 3–24 per bolt, though the dual-fire nature alters the practical damage output significantly. Shots per minute rise to 257.1. In infighting, the Doom 64 variant is aggressive enough to defeat barons of Hell, though typically at the cost of substantial health. Arachnotrons appear in MAP32: Hectic, MAP30: The Lair, and later in the normal campaign in MAP14: Eye of the Storm. MAP32 is the first map containing arachnotrons on all skill levels, while MAP30 and MAP32 occupy positions that chronologically precede their normal-campaign appearance.

IWAD distribution

The arachnotron appears across Doom II, TNT: Evilution, the Plutonia Experiment, Doom 64, and The Lost Levels. Plutonia uses the most arachnotrons of the classic Doom II-family IWADs, with up to 94 in its single-player maps on the highest skill levels, compared with 57 in TNT: Evilution and 64 in Doom II. Doom 64 tops out at 101 in single-player at maximum difficulty.

TNT: Evilution’s first arachnotron encounter also varies by skill: MAP06: Open Season contains the first arachnotrons on skills 1–2, MAP08: Metal on skill 3, and MAP04: Wormhole on skills 4–5.

The arachnotron in community WADs

The monster’s threatening silhouette and distinctive plasma sound have made it a staple of community level design. A notable example is MAP03: Arachnotron Arrival from the Ancient Aliens megawad, designed by Paul DeBruyne (skillsaw), where the arachnotron is thematically central enough to name the map. The level sequences its encounters so that arachnotrons serve as persistent environmental threats before a scripted resolution removes them at the finale.

Legacy and appearances beyond Doom II

The arachnotron has persisted as a recognizable Doom icon across multiple franchise entries, with redesigned versions appearing in Doom Eternal and Doom: The Dark Ages. Its combination of mechanical menace, sustained plasma fire, and spider-derived aesthetics made it one of the most visually distinctive additions introduced in Doom II.