Arachnotron

"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."

- Description in the Doom II manual

The Arachnotron is a spider-like cybrid monster introduced in Doom II. It is similar in appearance to the Spiderdemon, but smaller and with large eyes that change color depending on the monster's current state (moving, hurt, or attacking). It is supported by a metal chassis and a body that consists primarily of a large brain, along with two small arms. Instead of a chaingun, it wields a powerful plasma gun that fires bursts of yellow and green energy.

In Doom II, they make their first appearance in Dead Simple, where the player must fight them when they reinforce the fallen Mancubi.

Combat characteristics
An Arachnotron emits a crescendoing mechanical cry (akin to machinery powering up) when spotting a player. Like the Spiderdemon, it has audible footsteps, in their case sounding like buzzing machinery punctuated with occasional electronic crackling. When this monster dies, its head explodes into a morass of gore as its mechanical body collapses while unleashing an electronically-pitched dying tone that resembles failing machinery.

Tactical analysis
Once the Arachnotron engages an enemy it will continue to fire until the enemy is killed or moves out of its field of view, or until the monster itself flinches in pain or is destroyed - similar behavior can be observed with the Spiderdemon, nazi soldier and heavy weapon dude.

Although they may seem slow, Arachnotrons are actually faster than quite a few of the other monsters and can catch up with the player at an alarming rate. They have a medium pain chance, so rapid fire weapons such as the chaingun will work well, although using the chainsaw on them is very risky (unless the chainsaw-wielding player can rapidly reach and attack the monster before it opens fire). Their attacks are easier to avoid than the rapid-fire hitscan attacks of heavy weapon dudes and Spiderdemons, but their plasma bursts are among the fastest projectiles, and their high rate of fire can quickly drain the player's health. Additionally, the plasma bursts can obscure the player's vision upon a successful hit. If a player constantly attacks, moves in broad side-to-side movements, and tries to get out of the Arachnotron's line of fire periodically, then the player will sustain minimal damage.

Because of their sustained, extremely fast rate of fire, they are often more than capable of defeating almost any monster if provoked via monster infighting, especially when multiple Arachnotrons attack; however, stronger enemies such as a baron of hell will usually prevail in smaller battles because of their higher health.

Data

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 * 1) This table assumes that all calls to P_Random for damage, pain chance, impact animations, and backfire checks are consecutive. In real play, this is never the case: counterattacks and AI pathfinding must be handled, and of course the map may contain additional moving monsters and other randomized phenomena (such as flickering lights). Any resulting errors are probably toward the single-shot average, as they introduce noise into the correlation between the indices of "consecutive" calls.
 * 2) Assumes that direct hits are possible, which does not occur in any stock map.
 * 3) Hardcoded exception to infighting negates damage (excepting indirect damage caused by exploding barrels).

Appearance statistics
In classic Doom, the Arachnotron is first encountered on these maps:

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The IWADs contain the following numbers of Arachnotrons:

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Doom 64
In Doom 64 their physicality is considerably altered; the mechanical body is much bigger, with the brain head ducked down inside the machine itself instead of sitting atop it, also not having any organic arms like the classic version, and the mechanical legs (now six instead of only four) being longer and bent at upwards-pointing acute angles. Instead of a single plasma gun, the Doom 64 Arachnotrons possess two guns that fire out two blue plasma bolt shots simultaneously, but done so in spurts of five instead of a constant rate of fire, though it will continue firing as long as it can see the player. They first appear in MAP14: Eye of the Storm unless the player goes through MAP30: The Lair and MAP32: Hectic, which contain Arachnotrons but chronologically come first.

Doom Eternal
Two Arachnotrons can be seen walking in the official teaser trailer of Doom Eternal, but it's actual appearance in the game is yet to be confirmed.