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. -- Doom II manual
The Arachnotron monster is first introduced in Doom II in the seventh map, Dead Simple.
Arachnotron numbers by game
Broken down by game and difficulty.
Difficulty 1: I'm Too Young To DieDifficulty 2: Hey, Not Too RoughDifficulty 3: Hurt Me PlentyDifficulty 4: Ultra-Violence
Difficulty 5: Nightmare! (note: monsters respawn in this mode)