Despite their name, Fireworms resemble more caterpillars than worms. The have five body segments, each with a pair of spikes on the lower sides and a hatch that spits flames on top. Their head segment has a pair of black eyes and four spikes around where their mouth would be.
Sonic & Knuckles
In Sonic & Knuckles and its lock-on expansion Sonic the Hedgehog 3 & Knuckles, Fireworms are encountered in the tunnel sections of Lava Reef Zone. Usually found either by themselves or in groups, Fireworms will burst out of the walls and start floating back and forth, descending when turning left and ascending when turning right. While moving, their backs spew fire.
Since the Fireworms' backflames and belly spikes will inflict damage on the player should they attack their bodies from above or below. the player must attack the Fireworms' head with a Spin Jump to defeat them. However, Fireworms are some of the few Badniks that will respawn after being destroyed if the player wanders far enough away for their spawning point to get off-screen.
In Sonic Rivals, the Fireworm appears as a collectible card.
In Sonic Mania, and its expansion Sonic Mania Plus, the Fireworms appear as enemies. In terms of design, they look exactly like they did in Sonic & Knuckles. In these games, they appear in both Acts of Lava Reef Zone.
In gameplay, the Fireworms behave just like in their debut appearance: they float back and fourth while their backs spew fire and can only be damaged on their heads. Also, upon destroying a Fireworm, it will release an Animal.