Bayonets are swordlike weapons, usually affixed to the front of a rifle or other type of gun, used in melee combat. In the Metroid series, these weapons are explicitly named only twice, in Metroid Prime and Metroid Prime 2: Echoes.
Being melee weapons, bayonets are easily overpowered by long-range and projectile weapons, and are usually only used in close combat or when other options have been exhausted.
Galactic Federation Marines
After being stranded on Planet Aether, the Galactic Federation Marines of the GFMC Task Force Herakles were under constant attack by Splinters possessed by the Ing, and were quickly running low on ammunition. SPC F. Triplette predicted that the shortage would lead to reliance on bayonets in a personal log. Such use of bayonets is never shown, and their standard weapons don't appear to include a mounted bayonet. It's possible that the bayonets are retractable or a separate attachment that must be affixed manually as needed.
Elite Pirates and the Phazon Elite have retractable Wrist Bayonets in their arsenal, which they use in addition to their long-range weaponry. These bayonets appear as pairs of blue blades that do damage on contact. Pirates won't use these weapons unless pressed into extremely close combat, usually relying on Wave Quake Generators to dispatch foes.
- "Personal Log Entry, SPC F. Triplette
The sarge says those "Splinters" remind him of some killer bug he saw on another planet once. All I know is the things are fast and take a lot to drop. Pretty soon we'll have to go to bayonets. Everyone's low on ammo...even Brode, and he's the stingiest grunt I know." —Trooper Logs "SPC F. Triplette" (Metroid Prime 2: Echoes)
- "Recommended battle systems for Elite Pirates are as follows:
Shoulder-mounted Plasma Artillery Cannon, Repulsion Shield, Wave Quake projector, energy-siphon system, and retractable Wrist Bayonets." —Scan Data (Metroid Prime)