The Centipede has the same movement and health as the Firefly, but is armed with a better attack. The Centipede's attack applies A.C.I.D. and targets three tiles instead of one - the shot impacts one tile like the Firefly's, but also spreads from the impact point to hit two adjacent tiles for the same damage + A.C.I.D.
If you can safely keep a Centipede alive, its A.C.I.D. can be useful for neutralizing higher-health enemies, such as Alpha Vek or Beetles. If you can't convince a Centipede to hit Vek that are already on the board, try to encourage it to target the area of the board where Vek are emerging, since Vek that emerge on A.C.I.D. squares will immediately receive the A.C.I.D. debuff.