Originally an aquatic species, these bloodworms (order Glycera) steadily migrated from the coastal waters of the Atlantic coast after mutating, and have made their homes in the dry and dusty wasteland. Despite a total change in habitat, bloodworms have retained two of their original traits. The first of these is their burrowing ability; instead of burrowing into wet sand and silt, their four hollow jaws have become adept at plowing through the tough and dried soil. They use this ability quite a bit, ambushing prey from beneath the surface, and then returning underground to catch their prey unaware again.
The other trait that they kept, alluded to by their style of hunting, is that they are obligate carnivores, requiring meat to survive. They are roughly four to five feet (120 to 150 centimeters) long, so they have no problem ambushing and taking down megafauna such as humans.
Bloodworms that have mutated since the Great War, they are now approximately the size of an adult human being and have spines on the outside of their body in addition to their four-pronged maw. These worms react to the presence of intruders in their territory and will charge the intruders by burrowing through the dirt at high speeds. Upon reaching their prey, they will lunge forth from the ground and attempt to bite their target.
Bloodworm larvae will also inhabit the corpses of large prey, such as brahmin, and come out of the corpse to attack any new prey that wanders by. These brahmin can be spotted from a distance if watched, as they will twitch and bulge slowly.
Bloodworms have two targetable sections, one being their head and the other being their tail. Like mole rats and radscorpions, they can close the distance quite rapidly by burrowing into the ground and bursting forth closer to the Sole Survivor or their companions. However, unlike other creatures that are able to burrow underground, bloodworms have the unique ability to conceal their movement with a dirt cloud they produce while they move on the surface, making them hard to spot when they are about to strike. This won't make them immune to be targeted with V.A.T.S., though.
They are not usually dangerous, as most of the bloodworm species possess very low health, but if found in groups, and allowed to surround the player character, they might be a problem, especially if Action Points are unavailable to help spot them with V.A.T.S.
It is possible to prevent them from bursting close to the player character after burrowing underground by standing on materials other than earth, like rock, metal or wood when engaging them.
Shooting infested brahmin will leave the bloodworm larvae momentarily stunned as they right themselves before attacking.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
Bloodworms share a striking resemblance to the "Graboids" from the film series Tremors. This is further backed by Ricky's note, which has him describing his sense of impending doom due to unusual tremors.
End of information based on unverified behind the scenes information.