Place of Origin
They came from a Graboid that was about to metamorphose in the Sahara Dessert. They were captured by Meghan and several soldiers. They were taken to Perfection Valley to be trained to dig through dirt and save victims of earthquakes. Shriekers were being trained by a government agency to use their heat-seeking organs to locate people in disaster areas. However, these test subjects proved to be too much for the government agents and scientist to handle, and eventually went on a rampage. Within hours they multiplied from four individual Shriekers to forty. These Shriekers were specially trained to recognize even indirect sources of thermal radiation (a skill they taught to their offspring as well), and they also did not molt thanks to a unidentified treatment that stunted their development however it is unknown if this applied to their offspring. They were also modified with a chip in their brains that curbed their aggressive behavior in conjunction with their handling collars, making them the first cyborgs of their species.
Role in the Episode
Burt Gummer hears one of them shrieking, and he grabs a Heckler & Koch. He sees one of the up on a cliff and shoots him. When he gets up there he is held at gunpoint by a group of six soldiers. They don't tell him anything about what they are doing there. During a storm the Shriekers get nervous, but when a lightning bolt strikes their Behavoir Modification Unit (BMU), which controlled them, they revert back to their natural state.
They kill everyone except for Megan Flynn, Harold Baines, Otto and Karl Hartung. Burt, and all the others except Otto and Hartung (who are missing) head to Burt's house, where they set fires to draw in the shriekers, which have split up into three groups of forty. Hartung comes running to the bunker, while being chased by a Shrieker. The Shrieker kills him and Tyler and Burt kill the Shrieker. Armed with two M4-A1s and a shotgun Tyler, Burt and Rosalita head up the stairs and start to kill shriekers.
One of the Shriekers with a collar sneaks up behind Burt, but thanks to Nancy alerting him, Burt kills him. The second group attacks, but they along with another collared Shrieker are killed. While Burt, Tyler and Rosalita are up on the roof, Meghan burns all the ammo. They come back down, and find out that Nancy, Twitchell and Jodi are trapped in the safe room, because of a bullet jammed in the door. Burt tells Nancy, Jodi and Twitchell how to make a bomb to blow the door open.
Otto shows up and Meghan goes out to lure the Shriekers away, but they are killed as the Shriekers' collars are completely fried which Megan didn't expect. The remaining Shriekers eventually attack through Burt's escape tunnel, but Nancy kills them with a bomb they had been building to destroy the door to the safe room and free the group trapped in there.
Their role in the episode also shows that experienced Shriekers will sometimes be the leaders of a pack of Shriekers who defer to their greater experience and ability to find food.