Thresher maws are primarily found on large asteroids or on planets that are unsuitable for supporting carbon-based life such as humans. It is speculated that the beasts originated from the Krogan homeworld of Tuchanka, though this is unconfirmed. They are highly territorial, however, and will relentlessly attack anything that moves through their expansive nest areas.
As previously stated, thresher maws are constantly seeking food and will attack anything that moves. It should be noted that the body of a thresher maw has never been fully witnessed, most of it remaining below ground. When threshers attack, they will swipe with their two clawed limbs or simply try to engulf prey in their tremendous mouthes. They can also expel a highly corrosive substance that eats through just about anything. They launch this acid as projectiles with substantial physical force, strong enough to penetrate kinetic shields altogether.
Kalros is the ancient "mother of all thresher maws" described in Krogan legends. During the Reaper war of Mass Effect 3, it was revealed that Kalros made its home in the deserts of Tuchanka where an ancient Krogan city once stood. It is the largest thresher maw ever witnessed and said to be at least as old as the city ruins it guards. During the Tuchanka campaign of the Reaper War, Commander Shepard was able to activate an ancient maw hammer device to summon Kalros, which attacked and buried a Reaper Destroyer.