The Viridian Tank is an Insectoid tank. This tank is pretty much impossible to destroy but for a small discharge vent in the back where excess plasma from the cannon is vented.
The Insectoids are an insect-like alien race that evolved from giant insects, much like the ones that lived on Earth during the Carboniferous period, 300 million years ago. Their home world, known to humans as Arretus [ARE-EH-TUS], is much like Earth was at that time, with far more oxygen and far less carbon dioxide in the atmosphere than there is today on Earth. Aside from their ancestry, the Insectoids evolved very much like humans did. Over millions of years, insects on their world began to take a roughly humanoid form. They came to Mars on a colony mission and found humans living there already. They then decided to wipe out this scourge and complete their colonization of the planet.