When the Zanti prisoners attack Earth's nervous soldiers, a brutal firefight ensues, and all of the aliens are massacred.
The soldiers and airmen anxiously await the expected reprisal, but, instead, they receive a message of thanks from the Zanti leaders who explain that they were incapable of executing members of their own species so they sent them into the hands of a race who possessed no qualms about killing — the human race, referring to us as "practiced executioners".
Throughout history, various societies have tried various methods of exterminating those members who have proven their inability or unwillingness to live sanely amongst their fellow men.
During the Zantis' initial attack after landing on the roof of the command post, the creatures are seen descending an exterior wall; however, with the technology of the time, the stop-motion effect was not able to be used during this scene.
Here, the Zantis are mere models being lowered on wires, their movements erratic, with their legs not moving.
[3] Frank Zappa has released a piece titled Zanti Serenade on the 1992 album Playground Psychotics.