200 grasshoppers were used for the film. During the filming, they began to cannibalize one another, so by the time the last shots were done, only a dozen were left.
The National Guardsmen wear the insignia of the U.S. 33rd Infantry Division, which is indeed the designated unit of the Illinois National Guard. The soldiers in Chicago wear the insignia of the U.S. Fifth Army, which at the time was headquartered in Chicago, where the film takes place.
There really is a town called Ludlow, in the right area in Champaign County, IL, and the town Paxton is just north of it in Ford County. They are also near Chanute AFB, where reporter Audrey Aimes (Peggie Castle) was headed at the beginning of the film.