The phone in Audrey's car is a precursor to modern cell phones. It worked off of the Mobile Telephone Service, a VHF service with very limited availability. The user would be connected to an operator, who would then route the call to a second operator that could then route the call to the intended recipient.
The sound that Common True Katydids make is used for the giant grasshoppers.
A few scenes of deserted Chicago streets are actually still photos taken early in the day.
Footage of tornado destruction was used as the "aftermath" of the grasshopper attack on Ludlow, Illinois.
The National Guardsmen wear the insignia of the US Army's 33rd Infantry Division, which is the designated unit of the Illinois National Guard. The soldiers in Chicago wear the insignia of the US Fifth Army, which at the time was headquartered in Chicago, where the film takes place.