I have kids working on a model of a Boulton Paul Defiant, a WWII-era fighter plane. When we found images of this plane, it shows two rods sticking vertically down from the belly of the plane. Some of the images show them to be quite long-- the plane would have a hard time landing if they remained that length! After doing some searching, it seems like these are called "ventral masts". What are these for? Do they retract for landings? Thank you!
Normally, those masts are located on the upper fuselage and sometimes on the wings. They are used to string antennas between them and other airplane parts, or are antennas themselves.
In case of the Boulton-Paul Defiant, those antenna masts could not be located in the upper hemisphere in order to keep the field of fire of its gun turret free. Together with the Blackburn Roc and the Hawker Hotspur, those airplanes had a unique layout for a fighter with a single pilot and a gun turret placed behind the cockpit. How well this idea turned out in practice can be read in this answer.
In case of the Defiant, those antenna masts were made retractable to allow take-off and landing. The antennas were needed for communication and navigation.
During the era of the 2nd World War, some aircraft, like the Defiant, had radio masts that were used for communication and navigation. They could be extended while in flight to increase receptive range and retracted before approach to clear them out of the way for landing.
Aircraft Anatomy of World War II: Technical Drawings of Key Aircraft by Eden and Moeng is a good resource to see how elements like this were incorporated into the myriad different aircraft of the time period.
Hope this helps. Happy modeling.