While en route to spray a field 3 miles from the staging airstrip, the pilot heard an unusual noise in the cockpit. Seconds later, smoke and fire from under the instrument panel started filling the cockpit. The pilot made an emergency landing at the staging airstrip, and exited the burning airplane. The cockpit and center section of the airframe were fire damaged. Examination of the instrument panel disclosed that a fuel line for the fuel flow meter was burnt out of the instrument. Several fire damaged electrical wires were located in the immediate vicinity of the fuel line.