249.770.  First election of supervisors — terms. — Within one year after any sewer district has been organized and incorporated under the provisions of sections 249.760 to 249.810 there shall be elected a board of five supervisors, to be composed of voters in the district and the supervisors shall immediately by lot determine the terms of their office, which shall be respectively one, two, three, four and five years, and they shall serve until their successors are elected and qualified.