Mo. Rev. Stat. § 21.120: Writs of election, how directed.

If any vacancy happens in the senate, for a district composed of more than one county, the writ of election shall be directed to the election authority of the county first named in the report establishing the district; and if the vacancy happens in a senatorial district, which has been divided or altered after the general election next preceding the occurrence of the vacancy, the writ of election shall be directed to the election authority of the county first named in the old district and if any vacancy happens in either house, for any county which has been divided after the general election next preceding the occurrence of the vacancy, the writ of election shall be directed to the election authority of the old county.