Mo. Rev. Stat. § 21.145

Each senator or representative shall be reimbursed from the state treasury for actual and necessary expenses in an amount equal to eighty percent of the federal per diem established by the Internal Revenue Service for Jefferson City for each day on which the journal of the senate or house, respectively, shows the presence of such senator or representative. Upon certification by the president and secretary of the senate and by the speaker and chief clerk of the house of representatives as to the respective members thereof, the commissioner of administration shall approve and the state treasurer shall pay monthly such expense allowance.