The five nymphal instars of Mesouelia mulsanti can be separated by the number of rows of setae on the second abdominal tergum. Fifth instars can be separated from younger instars and from each other by the degree of development of the external genitalia.
McPherson, J. E. and Taylor, Steven J.
"Distinguishing Nymphal Instars of Mesovelia Mulsanti (Heteroptera: Mesoveliidae),"
The Great Lakes Entomologist, vol 26
Available at: https://scholar.valpo.edu/tgle/vol26/iss3/5