In order to determine the function of the white wing stigma in Calopteryx mandata, males and females were marked or left unaltered, pinned onto a stick and presented to free ranging males. Male responses to females with blackened stigmas were minimal whereas most males responded to unaltered females, unaltered males, and altered males. It is suggested that presence of the white stigma, in combination with wing transparency, is important for male discrimination between the sexes.
"Visual Recognition of Females by Male Calopteryx Maculata (Odonata: Calopterygidae),"
The Great Lakes Entomologist, vol 17
Available at: https://scholar.valpo.edu/tgle/vol17/iss4/3