The James 1933 article describing Proctacanthus rodecki says the following:

"This species may be separated from all other Nearctic forms except
P. duryi Hine by the white vestiture of the scutellum and palpi. It is a
much larger fly than P. duryi and differs from it in the color of the pile
of the thorax and the color of the femora. The length of the vein at the
base of the second posterior cell and the narrowing of the first posterior
cell are characteristic. In these respects it differs from P. milbertii
Macquart, which it closely approaches in appearance."