You will often see the size of a fly represented in hook size or in mm’s. I have never easily adjusted to metric and have a rough sense of flies in the 5mm to 60mm range. But, I need to actually see the graph to be sure. I enclose these charts for visuals, realizing you will refer to a ruler, etc.
So, notice a 5mm fly is about the equivalent of a size 18 to 22 hook. Many references world wide will always utilize mm references for insect size, pattern size, bead size or compare hook size (standard references) to an insect size (in mm’s)
“Of all tyrannies, a tyranny sincerely exercised for the good of its victims may be the most oppressive. It would be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be
satiated; but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience.”