I could be wrong , but I'd always thought it's the little white specks that are the eggs, and which give the undersides of the leaves a powdered look. The plants sometimes do get dried sap (maybe the brown spots) , which may be of various colors depending on the plant, which come from the wounds that the mites make in the leaves. I could be wrong about that, but whatever the case they've been the "bane" of my plant growing existance over these many years. Little buggers!!!