Wasserthal was able to stain the wings of Pieris rapae by injecting dye into the tip of the abdomen of a tethered, resting butterfly. Images of the wings at time intervals after dye injection demonstrate the movement of hemolymph into the wings along the veins. Wasserthal suggested that the fluid might oscillate somewhat but was mostly unidirectional flow from the hemolymph to the wings. Water lost to evaporation from the wings would be replaced by fluid from the hemolymph. This mechanism would allow the deposition in wing tissue, substances dissolved in hemolymph such as chemicals toxic to predators. The toxins could flow one way into the wing and accumulate there.
*Wasserthal, L.T. Haemolymph flows in the wings of Pierid butterflies visualized by vital staining. Zoomorphology(1983) 103:177-192.