Fig. 2. Inka cells of primitive aquatic insects stained with the PETH antiserum. A very large number of small Inka cells (stained orange) of the dragonfly Sympetrum was located on the surface of narrow tracheae branching from the major lateral tracheal trunk (A). A lower density of cells was found on the broad longitudinal tracheal trunk (B). (C) Numerous small Inka cells were scattered throughout the tracheal surface of the mayfly Epheorus. (D) Larger single or coupled Inka cells with cytoplasmic processes were present on the narrow tracheae of the stonefly Perla. Scale bar, 200 µm in A and 50 µm in B—D.