Critical curves (top row) and caustics (bottom row) of ellipsoidal NFW lenses with (κ_s, e) = (6, 0.9881) (left column) and (κ_s, e) = (8, 0.9986) (right column), with tangential and radial components marked green and purple, respectively. Both tangential critical curves are centrally bulged; both radial critical curves are flattened along the major axis. The inset in the bottom right panel shows a detail of the top part of the tangential caustic, revealing the three-cusped butterfly structure.