The membrane and actin cortex of a motile cell can autonomously differentiate into two states, one typical of the front, the other of the tail. On the substrate-attached surface of Dictyostelium cells, dynamic patterns of front-like and tail-like states are generated that are best suited to monitor transitions between these states. To image large-scale pattern dynamics independent of boundary effects, we produced giant cells by electric-pulse induced cell fusion. In these cells actin waves are coupled to the front and back bands of PIP3-rich bands that have a finite width. that are flanked at their front and back by an actin wave These composite waves propagate across the plasma membrane of the giant cells with undiminished velocity. After any disturbance, the bands of PIP3 are characterized by an return to their intrinsic width. to which they return after any disturbance Upon collision, the waves locally annihilate each other and change direction; at the cell border they are either extinguished or reflected. Accordingly, expanding areas of progressing PIP3 synthesis become unstable beyond a critical radius, their center switching from a front-like to a tail-like state. Our data suggest that PIP3 patterns in normal-sized cells are segments of the self-organizing patterns that evolve in giant cells.