Figure 4

Polarisation and arrangement of the bristle cells in the two zones of the segment. A right hemisegment is shown with the mature bristles on the left, the earliest stages on the right and other stages in between. Most probably, the orientations of the divisions of the SOP and PIIa are the same everywhere, while the divisions of the PIIb cells are different in the two zones; for details, see the text and Figure 3. Considering the neurons (green) and their processes: in the anterior zone, initially the dendrite grows anteriorly, while in the posterior zone the dendrite grows posteriorly. The dendrite becomes enwrapped by the shaft and socket cells and, in the anterior zone, the tip of the dendrite then forms a U-shape in order to enter the socket cell (yellow). In the posterior zone the dendrite grows straight along the AP axis. In the anterior zone, the axon grows posteriorly while in the posterior zone it grows anteriorly. Note on this right hemisegment, the U-shaped dendrites turn to the left. On the left hemisegment the U-shaped dendrites would turn to the right. This summary model is based on our observations and on [22,61].