Insect epidermal cells display planar polarity (i.e. polarity in the plane of the cell sheet) by secreting oriented cuticular
denticles and bristles before each moult. We investigate how cell polarities in an abdominal segment are uniformly oriented towards the posterior of the animal. Recently we have shown for the cotton bug Dysdercus that, in 180 degrees-rotated grafts pretreated with
colchicine, graft cells tend to adopt the orientation prevailing in surrounding host cells via an intermediate stage with outward oriented
denticles (Nübler-Jung and Grau, 1987). Here we show that, in untreated grafts that were transposed along the anteroposterior segment axis, the
denticles also always tend to point outwards. This independence of the polarity pattern from the direction of transposition is compatible neither with a gradient model for polarity control, nor with the assumption that epidermal cells orient according to the local sequence of distinctly differentiated cells. Instead we found that outward orientation of graft
denticles correlates with an elongation of epidermal cells along a host-graft border with divergent cell adhesiveness. We therefore propose that outward orientation in a graft results from a combination of two factors: epidermal cells stretch along an interface with divergent cell adhesiveness, and they form a
denticle perpendicular to their long axis. By analogy, the normal anteroposterior orientation of
denticles in a segment may result because epidermal cells tend to elongate parallel to the segment boundary and to form
denticles perpendicular to this mediolateral cell elongation, i.e. along the anteroposterior segment axis.