The structure and properties of a compartment border: the intersegmental boundary in Oncopeltus

García-Bellido et al. have described how groups of cells in developing Drosophila become subdivided into 'compartments'. Cells within any compartment have rigid prospective fates such that, while their progeny may give rise to variable regions within a compartment, they can never generate cells in any other compartment. Analysis of the position and shapes on clones allows definition of the compartment boundaries. I report studies on a compartment boundary (likewise demonstrated by clonal analysis) in the hemipteran insects Oncopeltus and Rhodnius. The advantage of this border is that it can easily be identified in the light and electron microscopes. There is an abrupt change of cell shape at the border, which has been analysed by means of serial electron microscope sections. The types of cell junctions at the border and elsewhere are compared and shown to have no qualitative differences. The border is an effective barrier to the growth of peripheral sensory axons, although not apparently to dendrites. The intersegmental boundary allows passage of information relating to cuticle deposition, wounding response, tracheolar movement and intercellular coupling. Making wounds across the border leads to greater effects on polarity of epidermal cells than making similar wounds elsewhere on the tergites.