ALT Diagram of the two layers of the wing disc in the third larval instar. The disc proper (DP, dark blue) is mostly composed of columnar cells and has characteristic buckles and folds (white lines); the peripodial epithelium (PE, light blue) is composed of squamous cells in the central region and cuboidal cells in more peripheral regions. Representative cell outlines of squamous cells are indicated. The diagram below shows a cross section through the wing disc along the dotted line above. Note the differences in cell shape between the two layers.