The segmentation pattern of the Drosophila wild-type embryo is characterized by a number of easily identifiable cuticular structures. They include skeletal elements of the involuted head and ventral denticle belts that define by size, pattern and orientation the anterior part of the three thoracic and eight abdominal segments. Further landmarks such as sensory organs and the posterior tracheal endings (‘Filzkörper’), in combination with the denticle belts, allow one to unequivocally determine the polarity and quality of each segment in preparations of the larval cuticle (see Fig. 1D).
The segmentation pattern of Drosophila is established at about blastoderm stage and it requires both maternally and zygotically active genes. Genetic analysis has identified a number of genes with zygotic activity that regulate key steps during pattern formation. Mutations in these genes cause specific defects in the segmental pattern of the embryo that allow the definition of classes of segmentation genes required for the subdivision of the embryo into segmental units (Nüsslein-Volhard & Wieschaus, 1980).