The body pattern of Drosophila varies a basic segmental morphology at different positions on the anterior–posterior axis to achieve an astonishing variety of related but very different structures. At early stages of development, the morphological organization of the posterior head is similar to that of the rest of the body, but thereafter it rapidly diverges. The morphological divisions that mark the segmental boundaries of the gnathal segments are the first to appear in the visible segmentation of the Drosophila body plan, arising at about 6.5 hours of development. Among the most prominent, and the most important for the purposes of this chapter, are the mouth hooks and cirri. The mouth hooks are serrated chitinous structures that function as the jawbones and teeth of the larva. Finally, the mouth hooks develop from cells of the maxillary segment, as do the cirri, the rows of triangular papillae that flank the opening of the larval mouth.
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