NUMBER, IDENTITY, AND SEQUENCE OF THE DROSOPHILA HEAD SEGMENTS AS REVEALED BY NEURAL ELEMENTS AND THEIR DELETION PATTERNS IN MUTANTS

The development of the insect head tagma involves massive rearrangemen ts and secondary fusions of segment anlagen during embryogenesis. Due to the lack of reliable morphological markers, the number, identity, a nd sequence of the head segments, particularly in the pregnathal regio n, are still a matter of ongoing debates. We examined the complex arra y of internal structures of the embryonic Drosophila melanogaster head such as the sensory structures and nerves of the peripheral and stoma togastric nervous systems, and we used embryonic head mutations causin g a lack of overlapping segment anlagen to unravel the segmental ident ity and the sequence of the neural elements. Our results provide evide nce for seven distinct segments in the Drosophila head, each character ized by a specific set of sensory neurons, consistent with the proposa l that insects, myriapods, and crustaceans share a monophyletic evolut ionary tree from a common annelid-like ancestor.