Embryonic development of the sensory innervation of the clypeo-labral complex: further support for serially homologous appendages in the locust

The clypeo-labrum, or upper lip, of insects is intimately involved in feeding behavior and is accordingly endowed with a rich sensory apparatus. In the present study we map the temporal appearance of all major clusters of sensory cells on this structure in the locust during the first half of embryogenesis. The identities of these sensory cell clusters were defined according to the origin of the branching point of their axons from the labral sensory nerve as seen at mid-embryogenesis. The first sensory cells to differentiate from the labral epithelium do so at stereotypic sites beginning at around 32% of embryogenesis. Bilaterally symmetrical clusters of differentiated neurons rapidly appear and pioneering of the labral sensory nerve on each side is performed by a specific cell from each cluster. This cell directs its axon anteriorly towards a bilaterally symmetrical pair of cells, the frontal commissure pioneers, on either side of the developing frontal ganglion. The final trajectory of the sensory nerve within the labrum closely matches the pattern of Repo-expressing glial cells. The majority of the sensory cell clusters differentiate during embryogenesis, but the number of sensory cells in some clusters are modified significantly during postembryonic development. Comparing the innervation pattern of the clypeo-labrum with that of other mouthparts and the leg at mid-embryogenesis, we find a striking similarity in organization which we interpret as support for the homologous appendage hypothesis.