Embryonic development of an insect sensory system, the abdominal cerci ofAcheta domesticus.

1. The embryonic development of the abdominal cerci of the house cricketAcheta domesticus is described from scanning and transmission electron microscope data. 2. A staged description of externally visible events in embryogenesis is tabulated as a context for describing the chronology of embryonic development of the abdominal cerci. 3. Three phases of cercal development are distinguished: differentiation of the cercal anlagen and secretion of the first embryonic cuticle; elongation of the cercus culminating in the secretion of the second embryonic cuticle after completion of a continuous epidermis at the time of dorsal closure; and differentiation of functional sensilla on the third embryonic (first instar) cuticle. 4. The first axon profiles appear in the cercus immediately before elongation of the cercus. These axons have dendrites with ciliary configuration in the lumen of the cercus. Glial cells associated with the pioneer axons may precede the axons in occupying dorsal and ventral luminal midlines of the cerci. 5. Trichoid sensilla appear in the integumet following apolysis of the second embryonic cuticle. 6. Axons are added to the dorsal and ventral pioneer fibre bundles shortly before sensilla become apparent. 7. The majority of sensory axons traverse the cercus during the final 15% of embryonic development. 8. The sensilla of the first instar cercus do not achieve their final orientation until the cercal cuticle is expanded following eclosion from the second embryonic cuticle that encloses the hatchling until it reaches a free surface. 9. The role of the pioneer fibres in establishing a pathway for the functional sensillar neurons is discussed in relation to other studies of sensillar development in insects.