MORPHOGENESIS OF THE ANTENNA OF THE MALE SILKMOTH, ANTHERAEA-POLYPHEMUS .5. DEVELOPMENT OF THE PERIPHERAL NERVOUS-SYSTEM

The imaginal antenna of the male silkmoth Antheraea polyphemus is a fe ather-shaped structure consisting of about 30 flagellomeres, each of w hich gives off two pairs of side branches. During the pupal stage (las ting for 3 weeks), the antenna develops from a leaf-shaped, flattened epidermal sac ('antennal blade') via two series of incisions which pro ceed from the periphery towards the prospective antennal stem. The dev elopment of the peripheral nervous system was studied by staining the neurons with an antibody against horseradish peroxidase as well as by electron microscopy. The epithelium is subdivided in segmentally arran ged sensillogenic regions alternating with non-sensillogenic regions. Immediately after apolysis, clusters consisting of 5 sensory neurons e ach and belonging to the prospective sensilla chaetica can be localize d at the periphery of the antennal blade in the sensiliogenic regions. During the first day following apolysis, the primordia of ca. 70 000 olfactory sensilla arise in the sensillogenic regions. Axons from thei r neurons are collected in segmentally arranged nerves which run towar ds the CNS along the dorsal as well as the ventral epidermis and are e nveloped by a glial sheath. This 'primary innervation pattern' is comp leted within the second day after apolysis. A first wave of incisions ('primary incisions') subdivide the antennal blade into segmental 'dou ble branches' without disturbing the innervation pattern. Then a secon d wave of incisions ('secondary incisions') splits the double branches into single antennal branches. During this process, the segmental ner ves and their glial sheaths are disintegrated. The axons are then redi stributed into single branch nerves while their glial sheath is recons tituted, forming the 'secondary', or adult, innervation pattern. The e pidermis is backed by a basal lamina which is degraded after outgrowth of the axons, but is reconstituted after formation of the single ante nnal branches.