Morphogenesis of the antenna of the male silkmoth, Antheraea polyphemus. V. Development of the peripheral nervous system

Abstract The imaginal antenna of the male silkmoth Antheraea polyphemus is a feather-shaped structure consisting of about 30 flagellomeres, each of which gives off two pairs of side branches. During the pupal stage (lasting for 3 weeks), the antenna develops from a leaf-shaped, flattened epidermal sac (‘antennal blade’) via two series of incisions which proceed from the periphery towards the prospective antennal stem. The development 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 arranged sensillogenic regions alternating with non-sensillogenic regions. Immediately after apolysis, clusters consisting of 5 sensory neurons each and belonging to the prospective sensilla chaetica can be localized at the periphery of the antennal blade in the sensillogenic regions. During the first day following apolysis, the primordia of ca. 70 000 olfactory sensilla arise in the sensillogenic regions. Axons from their neurons are collected in segmentally arranged nerves which run towards the CNS along the dorsal as well as the ventral epidermis and are enveloped by a glial sheath. This ‘primary innervation pattern’ is completed within the second day after apolysis. A first wave of incisions (‘primary incisions’) subdivide the antennal blade into segmental ‘double branches’ without disturbing the innervation pattern. Then a second wave of incisions (‘secondary incisions’) splits the double branches into single antennal branches. During this process, the segmental nerves and their glial sheaths are disintegrated. The axons are then redistributed into single branch nerves while their glial sheath is reconstituted, forming the ‘secondary’, or adult, innervation pattern. The epidermis is backed by a basal lamina which is degraded after outgrowth of the axons, but is reconstituted after formation of the single antennal branches.