The Formation of a Neurohaemal Organ During Insect Embryogenesis

This chapter describes the embryonic development of a simple neuroendocrine effector organ—the transverse nerve—in abdominal segments of Manduca. Each abdominal ganglion of the ventral nerve cord of larval Manduca consists of approximately 700 neurons. Within these segments, three nerves emanate from each ganglion to supply muscles and organs and to receive sensory information. The dorsal and ventral nerves are bilaterally paired. The third nerve is unpaired and is called the “transverse nerve” (TN); it originates from the small median nerve that lies between the longitudinal connectives. Under laboratory conditions (25°C), Manduca embryogenesis is completed within approximately 100 hours. The external morphology of developing embryos can be used to accurately stage animals to within 3 hours. The chapter uses the convention adopted by Tyrer and Bentley in staging grasshopper embryogenesis and refers to Manduca embryonic development in terms of percentages, where 100% is a fully completed embryo at the hatching stage. Conveniently, 1% of developmental time is equal to an hour of real time. The results suggest a pattern of stereotyped neuronal differentiation that includes specific cell interactions between developing neuroendocrine neurons and glial precursors. Numerous features of the TN trajectory and position appear to be “prefigured” by the prior arrival and assemblage of non-neuronal cells into cellular pathways that are later taken by the growing axons of the TN neurons.