Structure and distribution of dorsal unpaired median (DUM) neurones in the abdominal nerve cord of male and female locusts.

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Dorsal unpaired median (DUM) neurones in insects have been shown to modulate the activity of both skeletal and visceral muscle. It has been suggested that as a population they carry out a role analogous to that of the sympathetic nervous system in vertebrates; however, the extent of their distribution throughout the ventral nerve cord has not been assessed. This paper aims to fill this gap by systematically describing the number and morphology of DUM neurones in each of the abdominal ganglia of male and female locusts. To achieve this, the lateral nerves of each abdominal ganglion were backfilled to reveal the position of the somata of DUM neurones. To confirm their identity and reveal their structure, DUM somata were then impaled with microelectrodes and, after physiological characterization, the neurones were stained by intracellular injection with cobalt ions. In each of the first six abdominal ganglia of both sexes, two DUM neurones, one with axons in the tergal nerve and one with axons in the sternal nerve, were found. In the seventh abdominal, and the terminal, ganglion (composed of the eighth to eleventh neuromeres), there was considerable sexual dimorphism in DUM neurone distribution, which was most marked in those associated with some of the nerves innervating the genitalia. In the male, four clusters of somata in the seventh, eighth, and tenth segments have axons in the genital nerve. In the female, which lacks a genital nerve, clusters of DUM neurones, absent in the male, have axons in the seventh and eighth sternal nerves and the cercal nerve.