Symmetrically Organized Dorsal Unpaired Median (Dum) Neurones and Flash Control in the Male Firefly, Photuris Versicolor

Abstract

Male fireflies of the species Photuris versicolor produce a species-typical triple-pulsed flash which is used as a courtship signal. The neural anatomy was examined to determine if this complex behaviour could be attributed to the organization within the central nervous system. The lantern is innervated primarily by the two most posterior abdominal ganglia. Bilateral roots from these ganglia form a symmetrical pattern of innervation to both sides of the lantern tissue. With minor exceptions, this pattern is similar to that described for other firefly species. The neural organization within the lantern ganglia was determined by back-filling the roots with cobalt or Lucifer Yellow CH, and then examining the ganglia in whole mount. Clusters of three or four large dorsal unpaired median (DUM) neurone somata, each sending bilateral processes out of the lantern roots, were found in both lantern ganglia. The DUM neurone axons bifurcate several times and ramify throughout the dorsal surface of the lantern tissue. More than one DUM neurone may innervate a particular region of photogenic tissue. When dye was back-filled into peripheral branches of the lantern roots that do not innervate photogenic tissue, no DUM somata were stained. Instead, the fibres that filled carried the dye anteriorly up the nerve cord through the ipsilateral connective. No fibres were observed to cross the ganglion midline or exit from the contralateral root, nor were any fibres stained in the contralateral connectives. DUM neurones within the lantern ganglia have resting potentials between 30 and 45 mv and they exhibit multiple, as well as single-peaked spontaneous action potentials. The presence of multiple spikes might reflect the special bilateral morphology of these neurones. The lantern nervous system is organized in an arrangement capable of synchronizing the excitation of all the lantern photocytes. This neural organization could aid in the control of the complex flash pattern displayed by male Photuris versicolor fireflies.