Neuromodulatory unpaired median neurons in the New Zealand tree weta, Hemideina femorata

Wetas are ancient Gondwanan orthopterans (Anostostomatidae) with many species endemic to New Zealand. Like all Orthoptera they possess efferent neuromodulatory dorsal unpaired median (DUM) neurons, with bilaterally symmetrical axons, that are important components of motor networks. These neurons produce overshooting action potentials and are easily stimulated by a variety of external mechanosensory stimuli delivered to the body and appendages. In particular, stimulation of the antennae, mouth parts, tarsi and femora of the legs, abdomen, cerci and ovipositor is very effective in activating DUM neurons in the metathoracic ganglion of wetas. In addition, looming visual stimuli or light on-, light off-stimuli excite many metathoracic DUM neurons. These DUM sensory reflex pathways remain viable after the prothoracic to subesophageal connective is cut, whereas in locusts such reflex pathways are interrupted by the ablation. This suggests that, in wetas, sensory reflex pathways for DUM activation are organized in a less centralized fashion than in locusts, and may therefore reflect a plesiomorphic evolutionary state in the weta. In addition, many weta DUM neurons exhibit slow rhythmic bursting which also persists following the connective ablation.

Graphical abstractThe “Graphical abstract” shows a population of large median neuron somata (blue) in a metathoracic ganglion, of which individual cells exhibit features of unpaired median neurons with bilaterally symmetrical axons. Intracellular recordings from such a neuron reveal responses to touching the ovipositor (first panel A) which persist when both suboesophageal to prothoracic connectives are cut (second panel B).Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► New Zealand wetas possess octopaminergic neurons with bilaterally symmetrical axons. ► They respond to mechanosensory and visual stimuli. ► They also fire with voluntary leg movements as components of sensory-motor networks. ► Their connectivity pattern exhibits less centralized features than in other insects.