Mandibular movements and their control in the Weta, Hemideina Maori (Orthoptera : Ensifera : Stenopelmatidae)

The peripheral mechanisms involved in the control of mouthpart movements are investigated. All the behaviours observed to involve the mandibles are described. The morphology of the head capsule, mandibles, and mandibular muscles are described together with the innervation of the mandible from the suboesophageal ganglion. Myography and morphology are combined to investigate the mechanical functioning of the mandible. A survey of the mandibular sense organs revealed a number of sensilla associated with the cuticle, and three stretch receptors spanning the mandibular joint. Ultra structural examination showed two of these to be muscle receptors. The ventral muscle receptor organ (VMRO) is structurally complex. Its physiological responses are strongly dependent on efferent input, including reflexive input from the contralateral VMRO. Peripheral control in several behaviours was tested. The imposition of loads during inactivity and mastication revealed a load compensating capability. A control mechanism involving the VMRO as an error detector is proposed. The effect of VMRO ablation on the coordination of feeding and defensive biting was examined. In both of these, precise occlusion of the mandibles was impaired. By inducing defensive biting onto force transducers the bite strength of each mandible was recorded independently. The right mandible was found to have a dominant role in determining bite duration. The effects of VMRO ablation supported this. Bite duration under these restrained conditions appeared to be limited by the inability of the mandible to close. This again appears to be monitored by the VMRO in an error detector role. Other effects of ablation on biting are consistent with a position-monitoring function of the VMRO. This appears to involve a system of comparison involving the VMRO from each mandible. The proposed control mechanisms are all explicable in terms of the known receptor physiology