Insect mouthpart motor patterns: central circuits modified for highly derived appendages?

The interrelationships of motor patterns controlling the mouthparts and the salivary gland of the migratory locust were studied in a deafferented preparation activated by the muscarinic agonist pilocarpine. The aim of the study was to check whether motor output of different neuromeres of the suboesophageal ganglion and the brain is coherent and functionally adequate in the absence of sensory feedback. Our analysis shows that motor output to labial, maxillar, and labral muscles and to the salivary gland is strongly coupled to the mandibular motor pattern. Bilateral coupling is of similar strength. For a muscle of the labial palp, however, an independent pattern is shown. From our findings it is concluded that for stable coordination of most muscles involved in mouthpart movements sensory feedback is not essential. This is in contrast to motor patterns controlling thoracic appendages in similar insect model systems. As mouthparts are widely accepted to be homologous to thoracic appendages, it is concluded that during the evolutionary process which led to derived features of mouthparts also the central nervous networks controlling these structures were reconfigured accordingly.