Morphology and physiology of motor neurons innervating labral muscles of locusta migratoria

Major animal taxa, known as phyla, each represents a basic bauplan. Among the diversity of phyla, arthropods are the most dominant on the earth. The conspicuous feature of the arthropod bauplan is the grouping of body segments into specialized units by the phenomenon of tagmosis which in insect has given rise to three body regions; head, thorax and abdomen. But the number and the nature of segments involved in composition of the insect head is still a matter of dispute among today’s evolutionary biologists. From the beginning of the dispute, there was general agreement that three mouthparts segments (mandible, maxillae, and labium) take part in the head composition, and their ganglia fuse to form the suboesophageal ganglion of the insect nervous system. However, the nature of the segments anterior to the mandible (labral/ocular, antennal, and intercalary segments) remained controversial. Modern molecular, palaeontological, and neuroanatomical studies showed that the insect head is composed of six segments; occular, antennal, intercalary, and three gnathal segments. But conflicting views are still present regarding the nature and segmental affiliation of the insect labrum. Several such modern studies imply that the labrum is a highly modified appendage of the intercalary segment, and sensory innervation of the embryonic and the adult labrum shares many similarities with that of head and thoracic appendages. However, defining criterion of an appendage implies that every appendage must be innervated by its own segmental ganglion, and the innervation pattern must include both sensory and motor elements. The aim of thesis is to investigate the motor innervation pattern of muscles of the labrum in the adult locust to reveal the evolutionary changes underlying neuronal circuitry, which might help to decide whether the labrum represents a structure that has developed during the evolution by fusion of paired appendages associated with the intercalary segment. Using Neurobiotin as a retrograde neuronal tracer, specific ...