Abstract
Mechanosensory organs in legs play are crucial receptors in the feedback control of walking and in the detection of substrate-borne vibrations. Stick insects serve as a model for the physiological role of chordotonal organs and campaniform sensilla. This study documents, by axonal tracing, the neural innervation of the complex chordotonal organs and groups of campaniform sensilla in the proximal tibia of the midleg in Sipyloidea sipylus. In total, 6 nerve branches innervate the different sensory structures, and the innervation pattern associates different sensilla types by their position. Sensilla on the anterior and posterior tibia are innervated from distinct nerve branches. In addition, the variation in innervation is studied for five anatomical branching points. The most common variation is the innervation of the subgenual organ sensilla by two nerve branches rather than a single one. The fusion of commonly separated nerve branches also occurred. However, a common innervation pattern can be demonstrated, which is found in >75% of preparations. The variation did not include crossings of nerves between the anterior and posterior side of the leg. The study corrects the innervation of the posterior subgenual organ reported previously. The sensory neuroanatomy and innervation pattern can guide further physiological studies of mechanoreceptor organs and allow evolutionary comparisons to related insect groups.
Highlights
Insects have multiple types of mechanoreceptors and other sensilla on body appendages like legs or antenna [1,2,3,4,5,6,7]
Large internal organs are the chordotonal organs consisting of scolopidial sensilla [14,15], responding to mechanical forces that act on the insect body through substrate vibration, airborne sound, or joint movements, resulting in the stretch or tilting of the dendrites of the bipolar sensory neurons
The present study analyses the neuronal innervation of the subgenual organ complex and 6A/6B campaniform sensilla (CS) in the proximal tibia
Summary
Insects have multiple types of mechanoreceptors and other sensilla on body appendages like legs or antenna [1,2,3,4,5,6,7]. Large internal organs are the chordotonal organs consisting of scolopidial sensilla [14,15], responding to mechanical forces that act on the insect body through substrate vibration, airborne sound, or joint movements, resulting in the stretch or tilting of the dendrites of the bipolar sensory neurons. The femoral chordotonal organ (FCO) and the subgenual organ (SGO) are important chordotonal organs present in all leg pairs [15]. G. in stick insects, cockroaches, locusts) or intermediate organ (e.g., in bushcrickets) [17,18]. These organs respond to substrate vibrations and, in certain insects, to airborne sound [19]. By the position of these sensilla which is Insects 2020, 11, 40; doi:10.3390/insects11010040 www.mdpi.com/journal/insects
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