Mechanoreceptive sensillum fields at the tarsal tip of insect legs

Gradients in mechanotransduction of force and body weight in insects

Posture and walking require support of the body weight, which is thought to be detected by sensory receptors in the legs. Specificity in sensory encoding occurs through the numerical distribution, size and response range of sense organs. We have studied campaniform sensilla, receptors that detect forces as strains in the insect exoskeleton. The sites of mechanotransduction (cuticular caps) were imaged by light and confocal microscopy in four species (stick insects, cockroaches, blow flies and Drosophila). The numbers of receptors and cap diameters were determined in projection images. Similar groups of receptors are present in the legs of each species (flies lack Group 2 on the anterior trochanter). The number of receptors is generally related to the body weight but similar numbers are found in blow flies and Drosophila, despite a 30 fold difference in their weight. Imaging data indicate that the gradient (range) of cap sizes may more closely correlate with the body weight: the range of cap sizes is larger in blow flies than in Drosophila but similar to that found in juvenile cockroaches. These studies support the idea that morphological properties of force-detecting sensory receptors in the legs may be tuned to reflect the body weight.

Neuronal Innervation of the Subgenual Organ Complex and the Tibial Campaniform Sensilla in the Stick Insect Midleg

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.