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Abstract During walking, sensory information is measured and monitored by sensory organs that can be found on and within various limb segments. Strain can be monitored by insect load sensors, campaniform sensilla (CS), which have components embedded within the exoskeleton. CS vary in eccentricity, size, and orientation, which can affect their sensitivity to specific strains. Directly investigating the mechanical interfaces that these sensors utilize to encode changes in load bears various obstacles, such as modeling of viscoelastic properties. To circumvent the difficulties of modeling and performing biological experiments in small insects, we developed 3-dimensional printed resin models based on high-resolution imaging of CS. Through the utilization of strain gauges and a motorized tensile tester, physiologically plausible strain can be mimicked while investigating the compression and tension forces that CS experience; here, this was performed for a field of femoral CS inDrosophila melanogaster. Different loading scenarios differentially affected CS compression and the likely neuronal activity of these sensors and elucidate population coding of stresses acting on the cuticle. 

Insects use various sensory organs to monitor proprioceptive and exteroceptive information during walking. The measurement of forces in the exoskeleton is facilitated by campaniform sensilla (CS), which monitor resisted muscle forces through the detection of exoskeletal strains. CS are commonly found in leg segments arranged in fields, groups, or as single units. Most insects have the highest density of sensor locations on the trochanter, a proximal leg segment. CS are arranged homologously across species, suggesting comparable functions despite noted morphological differences. Furthermore, the trochanter–femur joint is mobile in some species and fused in others. To investigate how different morphological arrangements influence strain sensing in different insect species, we utilized two robotic models of the legs of the fruit fly Drosophila melanogaster and the stick insect Carausius morosus. Both insect species are past and present model organisms for unraveling aspects of motor control, thus providing extensive information on sensor morphology and, in-part, function. The robotic models were dynamically scaled to the legs of the insects, with strain gauges placed with correct orientations according to published data. Strains were detected during stepping on a treadmill, and the sensor locations and leg morphology played noticeable roles in the strains that were measured. Moreover, the sensor locations that were absent in one species relative to the other measured strains that were also being measured by the existing sensors. These findings contributed to our understanding of load sensing in animal locomotion and the relevance of sensory organ morphology in motor control. 

Sensory feedback from sense organs during animal locomotion can be heavily influenced by an organism’s mechanical structure. In insects, the interplay between sensing and mechanics can be demonstrated in the campaniform sensilla (CS) strain sensors located across the exoskeleton. Leg CS are highly sensitive to the loading state of the limb. In walking, loading is primarily influenced by ground reaction forces (GRF) mediated by the foot, or tarsus. The complex morphology of the tarsus provides compliance, passive and active substrate grip, and an increased moment arm for the GRF, all of which impact leg loading and the resulting CS discharge. To increase the biomimicry of robots we use to study strain feedback during insect walking, we have developed a series of tarsi for our robotic model of a Carausius morosus middle leg. We seek the simplest design that mimics tarsus functionality. Tarsi were designed with varying degrees of compliance, passive grip, and biomimetic structure. We created elastic silicone tarsal joints for several of these models and found that they produced linear stiffness within joint limits across different joint morphologies. Strain gauges positioned in CS locations on the trochanterofemur and tibia recorded strain while the leg stepped on a treadmill. Most, but not all, designs increased axial strain magnitude compared to previous data with no tarsus. Every tarsus design produced positive transversal strain in the tibia, indicating axial torsion in addition to bending. Sudden increases in tibial strain reflected leg slipping during stance. This data show how different aspects of the tarsus may mediate leg loading, allowing us to improve the mechanical biomimicry of future robotic test platforms. 

ABSTRACT Legged locomotion in terrestrial animals is often essential for mating and survival, and locomotor behavior must be robust and adaptable to be successful. This adaptability is largely provided by proprioceptors monitoring positions and movements of body parts and providing feedback to other components of locomotor networks. In insects, proprioceptive chordotonal organs span joints and encode parameters of relative movement between segments. Previous studies have used whole-organ ablation, reduced preparations or broad physiological manipulations to impair the function of the femoral chordotonal organ (fCO), which monitors the femur–tibia joint, and have demonstrated its contribution to interleg coordination and walking behavior. The fCO in Drosophila melanogaster comprises groups of neurons that differ in their morphology and encoding properties (club, hook, claw); sub-population-level manipulations of fCO function have not been methodologically accessible. Here, we took advantage of the genetic toolkit available in D. melanogaster to identify sub-populations of fCO neurons and used transient optogenetic inhibition to investigate their roles in locomotor coordination. Our findings demonstrate that optogenetic inhibition of a subset of club and hook neurons replicates the effects of inhibiting the whole fCO; when inhibited alone, however, the individual subset types did not strongly affect spatial aspects of single-leg kinematics. Moreover, fCO subsets seem to play only a minor role in interleg temporal coordination. Thus, the fCO contains functionally distinct subgroups, and this functional classification may differ from those based on anatomy and encoding properties; this should be investigated in future studies of proprioceptors and their involvement in locomotor networks. 

Insect load sensors, called campaniform sensilla (CS), measure strain changes within the cuticle of appendages. This mechanotransduction provides the neuromuscular system with feedback for posture and locomotion. Owing to their diverse morphology and arrangement, CS can encode different strain directions. We used nano-computed tomography and finite-element analysis to investigate how different CS morphologies within one location—the femoral CS field of the leg in the fruit fly Drosophila —interact under load. By investigating the influence of CS substructures' material properties during simulated limb displacement with naturalistic forces, we could show that CS substructures (i.e. socket and collar) influence strain distribution throughout the whole CS field. Altered socket and collar elastic moduli resulted in 5% relative differences in displacement, and the artificial removal of all sockets caused differences greater than 20% in cap displacement. Apparently, CS sockets support the distribution of distal strain to more proximal CS, while collars alter CS displacement more locally. Harder sockets can increase or decrease CS displacement depending on sensor location. Furthermore, high-resolution imaging revealed that sockets are interconnected in subcuticular rows. In summary, the sensitivity of individual CS is dependent on the configuration of other CS and their substructures.