The cuticle exoskeleton plays a key role in facilitating the evolutionary success of insects. Since the mid of the last century, many different biomechanical properties of exoskeletons have been investigated, always utilizing the most sophisticated scientific methods available at the time. So far, information on the biomechanical properties of cuticle seems to be as diverse as the methods used to measure them. As a consequence, insect cuticle is often considered to exhibit the most complex and diverse biomechanical properties of any biological material. However, it remains unclear which role the respective measurement methods and sample treatments used in previous studies play in supporting this claim. This review provides a broad overview of examination techniques used to study biomechanical properties of insect exoskeletons and discusses their respective advantages and disadvantages in describing the properties of a complex material such as cuticle. Our meta-analysis of the present data confirms significant effects of the respective measurement methods, sample treatments and body parts on the obtained mechanical properties. Based on our findings, we highlight research gaps and point out important factors which should be taken into account in future studies on insect cuticle.

The antennae of mosquitoes are model systems for acoustic sensation, in that they obey general principles for sound detection, using both active feedback mechanisms and passive structural adaptations. However, the biomechanical aspect of the antennal structure is much less understood than the mechano-electrical transduction. Using confocal laser scanning microscopy, we measured the fluorescent properties of the antennae of two species of mosquito—Toxorhynchites brevipalpisandAnopheles arabiensis—and, noting that fluorescence is correlated with material stiffness, we found that the structure of the antenna is not a simple beam of homogeneous material, but is in fact a rather more complex structure with spatially distributed discrete changes in material properties. These present as bands or rings of different material in each subunit of the antenna, which repeat along its length. While these structures may simply be required for structural robustness of the antennae, we found that in FEM simulation, these banded structures can strongly affect the resonant frequencies of cantilever-beam systems, and therefore taken together our results suggest that modulating the material properties along the length of the antenna could constitute an additional mechanism for resonant tuning in these species.

Campaniform sensilla (CS) are mechanosensors embedded in the cuticle of insects. They are often found at locations near the joints of leg segments. On legs, CS are generally considered to respond directionally to cuticle bending during legged locomotion. It is currently unclear how CS locations affect strain levels at the CS, but this information is crucial for understanding how CS respond to stimuli. Here we present a parametric finite element model of the femoral CS field for Drosophila hind legs with 12 general and seven CS-specific parameters each. This model allows testing how changes in CS location, orientation and material property affect strain levels at each CS. We used experimentally acquired kinematic data and computed ground reaction forces to simulate in vivo -like forward stepping. The displacements found in this study at the physiological CS field location near the trochanter–femur joint are smaller than those necessary for conformation changes of ion channels involved in signal elicitation. Also, variation of material properties of the CS had little influence on displacement magnitudes at the CS cap where the sensory neuron attaches. Thus, our results indicate that ground reaction forces alone are unlikely to serve CS field activation during forward walking.

Small-scale bioacoustic sensors, such as antennae in insects, are often considered, biomechanically, to be not much more than the sum of their basic geometric features. Therefore, little is known about the fine structure and material properties of these sensors—even less so about the degree to which the well-known sexual dimorphism of the insect antenna structure affects those properties. By using confocal laser scanning microscopy (CLSM), we determined material composition patterns and estimated distribution of stiffer and softer materials in the antennae of males and females of the non-biting midge Chironomus riparius. Using finite element modelling (FEM), we also have evidence that the differences in composition of these antennae can influence their mechanical responses. This study points to the possibility that modulating the elastic and viscoelastic properties along the length of the antennae can affect resonant characteristics beyond those expected of simple mass-on-a-spring systems—in this case, a simple banded structure can change the antennal frequency sensitivity. This constitutes a simple principle that, now demonstrated in another Dipteran group, could be widespread in insects to improve various passive and active sensory performances.

Campaniform sensilla (CS) are mechanosensors embedded within the cuticle of many insects at key locations such as nearby leg segment joints or halters. CS located at leg segments were found to respond to cuticle bending which can be induced by walking or jumping movements or by the underlying tensile forces of the muscles. For Drosophila it is unclear how CS location and material property variation influence stress levels within and around CS but this information is crucial to understand how flies might use CS input to adjust walking behaviour. Here, we designed a parametric model of the femoral CS field for Drosophila to allow for a systematic testing of the influence of CS location, orientation and material property variation on stress levels. The model consists of 7 changeable parameters per CS and 12 which can be changed for the CS field. Simulations of leg bending are in line with general beam bending theory: At the specific proximal CS field location nearby the trochantero-femoral leg joint, displacements are smaller than distal, while stresses are higher. When changing CS location towards more distal leg parts the situation changes towards more displacement and less stress. Changes in material property values for CS substructures or whole CS fields have a very low influence on stress or displacement magnitudes (regarding curve shape and amplitude) at the CS caps to which the nerve cells attach. Taken together, our simulation results indicate that for CS fields located at proximal leg parts, the displacements induced by other sources such as muscle tensile forces might be more relevant stimuli than the overall leg bending induced by typical locomotion scenarios. Future parametric finite element models should contain experimentally validated information on the anisotropic and viscoelastic properties of materials contained in this sensory system to further our understanding of CS activation patterns.

During their development, phasmid eggs with their long developmental periods face numerous biotic and abiotic challenges. These insect eggs are unique for their thick egg capsule and a specialised escape structure called the operculum/lid. This operculum needs to be easy to open from the inside, yet must not be a ‘weak spot’ for potential predators. One possible solution to this biomechanical dilemma could be a change in the operculum's mechanical properties over time. To investigate this hypothesis, we analysed the effect of age and storage conditions on the biomechanical properties of the egg and the operculum. We also performed high-resolution X-ray microscopy (XRM) and synchrotron studies to analyse the ultrastructure of the eggshell and operculum. Our results show that the eggs of Carausius morosus were able to resist a compressive force of up to 2 N. During maturation, the force required to open the operculum was significantly reduced from 0.14 N to 0.09 N. The properties of the eggshell itself, however, were not affected by humidity and did not change during maturation. Interestingly, the egg properties were affected by the mother's age. Our results thus indicate that the operculum is not a primary fracture site for externally applied stress; however, structural changes in the operculum during the developmental process of the embryo facilitate the hatching process.