Stiffness gradient of the beetle penis facilitates propulsion in the spiraled female spermathecal duct

Mechanical properties of a female reproductive tract of a beetle and implications for penile penetration

Coevolution of male and female genitalia is widespread in animals. Nevertheless, few studies have examined the mechanics of genital interactions during mating. We characterized the mechanical properties of the elongated female genitalia, the spermathecal duct, of the small cassidine beetle, Cassida rubiginosa. The data were compared with the mechanical properties of the elongated male genitalia, the flagellum. We analysed the material distributions of the spermathecal duct using a microscopy technique, established a tensile test setup under a light microscope and conducted tensile tests. Diameter and tensile stiffness gradients were present along the spermathecal duct, but its Young's modulus and material distribution were more or less homogeneous. The results confirmed the hypothesis based on numerical simulations that the spermathecal duct is more rigid than the flagellum. In the study species, the penile penetration force is simply applied to the base of the hyper-elongated flagellum and conveyed along the flagellum to its tip. Considering this simple penetration mechanism, the relatively low flexibility of the spermathecal duct, compared to the flagellum, is likely to be essential for effective penetration of the flagellum.

Penetration mechanics of elongated female and male genitalia of earwigs

We unveiled the penile penetration mechanics of two earwig species, Echinosoma horridum, whose intromittent organ, termed virga, is extraordinarily long, and E. denticulatum, whose virga is conversely short. We characterised configuration, geometry, material and bending stiffness for both virga and spermatheca. The short virga of E. denticulatum has a material gradient with the stiffer base, whereas the long virga of E. horridum and the spermathecae of both species are homogeneously sclerotised. The long virga of E. horridum has a lower bending stiffness than the spermatheca. The virga of E. denticulatum is overall less flexible than the spermatheca. We compared our results to a previous study on the penetration mechanics of elongated beetle genitalia. Based on the comparison, we hypothesised that the lower stiffness of the male intromittent organ comparing to the corresponding female structure is a universal prerequisite for the penetration mechanics of the elongated intromittent organ in insects.

The metendosternite and penile flagellum: two unexplored character systems of pleasing fungus beetles (Coleoptera: Erotylidae)

Erotylidae (Cucujoidea) are currently divided into six subfamilies, which are regarded as monophyletic. However, there are doubts on the monophyly of lower ranked taxa within Erotylidae, including its most diverse tribe (Tritomini) and the highly diverse genus Mycotretus (Tritomini). The next steps in phylogenetic studies on these taxa rely on better sampling and on studies of unexplored character sets. Here, we conduct a comparative morphological study of the metendosternite and the penile flagellum within Erotylidae, with emphasis on Tritomini and Mycotretus, establishing a naming system for these structures. Representatives of 56 species belonging to all subfamilies of Erotylidae were analysed. A total of 17 genera and 45 species of Tritomini were selected, of which 21 species were Mycotretus. A total of 17 characters (eight of the metendosternite and nine of the penile flagellum) with potential phylogenetic value were recognized. Within Tritomini there is evidence of phylogenetic signal for the presence or absence of the metendosternal lamina at the generic level and above. On the other hand, the penile flagellum may be more informative for levels below the genus and morphological features of the flagellar ‘head’ seem to have phylogenetic signal for groups of potentially related species of Mycotretus.

Resilin Distribution and Sexual Dimorphism in the Midge Antenna and Their Influence on Frequency Sensitivity

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.

Functional significance of graded properties of insect cuticle supported by an evolutionary analysis

The exoskeleton of nearly all insects consists of a flexible core and a stiff shell. The transition between these two is often characterized by a gradual change in the stiffness. However, the functional significance of this stiffness gradient is unknown. Here by combining finite-element analysis and multi-objective optimization, we simulated the mechanical response of about 3000 unique gradients of the elastic modulus to normal contacts. We showed that materials with exponential gradients of the elastic modulus could achieve an optimal balance between the load-bearing capacity and resilience. This is very similar to the elastic modulus gradient observed in insect cuticle and, therefore, suggests cuticle adaptations to applied mechanical stresses; this is likely to facilitate the function of insect cuticle as a protective barrier. Our results further indicate that the relative thickness of compositionally different regions in insect cuticle is similar to the optimal estimation. We expect our findings to inform the design of engineered materials with improved mechanical performance.

Evolution of a hyper-complex intromittent organ in rove beetles – the endophallus of Xantholinini (Staphylinidae: Coleoptera)

Studies on the functional morphology and evolution of genitalia have been crucial to understanding sexual traits in speciation, reproductive isolation and sexual selection in Coleoptera and insects in general. However, the focus of investigation of the intromittent organ of beetles was largely confined to the sclerotized elements of the aedeagus, whereas the membranous structures of the endophallus (=internal sac) have often not been adequately considered. Using a micro-operating technique, we observed living male rove beetles and found five different types of endophallus eversion and related morphological modifications. Analysing genital data of a larger sample of Xantholinini, we could demonstrate that endophallus complexity and modifications tend to vary inversely with the median lobe (penis: intromittent organ). Our comparative morphological study, combined with a molecular phylogenetic analysis, suggests that endophallus spiralling occurring after endophallus eversion is an innovation in beetle evolution.

A simple, high-resolution, non-destructive method for determining the spatial gradient of the elastic modulus of insect cuticle

Nature has evolved structures with high load-carrying capacity and long-term durability. The principles underlying the functionality of such structures, if studied systematically, can inspire the design of more efficient engineering systems. An important step in this process is to characterize the material properties of the structure under investigation. However, direct mechanical measurements on small complex-shaped biological samples involve numerous technical challenges. To overcome these challenges, we developed a method for estimation of the elastic modulus of insect cuticle, the second most abundant biological composite in nature, through simple light microscopy. In brief, we established a quantitative link between the autofluorescence of different constituent materials of insect cuticle, and the resulting mechanical properties. This approach was verified using data on cuticular structures of three different insect species. The method presented in this study allows three-dimensional visualisation of the elastic modulus, which is impossible with any other available technique. This is especially important for precise finite-element modelling of cuticle, which is known to have spatially graded properties. Considering the simplicity, ease of implementation and high-resolution of the results, our method is a crucial step towards a better understanding of material-function relationships in insect cuticle, and can potentially be adapted for other graded biological materials.

Material stiffness variation in mosquito antennae

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 brevipalpis and Anopheles 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.

Fitness advantages of the biased use of paired laterally symmetrical penises in an insect

The evolution of laterality, that is the biased use of laterally paired, morphologically symmetrical organs, has attracted the interest of researchers from a variety of disciplines. It is, however, difficult to quantify the fitness benefits of laterality because many organs, such as human hands, possess multimodal functions. Males of the earwig Labidura riparia (Insecta: Dermaptera: Labiduridae) have morphologically similar laterally paired penises, only one of which is used for inseminating the female during a single copulation bout, and thus provide a rare opportunity to address how selection pressure may shape the evolution of population-level laterality. Our population studies revealed that in 10 populations, located at 2.23-43.3° north, the right penis is predominantly used for copulating (88.6%). A damaged penis was found in 23% of rare left-handers, suggesting that the left penis can function as a spare when the right one is damaged. By pairing L. riparia females with surgically manipulated males, we found that males forced to use the right penis outperformed left-handed males in copulation (the probability of establishing genital coupling during the 1-hr observation period: odds ratio [OR] of 3.50) and insemination (probability of transferring a detectable amount of sperm: OR of 2.94). This right-handed advantage may be due to the coiled morphology of the sperm storage organ with a right-facing opening. Thus, female genital morphology may play a significant role in the evolution of handedness and may have acted as a driving force to reduce penis number in related taxa.

Stiffness distribution in insect cuticle: a continuous or a discontinuous profile?

Insect cuticle is a biological composite with a high degree of complexity in terms of both architecture and material composition. Given the complex morphology of many insect body parts, finite-element (FE) models play an important role in the analysis and interpretation of biomechanical measurements, taken by either macroscopic or nanoscopic techniques. Many previous studies show that the interpretation of nanoindentation measurements of this layered composite material is very challenging. To develop accurate FE models, it is of particular interest to understand more about the variations in the stiffness through the thickness of the cuticle. Considering the difficulties of making direct measurements, in this study, we use the FE method to analyse previously published data and address this issue numerically. For this purpose, sets of continuous or discontinuous stiffness profiles through the thickness of the cuticle were mathematically described. The obtained profiles were assigned to models developed based on the cuticle of three insect species with different geometries and layer configurations. The models were then used to simulate the mechanical behaviour of insect cuticles subjected to nanoindentation experiments. Our results show that FE models with discontinuous exponential stiffness gradients along their thickness were able to predict the stress and deformation states in insect cuticle very well. Our results further suggest that, for more accurate measurements and interpretation of nanoindentation test data, the ratio of the indentation depth to cuticle thickness should be limited to 7% rather than the traditional '10% rule'. The results of this study thus might be useful to provide a deeper insight into the biomechanical consequences of the distinct material distribution in insect cuticle and also to form a basis for more realistic modelling of this complex natural composite.

Penetration mechanics of a beetle intromittent organ with bending stiffness gradient and a soft tip

Hyper-elongated structures and their penetration are widespread among insects, for example, intromittent organs, ovipositors, and piercing-sucking mouthparts. The penetration of thin structures with high aspect ratio without buckling and rupturing is mechanically very challenging. However, this problem is economically solved in nature, and the solutions might be helpful for, for example, in the development of harmless catheters. We focus on the penetration process of a hyper-elongated structure of a cassidine beetle intromittent organ, termed a flagellum. We applied a three-point bending test for the flagellum to measure its bending stiffness along the entire flagellum. We demonstrated the bending stiffness gradient, in which the basal half is relatively stiff and the apical half is softer, whose good performance during copulation had been previously numerically demonstrated. The stiffness gradient is the result of the flagellum shape, which is cylindrical and tapered toward the tip. Moreover, the curved tip comprises a harder outer curve and a softer inner curve. Considering the findings of preceding studies, the flagellum works in the following way: (i) the bending stiffness gradient supports the flagellum, easily fitting to a shape of a highly coiled spermathecal duct, (ii) the stiffness property of the very tip may make the tip tougher, and (iii) the curled tip and homogeneously cylindrical shape of the organ help the very tip to fit the shape of the spermathecal duct of the female. Our study shows that the apparently simple flagellum penetration is achieved with numerous elaborate mechanical adaptations.