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

Performance of biological food processing interfaces: Perspectives on the science of mollusc radula

The Mollusca comprises a diverse range of organisms, with the class Gastropoda alone boasting approximately 80 000 extant species. Their adaptability across various habitats is facilitated by the evolution of the radula, a key structure for food acquisition. The radula's composition and mechanical properties, including its chitinous membrane, teeth, and supporting structures, enable efficient food gathering and processing. Through adaptive tooth morphology and composition, an interplay between radular components is facilitated, which results in collective effects to withstand forces encountered during feeding and reduce structural failure, with the broad range of variations reflecting ecological niches. Furthermore, teeth consist of composite materials with sometimes high contents of iron, calcium, or silicon to reduce wear. During interaction with the food, the radula performs complex three-dimensional motions, challenging to document. Here, we provide a review on the morphology, the mechanical properties, the composition, and various other parameters that contribute to radular performance. Due to, e.g., the smallness of these structures, there are, however, limitations to radular research. However, numerical simulations and physical models tested on substrates offer avenues for further understanding radular function and performance during feeding. These studies not only advance our knowledge of molluscan biology and ecology but also provide inspirations for biomimetic design and further advances in materials engineering.

Mandible mechanical properties and composition of the larval Glossosoma boltoni (Trichoptera, Insecta)

Insect feeding structures, such as mandibles, interact with the ingesta (food or/and substrate) and can be adapted in morphology, composition of material and mechanical properties. The foraging on abrasive ingesta, as on algae covering rocks, is particularly challenging because the mandibles will be prone to wear and structural failure, thus suggesting the presence of mandibular adaptations to accompany this feeding behavior. Adaptations to this are well studied in the mouthparts of molluscs and sea urchins, but for insects there are large gaps in our knowledge. In this study, we investigated the mandibles of a grazing insect, the larvae of the trichopteran Glossosoma boltoni. Using scanning electron microscopy, wear was documented on the mandibles. The highest degree was identified on the medial surface of the sharp mandible tip. Using nanoindentation, the mechanical properties, such as hardness and Young's modulus, of the medial and lateral mandible cuticles were tested. We found, that the medial cuticle of the tip was significantly softer and more flexible than the lateral one. These findings indicate that a self-sharpening mechanism is present in the mandibles of this species, since the softer medial cuticle is probably abraded faster than the harder lateral one, leading to sharp mandible tips. To investigate the origins of these properties, we visualized the degree of tanning by confocal laser scanning microscopy. The autofluorescence signal related to the mechanical property gradients. The presence of transition and alkaline earth metals by energy dispersive X-ray spectroscopy was also tested. We found Ca, Cl, Cu, Fe, K, Mg, Mn, P, S, Si, and Zn in the cuticle, but the content was very low and did not correlate with the mechanical property values.

Elemental composition and material properties of radular teeth in the heterobranch snail Gastropteron rubrum (Mollusca, Gastropoda, Cephalaspidea) foraging on hard organisms

The molluscan feeding structure is the radula, a chitinous membrane with teeth, which are highly adapted to the food and the substrate to which the food is attached. In Polyplacophora and Patellogastropoda, the handling of hard ingesta can be facilitated by high content of chemical compounds containing Fe or Si in the tooth cusps. Other taxa, however, possess teeth that are less mineralized, even though animals have to avoid structural failure or high wear during feeding as well. Here, we investigated the gastropod Gastropteron rubrum, feeding on hard Foraminifera, diatoms and Porifera. Tooth morphologies and wear were documented by scanning electron microscopy and their mechanical properties were tested by nanoindentation. We determined that gradients of hard- and stiffness run along each tooth, decreasing from cusp to basis. We also found that inner lateral teeth were harder and stiffer than the outer ones. These findings allowed us to propose hypotheses about the radula-ingesta interaction. In search for the origins of the gradients, teeth were visualized using confocal laser scanning microscopy, to determine the degree of tanning, and analyzed with energy-dispersive X-ray spectroscopy, to test the elemental composition. We found that the mechanical gradients did not have their origins in the elemental content, as the teeth did not contain high proportions of metals or other minerals. This indicates that their origin might be the degree of tanning. However, in the tooth surfaces that interact with the ingesta high Si and Ca contents were determined, which is likely an adaptation to reduce wear.

Coping with abrasive food: diverging composition of radular teeth in two Porifera-consuming nudibranch species (Mollusca, Gastropoda)

Molluscs forage with their radula, a chitinous membrane with teeth. Adaptations to hard or abrasive ingesta were well studied in Polyplacophora and Patellogastropoda, but for other taxa there are large gaps in knowledge. Here, we investigated the nudibranch gastropods Felimare picta and Doris pseudoargus, both of which feed on Porifera. Tooth morphologies were documented by scanning electron microscopy, and mechanical properties were tested by nanoindentation. We found that these parameters are rather similar in both species, indicating that teeth are similar in their function. To study the composition, teeth were visualized using confocal laser scanning microscopy (CLSM), to determine the degree of tanning, and analysed with energy-dispersive X-ray spectroscopy, to test the elemental composition. The emitted autofluorescence signal and the inorganic content differed between the species. This was especially prominent when studying the inner and outer tooth surfaces (leading and trailing edges). In F. picta, we detected high proportions of Si, whereas teeth of D. pseudoargus contained high amounts of Ca, which influenced the autofluorescence signal in CLSM. Employing nanoindentation, we determined high Young's modulus and hardness values for the leading edges of teeth, which relate to the Si and Ca content. This highlights that teeth with a similar morphology and mechanical properties can be mechanically enhanced via different chemical pathways in Nudibranchia.

Material gradients in gastropod radulae and their biomechanical significance: a combined approach on the paludomid Lavigeria grandis

The radula, a chitinous membrane spiked with teeth, is the molluscan autapomorphy for the gathering and processing of food. The teeth, as actual interfaces between the organism and the ingesta, act as load transmitting regions and have to withstand high stresses during foraging - without structural failure or high degrees of wear. Mechanisms contributing to this were studied previously in paludomid gastropods from Lake Tanganyika. For some species, gradients in hardness and Young's modulus along the teeth were detected, enabling the bending and relying of teeth onto the next row, distributing the stresses more equally. The here presented study on one of them - Lavigeria grandis - aims at shedding light on the origin of these functional gradients. The mechanical properties were identified by nanoindentation technique and compared to the elemental composition, determined by elemental dispersive X-ray spectroscopy (EDX, EDS). This was done for the complete radular (mature and immature tooth rows), resulting in overall 236 EDX and 700 nanoindentation measurements. Even though teeth showed regional differences in elemental composition, we could not correlate the mechanical gradients with the elemental proportions. By applying confocal laser scanning microscopy (CLSM), we were finally able to relate the mechanical properties with the degree of tanning. CLSM is a common technique used on arthropod cuticle, but was never applied on radular teeth before. In general, we found that nanoindentation and CLSM techniques complement one another, as for example, CLSM is capable of revealing heterogeneities in material or micro-gradients, which leads to a better understanding of the functionalities of biological materials and structures.

Mechanical properties, degree of sclerotisation and elemental composition of the gastric mill in the red swamp crayfish Procambarus clarkii (Decapoda, Crustacea)

The gastric mill of Decapoda is a unique feature, which comprises teeth, stabilizing ossicles, and particle sorting setae. Involved in the fragmentation and sorting of the food, this structure serves as interface between the organism and its environment. As material properties complement morphology and hold information about function and trophic preferences, we here provide a basis for more comparative research on gastric mills. For gastric mill components of the adult red swamp crayfish Procambarus clarkii, we studied (a) the micro-structure via scanning electron microscopy, (b) the elemental composition by energy-dispersive X-ray spectroscopy, (c) the heterogeneities in material properties and degree of tanning (autofluorescence) by confocal laser scanning microscopy, and (d) the mechanical properties hardness and elasticity by nanoindentation technique. The morphology and micro-structure were previously described for this species, but the mechanical properties and the autofluorescence were not studied before. As epicuticle and exocuticle could be analyzed individually, material property gradients, with values decreasing from the interacting surface towards interior, could be determined. Finally, we were able to relate the mechanical property data with the elemental composition and the degree of tanning. We found that the epicuticle of the teeth is among the hardest and stiffest biological materials in invertebrates due to the incorporations of high proportions of silicon.

Exploration of the homology among the muscles associated with the female genitalia of the three suborders of Psocodea (Insecta)

By using μCT technology, we reconstructed 3D models of the female genital structures and associated muscles of seven species from three suborders of Psocodea (free-living species only, formerly known as independent insect order "Psocoptera"). The homology of the female genital structures and associated muscles of different species is discussed. A total of 21 muscle groups were observed, and except for one muscle, all were homologized throughout the order. Moreover, some of the homologous muscles could be identified confidently in holometabolan insects. Using the muscles as landmarks, we discuss the homology of the ovipositor valves between Psocodea and other neopteran insects. Most importantly, the ovipositor of the suborder Trogiomorpha was identified to consist of the well-developed external valve (V3) plus a remnant of the dorsal valve (V2). We also examined the phylogenetic information included in the female genital muscles and found that certain muscles provide useful information and support deeper nodes (e.g., monophyly of the suborder Psocomorpha). The present study of female genital muscles not only helps us to better understand the phylogeny of Psocodea but also provides a solid foundation for research on muscle evolution.