Selection for functional performance in the evolution of cuticle hardening mechanisms in insects

Mandible composition and properties in two selected praying mantises (Insecta, Mantodea)

Insects process their food with their cuticle-based mouthparts. These feeding structures reflect their diversity and can, in some cases, showcase adaptations in material composition, mechanical properties, and shape to suit their specific dietary preferences. To pave the way to deeply understand the interaction between mouthparts and food and to determine potential adaptations of the structures to the food, this study focuses on the mandibles of two praying mantis species. Gongylus gongylodes feeds mainly on Diptera, and Sphodromantis lineola forages on larger prey. Employing scanning electron microscopy, the mandibular morphologies were analyzed. The degree of the cuticle tanning was tested using confocal laser scanning microscopy. Furthermore, the contents of transition and alkaline earth metals in the mandible cuticle were studied using energy-dispersive X-ray spectroscopy and the mechanical properties tested by nanoindentation. We found that S. lineola mandibles show pronounced gradients of Young's modulus and hardness from the basis to the tip, which might be an adaptation against high stresses during biting and chewing. G. gongylodes, in contrast, did not show pronounced gradients, which may indicate that there is less stress involved in feeding-necessary to test in future studies. The mechanical properties of manidibles in both species are related to the degree of cuticle tanning but also positively correlate with the content of magnesium. These findings enrich our understanding of insect cuticle biology but also present new sets of data on praying mantis structures.

Interspecific and intraspecific variation in grasshopper (Orthoptera: Acrididea) molar form: implications for dietary ecology

Like many mammals, grasshoppers (infraorder Acrididea) chew using molariform structures. Despite decades of research on mammals, little is known about grasshopper molar form and how it relates to grasshopper feeding biomechanics, diet, dietary ecology and evolution. Here, we develop a method for quantifying molar form and apply it to two species of distantly related grasshoppers with different diets (Phymateus saxosus, seven females; Valanga nigricornis, seven females, 11 males). We show that there are quantifiable differences in molar form, potentially related to diet. There are some differences in molar shape between left and right molars in both species and sexes, and significant differences in molar size, potentially due to scaling. Like in mammals, molar wear can cause large differences in molar shape. Species differences in molar shape did not match what was expected based on mammalian molar functional morphology. Dental topographic analysis is a promising new avenue for quantifying molar form in grasshoppers and a distinct advantage over traditional two-dimensional microscopy methods, and promises to reveal much about the biology, biomechanics and evolution of Acrididea.

The Fast Evolution of the Stenobothrini Grasshoppers (Orthoptera, Acrididae, and Gomphocerinae) Revealed by an Analysis of the Control Region of mtDNA, with an Emphasis on the Stenobothrus eurasius Group

The two cryptic grasshopper species of the genus Stenobothrus, S. eurasius and S. hyalosuperficies, demonstrate different acoustic behavior despite a strong similarity in morphology. A hybridization between these species is possible in the contact zone; however, there are little molecular data about the relationships of these species. The analysis of the mtDNA control region (CR) reveals that haplotypes of S. hyalosuperficies have more in common with the more distant Stenobothrus species than with the closely related S. eurasius. In the contact zone, S. eurasius has mt-haplotypes shared with S. hyalosuperficies, which might indicate an introgression of mtDNA from S. hyalosuperficies to the S. eurasius gene pool. We also analyze the structure and evolutionary rate of the mtDNA CR for the Stenobothrus genus and estimate the time of divergence of the species within the genus. The phylogenetic tree of the tribe Stenobothrini reconstructed with either the CR or COI gave the same four groups. The phylogenetic tree of the Stenobothrus genus has a star-like topology with each mtDNA haplotype found in any analyzed species, except S. eurasius, which forms a separate branch. The maximum degree of incomplete lineage sorting can demonstrate either ancestral polymorphism or introgression.

Mandible elemental composition and mechanical properties from distinct castes of the leafcutter ant Atta laevigata (Attini; Formicidae)

Leafcutter ant colonies are divided into castes with the individuals performing different tasks, based mostly on size. With the mandibles, the small minims care for the brood or the fungus, whereas the larger minors and mediae cut and transport plant material, with the ant size positively related to the material size. The mechanical properties and composition of the mandible cuticle have been previously tested in the soldiers as the largest caste, revealing that the cutting edges contained high contents of the cross-linking transition metal zinc (Zn). With regard to the smaller castes, no data are present. To study how the mandible size and function relates to its mechanical properties, we here tested the mandibles of minims, minors and mediae by nanoindentation. We found that the hardness (H) and Young's modulus (E) values increased with increasing ant size and that the mandible cutting edges in each caste have the highest H- and E-values. To gain insight into the origins of these properties, we characterized the elemental composition by energy-dispersive X-ray analysis, revealing that minors and mediae possessed higher content of Zn in the cutting edges in contrast to the minims containing significantly less Zn. This shows, that Zn content relates to higher mechanical property values. Additionally, it shows that all of these parameters can differ within a single species.

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.

Comparative transcriptome analysis to unveil genes affecting the host cuticle destruction in Metarhizium rileyi

Insect pathogenic fungi, also known as entomopathogenic fungi, are one of the largest insect pathogenic microorganism communities, represented by Beauveria spp. and Metarhizium spp. Entomopathogenic fungi have been proved to be a great substitute for chemical pesticide in agriculture. In fact, a lot of functional genes were also already characterized in entomopathogenic fungi, but more depth of exploration is still needed to reveal their complicated pathogenic mechanism to insects. Metarhizium rileyi (Nomuraea rileyi) is a great potential biocontrol fungus that can parasitize more than 40 distinct species (mainly Lepidoptera: Noctuidae) to cause large-scale infectious diseases within insect population. In this study, a comparative analysis of transcriptome profile was performed with topical inoculation and hemolymph injection to character the infectious pattern of M. rileyi. Appressorium and multiple hydrolases are indispensable constituents to break the insect host primary cuticle defense in entomopathogenic fungi. Within our transcriptome data, numerous transcripts related to destruction of insect cuticle rather growth regulations were obtained. Most importantly, some unreported ribosomal protein genes and novel unannotated protein (hypothetical protein) genes were proved to participate in the course of pathogenic regulation. Our current data provide a higher efficiency gene library for virulence factors screen in M. rileyi, and this library may be also useful for furnishing valuable information on entomopathogenic fungal pathogenic mechanisms to host.