Cuticular colour reflects underlying architecture and is affected by a limiting resource

Microbial bases of herbivory in beetles

The ecological radiation of herbivorous beetles is among the most successful in the animal kingdom. It coincided with the rise and diversification of flowering plants, requiring beetles to adapt to a nutritionally imbalanced diet enriched in complex polysaccharides and toxic secondary metabolites. In this review, we explore how beetles overcame these challenges by coopting microbial genes, enzymes, and metabolites, through both horizontal gene transfer (HGT) and symbiosis. Recent efforts revealed the functional convergence governing both processes and the unique ways in which microbes continue to shape beetle digestion, development, and defense. The development of genetic and experimental tools across a diverse set of study systems has provided valuable mechanistic insights into how microbes spurred metabolic innovation and facilitated an herbivorous transition in beetles.

Cuticular nitrogen economy during development in the cockroach Cryptocercus punctulatus and the termite Neotermes jouteli

The role of nitrogen during insect development and reproduction is key in the success of a species, and is of primary importance in wood feeding taxa. Based on comparison of xylophagous, one-piece termites to the termite sister group, subsocial wood-feeding cockroaches in the genus Cryptocercus, it has been proposed that the evolution of termite eusociality involved a fundamental shift in nitrogen allocation strategies. Cryptocercus exhibits a nitrogen storage economy, with individuals gradually increasing in size and cuticular density over a years-long developmental period. Termites, however, remain in a juvenilized morphotype with minimal investment into cuticle, suggesting that nitrogen is conserved and circulated according to the needs of the colony via behaviors such as trophallaxis and cannibalism. We examined the nitrogen economy of Cryptocercus punctulatus and the dampwood termite Neotermes jouteli, focusing on cuticular nitrogen investment during development and exuvial nitrogen losses resulting from molting. Cryptocercus progressively changes from a pale, thin, soft cuticle at hatch to a dark, thick, heavily sclerotized cuticle in adults; increases in N/mg cuticle and the quantity of cuticular catecholamines are correlated with these ontogenetic color changes. There were significant differences in the nitrogen content of two successive age classes of early stage juveniles and in their discarded exuvia at molt. Soldier and alate castes of N. jouteli exhibited the highest sclerotization/melanization indices; pseudergates had levels comparable to those measured in Class I (3rd and 4th instar) juveniles of C. punctulatus. Exuvia of N. jouteli contained 0.19 μgN/mg, while exuvia of approximately two- and three-year-old C. punctulatus had 72.9 and 82.6 μgN/mg, respectively. Our data support the hypothesis that the evolution of termite eusociality from subsocial cockroach ancestors was rooted in chronic fitness limitations imposed by their low nitrogen diet.

Polarization and reflectance are linked to climate, size and mechanistic constraints in a group of scarab beetles

Beetles exhibit an extraordinary diversity of brilliant and colourful appearances and optical effects invisible to humans. Their underlying mechanisms have received some attention, but we know little about the ecological variables driving their evolution. Here we investigated environmental correlates of reflectivity and circular polarization in a group of optically diverse beetles (Scarabaeidae-Rutelinae). We quantified the optical properties of 261 specimens representing 46 species using spectrophotometry and calibrated photographs. Then, we examined associations between these properties and environmental variables such as temperature, humidity and vegetation cover, controlling for body size and phylogenetic relatedness. Our results showed larger beetles have higher visible reflectivity in drier environments. Unexpectedly, near-infrared (NIR) reflectivity was not correlated with ecological variables. However, we found a correlation between humidity and polarization (chiral nanostructures). We identified trade-offs between optical properties: beetles without polarization-associated nanostructures had higher NIR reflectivity. By contrast, visible reflectivity was negatively correlated with the accumulation of pigments such as melanin. Our study highlights the value of a macroecological approach for testing alternative hypotheses to explain the diversity of optical effects in beetles and to understand the link between structure and function.

Resource-based trade-offs and the adaptive significance of seasonal plasticity in butterfly wing melanism

Phenotypic plasticity is the ability of an organism to alter its phenotype in response to environmental cues. This can be adaptive if the cues are reliable predictors of impending conditions and the alterations enhance the organism's ability to capitalize on those conditions. However, since traits do not exist in isolation but as part of larger interdependent systems of traits (phenotypic integration), trade-offs between correlated plastic traits can make phenotypic plasticity non- or maladaptive. We examine this problem in the seasonally plastic wing melanism of a pierid (Order Lepidoptera, Family Pieridae) butterfly, Pieris rapae L. Several wing pattern traits are more melanized in colder than in warmer seasons, resulting in effective thermoregulation through solar absorption. However, other wing pattern traits, the spots, are less melanized during colder seasons than in warmer seasons. Although spot plasticity may be adaptive, reduced melanism of these spots could also be explained by resource-based trade-offs. Theory predicts that traits involved in resource-based trade-offs will be positively correlated when variation among individuals in resource acquisition is greater than variation among individuals in resource allocation strategies, and negatively correlated when variation in allocation is greater than variation in acquisition. Using data from both field studies and laboratory studies that manipulate dietary tyrosine, a melanin precursor, we show that when allocation to thermoregulatory melanism (ventral hindwing, and basal dorsal fore- and hindwing "shading") varies substantially this trait is negatively correlated with spot melanism. However, when there is less variation in allocation to thermoregulatory melanism we find these traits to be positively correlated; these findings are consistent with the resource-based trade-off hypothesis, which may provide a non- or maladaptive hypothesis to explain spot plasticity. We also show that increased dietary tyrosine results in increased spot melanism under some conditions, supporting the more general idea that melanism may involve resource-based costs.

Endosymbiosis allows Sitophilus oryzae to persist in dry conditions

Insects frequently associate with intracellular microbial symbionts (endosymbionts) that enhance their ability to cope with challenging environmental conditions. Endosymbioses with cuticle-enhancing microbes have been reported in several beetle families. However, the ecological relevance of these associations has seldom been demonstrated, particularly in the context of dry environments where high cuticle quality can reduce water loss. Thus, we investigated how cuticle-enhancing symbionts of the rice-weevil, Sitophilus oryzae contribute to desiccation resistance. We exposed symbiotic and symbiont-free (aposymbiotic) beetles to long-term stressful (47% RH) or relaxed (60% RH) humidity conditions and measured population growth. We found that symbiont presence benefits host fitness especially under dry conditions, enabling symbiotic beetles to increase their population size by over 33-fold within 3 months, while aposymbiotic beetles fail to increase in numbers beyond the starting population in the same conditions. To understand the mechanisms underlying this drastic effect, we compared beetle size and body water content and found that endosymbionts confer bigger body size and higher body water content. While chemical analyses revealed no significant differences in composition and quantity of cuticular hydrocarbons after long-term exposure to desiccation stress, symbiotic beetles lost water at a proportionally slower rate than did their aposymbiotic counterparts. We posit that the desiccation resistance and higher fitness observed in symbiotic beetles under dry conditions is due to their symbiont-enhanced thicker cuticle, which provides protection against cuticular transpiration. Thus, we demonstrate that the cuticle enhancing symbiosis of Sitophilus oryzae confers a fitness benefit under drought stress, an ecologically relevant condition for grain pest beetles. This benefit likely extends to many other systems where symbiont-mediated cuticle synthesis has been identified, including taxa spanning beetles and ants that occupy different ecological niches.

Triazine Herbicide and NPK Fertilizer Exposure: Accumulation of Heavy Metals and Rare Earth Elements, Effects on Cuticle Melanization, and Immunocompetence in the Model Species Tenebrio molitor

The increasing use of agrochemicals, including fertilizers and herbicides, has led to worrying metal contamination of soils and waters and raises serious questions about the effects of their transfer to different levels of the trophic web. Accumulation and biomagnification of essential (K, Na, Mg, Zn, Ca), nonessential (Sr, Hg, Rb, Ba, Se, Cd, Cr, Pb, As), and rare earth elements (REEs) were investigated in newly emerged adults of Tenebrio molitor exposed to field-admitted concentrations of a metribuzin-based herbicide and an NPK blend fertilizer. Chemical analyses were performed using inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) supported by unsupervised pattern recognition techniques. Physiological parameters such as cuticle melanization, cellular (circulating hemocytes), and humoral (phenoloxidase enzyme activity) immune responses and mass loss were tested as exposure markers in both sexes. The results showed that NPK fertilizer application is the main cause of REE accumulation in beetles over time, besides toxic elements (Sr, Hg, Cr, Rb, Ba, Ni, Al, V, U) also present in the herbicide-treated beetles. The biomagnification of Cu and Zn suggested a high potential for food web transfer in agroecosystems. Gender differences in element concentrations suggested that males and females differ in element uptake and excretion. Differences in phenotypic traits show that exposure affects metabolic pathways involving sequestration and detoxification during the transition phase from immature-to-mature beetles, triggering a redistribution of resources between sexual maturation and immune responses. Our findings highlight the importance of setting limits for metals and REEs in herbicides and fertilizers to avoid adverse effects on species that provide ecosystem services and contribute to soil health in agroecosystems.

Nutritional symbionts enhance structural defence against predation and fungal infection in a grain pest beetle

Many insects benefit from bacterial symbionts that provide essential nutrients and thereby extend the hosts' adaptive potential and their ability to cope with challenging environments. However, the implications of nutritional symbioses for the hosts' defence against natural enemies remain largely unstudied. Here, we investigated whether the cuticle-enhancing nutritional symbiosis of the saw-toothed grain beetle Oryzaephilus surinamensis confers protection against predation and fungal infection. We exposed age-defined symbiotic and symbiont-depleted (aposymbiotic) beetles to two antagonists that must actively penetrate the cuticle for a successful attack: wolf spiders (Lycosidae) and the fungal entomopathogen Beauveria bassiana. While young beetles suffered from high predation and fungal infection rates regardless of symbiont presence, symbiotic beetles were able to escape this period of vulnerability and reach high survival probabilities significantly faster than aposymbiotic beetles. To understand the mechanistic basis of these differences, we conducted a time-series analysis of cuticle development in symbiotic and aposymbiotic beetles by measuring cuticular melanisation and thickness. The results reveal that the symbionts accelerate their host's cuticle formation and thereby enable it to quickly reach a cuticle quality threshold that confers structural protection against predation and fungal infection. Considering the widespread occurrence of cuticle enhancement via symbiont-mediated tyrosine supplementation in beetles and other insects, our findings demonstrate how nutritional symbioses can have important ecological implications reaching beyond the immediate nutrient-provisioning benefits.

Color lightness of velvet ants (Hymenoptera: Mutillidae) follows an environmental gradient

Color traits are highly influenced by environmental conditions along the distributional range of many species. Studies on the variation of animal coloration across different geographic gradients are, therefore, fundamental for a better understanding of the ecological and evolutionary processes that shape color variation. Here, we address whether color lightness in velvet ants (Hymenoptera: Mutillidae) responds to latitudinal gradients and bioclimatic variations, testing three ecogeographic rules: The Thermal melanism hypothesis; the Photoprotection hypothesis; and Gloger's rule. We test these hypotheses across the New World. We used photographs of 482 specimens (n = 142 species) of female mutillid wasps and extracted data on color lightness (V). We analyzed whether variation in color is determined by bioclimatic factors, using Phylogenetic Generalized Least Square analysis. Our explanatory variables were temperature, ultraviolet radiation, humidity, and forest indicators. Our results were consistent with the Photoprotection hypothesis and Gloger's rule. Species with darker coloration occupied habitats with more vegetation, higher humidity, and UV-B radiation. However, our results refute one of the initial hypotheses suggesting that mutillids do not respond to the predictions of the Thermal melanism hypothesis. The results presented here provide the first evidence that abiotic components of the environment can act as ecological filters and as selective forces driving the body coloration of velvet ants. Finally, we suggest that studies using animals with melanin-based colors as a model for mimetic and aposematic coloration hypotheses consider that this coloration may also be under the influence of climatic factors and not only predators.

Immune function differences between two color morphs of the red palm weevil Rhynchophorus ferrugineus (Coleoptera: Curculionidae) at different life stages

Several studies demonstrated that in insects cuticle melanism is interrelated with pathogen resistance, as melanin-based coloration and innate immunity possess similar physiological pathways. For some insects, higher pathogen resistance was observed in darker individuals than in individuals with lighter cuticular coloration. Here, we investigated the difference in immune response between two color morphs (black and red) and between the life stages (pupa and adult) of the red palm weevil Rhynchophorus ferrugineus (Coleoptera: Curculionidae). Here in this study, cuticle thickness, microbial test (antimicrobial activity, phenoloxidase activity, and hemocyte density), and immune-related gene expression were evaluated at different stages of RPW. Study results revealed that cuticle thickness of black phenotype was thicker than red phenotype at old-pupa stage, while no significant difference found at adult stage. These results may relate to the development processes of epidermis in different stages of RPW. The results of antimicrobial activity, phenoloxidase (PO) activity, and hemocyte density analyses showed that adults with a red phenotype had stronger pathogen resistance than those with a black phenotype. In addition to antimicrobial activity and PO activity, we tested relative gene expression in the fat body of old pupae. The results of hemolymph antimicrobial analysis showed that old pupae with a red phenotype were significantly different from those with a black phenotype at 12 hr after Staphylococcus aureus injection, suggesting that red phenotype pupae were more sensitive to S. aureus. Examination of gene expression in the fat body also revealed that the red phenotype had a higher immune response than the black phenotype. Our results were inconsistent with the previous conclusion that dark insects had increased immune function, suggesting that the relationship between cuticle pigmentation and immune function in insects was not a direct link. Additional possible factors that are associated with the immune response, such as life-history, developmental, physiological factors also need to be considered.

The evolution of insect body coloration under changing climates

Insects have been influential models in research on color variation, its evolutionary drivers and the mechanistic basis of such variation. More recently, several studies have indicated that insect color is responding to rapid climate change. However, it remains challenging to ascertain drivers of color variation among populations and species, and across space and time, as multiple biotic and abiotic factors can interact and mediate color change. Here, we describe some of the challenges and recent advances made in this field. First, we outline the main alternative hypotheses that exist for insect color variation in relation to climatic factors. Second, we review the existing evidence for contemporary adaptive evolution of insect color in response to climate change and then discuss factors that can promote or hinder the evolution of color in response to climate change. Finally, we propose future directions and highlight gaps in this research field. Pigments and structures producing insect color can vary concurrently or independently, and may evolve at different rates, with poorly understood effects on gene frequencies and fitness. Disentangling multiple competing hypotheses explaining insect coloration should be key to assign color variation as an evolutionary response to climate change.

Benefits of insect colours: a review from social insect studies

Insect colours assist in body protection, signalling, and physiological adaptations. Colours also convey multiple channels of information. These channels are valuable for species identification, distinguishing individual quality, and revealing ecological or evolutionary aspects of animals' life. During recent years, the emerging interest in colour research has been raised in social hymenopterans such as ants, wasps, and bees. These insects provide important ecosystem services and many of those are model research organisms. Here we review benefits that various colour types give to social insects, summarize practical applications, and highlight further directions. Ants might use colours principally for camouflage, however the evolutionary function of colour in ants needs more attention; in case of melanin colouration there is evidence for its interrelation with thermoregulation and pathogen resistance. Colours in wasps and bees have confirmed linkages to thermoregulation, which is increasingly important in face of global climate change. Besides wasps use colours for various types of signalling. Colour variations of well chemically defended social insects are the mimetic model for unprotected organisms. Despite recent progress in molecular identification of species, colour variations are still widely in use for species identification. Therefore, further studies on variability is encouraged. Being closely interconnected with physiological and biochemical processes, insect colouration is a great source for finding new ecological indicators and biomarkers. Due to novel digital imaging techniques, software, and artificial intelligence there are emerging possibilities for new advances in this topic. Further colour research in social insects should consider specific features of sociality.

Gene functions in adult cuticle pigmentation of the yellow mealworm, Tenebrio molitor

In many arthropod species including insects, the cuticle tanning pathway for both pigmentation and sclerotization begins with tyrosine and is responsible for production of both melanin- and quinoid-type pigments, some of which are major pigments for body coloration. In this study we identified and cloned cDNAs of the yellow mealworm, Tenebrio molitor, encoding seven key enzymes involved in this pathway including tyrosine hydroxylase (TmTH), DOPA decarboxylase (TmDDC), laccase 2 (TmLac2), Yellow-y (TmY-y), arylalkylamine N-acetyltransferase (TmAANAT1), aspartate 1-decarboxylase (TmADC) and N-β-alanyldopamine synthase (Tmebony). Expression profiles of these genes during development were analyzed by real-time PCR, revealing development-specific patterns of expression. Loss of function mediated by RNAi of either 1) TmTH or TmLac2, 2) TmDDC or TmY-y, and 3) TmAANAT1, TmADC or Tmebony resulted in pale/white, light yellow/brown and dark/black adult body coloration, respectively. In addition, there are three distinct layer/regional pigmentation differences in rigid types of adult cuticle, a brownish outer exocuticle (EX), a dark pigmented middle mesocuticle (ME) and a transparent inner endocuticle (EN). Decreases in pigmentation of the EX and/or ME layers were observed after RNAi of TmDDC or TmY-y. In TmADC- or Tmebony-deficient adults, a darker pigmented EX layer was observed. In TmAANAT1-deficient adults, trabeculae formed between the dorsal and ventral elytral cuticles as well as the transparent EN layer became highly pigmented. These results demonstrate that knocking down the level of gene expression of specific enzymes of this tyrosine metabolic pathway leads to abnormal pigmentation in individual layers and substructure of the rigid adult exoskeleton of T. molitor.

Immune Defenses of a Beneficial Pest: The Mealworm Beetle, Tenebrio molitor

The mealworm beetle, Tenebrio molitor, is currently considered as a pest when infesting stored grains or grain products. However, mealworms are now being promoted as a beneficial insect because their high nutrient content makes them a viable food source and because they are capable of degrading polystyrene and plastic waste. These attributes make T. molitor attractive for mass rearing, which may promote disease transmission within the insect colonies. Disease resistance is of paramount importance for both the control and the culture of mealworms, and several biotic and abiotic environmental factors affect the success of their anti-parasitic defenses, both positively and negatively. After providing a detailed description of T. molitor's anti-parasitic defenses, we review the main biotic and abiotic environmental factors that alter their presentation, and we discuss their implications for the purpose of controlling the development and health of this insect.

Weak and inconsistent associations between melanic darkness and fitness-related traits in an insect

The idea that the fitness value of body coloration may be affected by biochemically mediated trade-offs has received much research attention. For example, melanization is believed to interact with other fitness-related traits via competition for substrates, costs associated with the synthesis of melanin or pleiotropic effects of the involved genes. However, genetic correlations between coloration and fitness-related traits remain poorly understood. Here, we present a quantitative-genetic study of a coloration trait correlated to melanin-based cuticular darkness ('darkness', hereafter) in a geometrid moth, Ematurga atomaria. This species has considerable variation in larval appearance. We focus on correlations between larval darkness and fitness-related growth performance traits. Both a half-sib analysis and an 'animal model' approach revealed moderately high heritabilities of larval darkness and indices of growth performance. Heritability estimates of darkness derived from the animal model were, however, considerably higher than those based on the half-sib model suggesting that the determination of coloration includes genetic interactions and epigenetic effects. Importantly, on the host plant with the largest sample size, we found no evidence for either genetic or environmental correlations between darkness and growth parameters. On an alternative host plant, there was some indication of positive genetic and negative environmental correlation between these traits. This shows that respective relationships are environment-specific. Nevertheless, the overall pattern of weak and inconsistent correlations between larval coloration and growth parameters does not support universal trade-offs between these traits and suggests that physiological costs of producing colour patterns do not necessarily interfere with adaptive evolution of coloration.

Immune deployment increases larval vulnerability to predators and inhibits adult life-history traits in a dragonfly

While deploying immune defences early in ontogeny can trade-off with the production and maintenance of other important traits across the entire life cycle, it remains largely unexplored how features of the environment shape the magnitude or presence of these lifetime costs. Greater predation risk during the juvenile stage may particularly influence such costs by (1) magnifying the survival costs that arise from any handicap of juvenile avoidance traits and/or (2) intensifying allocation trade-offs with important adult traits. Here, we tested for predator-dependent costs of immune deployment within and across life stages using the dragonfly, Pachydiplax longipennis. We first examined how larval immune deployment affected two traits associated with larval vulnerability to predators: escape distance and foraging under predation risk. Larvae that were induced to mount an immune response had shorter escape distances but lower foraging activity in the presence of predator cues. We also induced immune responses in larvae and reared them through emergence in mesocosms that differed in the presence of large predatory dragonfly larvae (Aeshnidae spp.). Immune-challenged larvae had later emergence overall and lower survival in pools with predators. Immune-challenged males were also smaller at emergence and developed less sexually selected melanin wing coloration, but these effects were independent of predator treatment. Overall, these results highlight how mounting an immune defence early in ontogeny can have substantial ecological and physiological costs that manifest both within and across life stages.

Sustainable farming of the mealworm Tenebrio molitor for the production of food and feed

The farming of edible insects is an alternative strategy for the production of protein-rich food and feed with a low ecological footprint. The industrial production of insect-derived protein is more cost-effective and energy-efficient than livestock farming or aquaculture. The mealworm Tenebrio molitor is economically among the most important species used for the large-scale conversion of plant biomass into protein. Here, we review the mass rearing of this species and its conversion into food and feed, focusing on challenges such as the contamination of food/feed products with bacteria from the insect gut and the risk of rapidly spreading pathogens and parasites. We propose solutions to prevent the outbreak of infections among farmed insects without reliance on antibiotics. Transgenerational immune priming and probiotic bacteria may provide alternative strategies for sustainable insect farming.

Nuclear receptor HR3 controls locust molt by regulating chitin synthesis and degradation genes of Locusta migratoria

During growth and development of insects, the steroid hormone 20-Hydroxyecdysone (20E) regulates the molting process through activation of a series of genes including E74, E75 and HR3 by the 20E receptor EcR. Here, we analyzed the function of LmHR3 in the migratory locust Locusta migratoria. By sequence comparison, we first identified and characterized the putative nuclear receptor protein (LmHR3) based on L. migratoria transcriptome data. The full length cDNA is 2272 bp long encoding a protein of 455 amino acids that contains a DNA binding domain (zinc finger) and a ligand binding domain. Phylogenetic analyses showed that LmHR3 has a high homology with the ortholog from Blattaria. RT-qPCR results revealed that LmHR3 has a low level expression in the early days of 5th instar nymphs, and then increases and peaks at day 6, followed by a decrease to low levels before ecdysis. The LmHR3, hence, coincides with the profile of circulating 20E levels. Indeed, we show that transcription of LmHR3 is induced by 20E in vivo, and significantly suppressed by successfully knocking down expression of LmEcR. After injection of dsRNA for LmHR3 (dsLmHR3) at day 2 of earlier instar nymphs (3rd and 4th instar) and final instar nymphs (5th instar), none of the nymphs were able to molt normally, and eventually died. Chitin staining and ultra-structural analysis showed that both the synthesis of the new cuticle and the degradation of the old cuticle were blocked in the dsLmHR3 treated nymphs. Especially, chitin synthesis genes (LmUAP1 and LmCHS1) and chitinase genes (LmCHT5 and LmCHT10) were significantly down-regulated in the dsLmHR3 treatment group. Together, our results suggest that LmHR3 is involved in the control of chitin synthesis and degradation during L. migratoria molting.

Age, sex, mating status, but not social isolation interact to shape basal immunity in a group-living insect

Immunity is a crucial but costly trait. Individuals should therefore adjust their investment into immunity to their condition and infection risks, which are often determined by their age, sex, mating status and social environment. However, whether and how these four key factors can interact to shape basal immunity remains poorly understood. Here, we tested the simultaneous effects of these factors on hemocyte concentration and phenoloxidase activity in adults of the European earwig. We found that hemocyte concentration increased with age, and that this increase was stronger in males. We also found an age-dependent increase in phenoloxidase activity in males and virgin females, but not in mated females. However, the two immune parameters were independent of social isolation. Overall, our results reveal that a complex interplay between age, sex and mating status determines basal immunity and stress the importance of interactions in our understanding of immune investment.