Spiders do have melanin after all

The moulting arthropod: a complete genetic toolkit review

Exoskeletons are a defining character of all arthropods that provide physical support for their segmented bodies and appendages as well as protection from the environment and predation. This ubiquitous yet evolutionarily variable feature has been instrumental in facilitating the adoption of a variety of lifestyles and the exploitation of ecological niches across all environments. Throughout the radiation that produced the more than one million described modern species, adaptability afforded by segmentation and exoskeletons has led to a diversity that is unrivalled amongst animals. However, because of the limited extensibility of exoskeleton chitin and cuticle components, they must be periodically shed and replaced with new larger ones, notably to accommodate the growing individuals encased within. Therefore, arthropods grow discontinuously by undergoing periodic moulting events, which follow a series of steps from the preparatory pre-moult phase to ecdysis itself and post-moult maturation of new exoskeletons. Each event represents a particularly vulnerable period in an arthropod's life cycle, so processes must be tightly regulated and meticulously executed to ensure successful transitions for normal growth and development. Decades of research in representative arthropods provide a foundation of understanding of the mechanisms involved. Building on this, studies continue to develop and test hypotheses on the presence and function of molecular components, including neuropeptides, hormones, and receptors, as well as the so-called early, late, and fate genes, across arthropod diversity. Here, we review the literature to develop a comprehensive overview of the status of accumulated knowledge of the genetic toolkit governing arthropod moulting. From biosynthesis and regulation of ecdysteroid and sesquiterpenoid hormones, to factors involved in hormonal stimulation responses and exoskeleton remodelling, we identify commonalities and differences, as well as highlighting major knowledge gaps, across arthropod groups. We examine the available evidence supporting current models of how components operate together to prepare for, execute, and recover from ecdysis, comparing reports from Chelicerata, Myriapoda, Crustacea, and Hexapoda. Evidence is generally highly taxonomically imbalanced, with most reports based on insect study systems. Biases are also evident in research on different moulting phases and processes, with the early triggers and late effectors generally being the least well explored. Our synthesis contrasts knowledge based on reported observations with reasonably plausible assumptions given current taxonomic sampling, and exposes weak assumptions or major gaps that need addressing. Encouragingly, advances in genomics are driving a diversification of tractable study systems by facilitating the cataloguing of putative genetic toolkits in previously under-explored taxa. Analysis of genome and transcriptome data supported by experimental investigations have validated the presence of an "ultra-conserved" core of arthropod genes involved in moulting processes. The molecular machinery has likely evolved with elaborations on this conserved pathway backbone, but more taxonomic exploration is needed to characterise lineage-specific changes and novelties. Furthermore, linking these to transformative innovations in moulting processes across Arthropoda remains hampered by knowledge gaps and hypotheses based on untested assumptions. Promisingly however, emerging from the synthesis is a framework that highlights research avenues from the underlying genetics to the dynamic molecular biology through to the complex physiology of moulting.

Xanthurenic Acid in the Shell Purple Patterns of Crassostrea gigas: First Evidence of an Ommochrome Metabolite in a Mollusk Shell

Ommochromes are one of the least studied groups of natural pigments, frequently confused with melanin and, so far, exclusively found in invertebrates such as cephalopods and butterflies. In this study focused on the purple color of the shells of a mollusk, Crassostrea gigas, the first evidence of a metabolite of ommochromes, xanthurenic acid (XA), was obtained by liquid chromatography combined with mass spectrometry (UPLC-MS). In addition to XA and various porphyrins previously identified, a second group of high molecular weight acid-soluble pigments (HMASP) has been identified with physicochemical and structural characteristics similar to those of ommochromes. In addition, fragmentation of HMASP by tandem mass spectrometry (MS/MS) has revealed a substructure common to XA and ommochromes of the ommatin type. Furthermore, the presence of melanins was excluded by the absence of characteristic by-products among the oxidation residues of HMASP. Altogether, these results show that the purple color of the shells of Crassostrea gigas is a complex association of porphyrins and ommochromes of potentially ommatin or ommin type.

The spider cuticle: a remarkable material toolbox for functional diversity

Engineered systems are typically based on a large variety of materials differing in composition and processing to provide the desired functionality. Nature, however, has evolved materials that are used for a wide range of functional challenges with minimal compositional changes. The exoskeletal cuticle of spiders, as well as of other arthropods such as insects and crustaceans, is based on a combination of chitin, protein, water and small amounts of organic cross-linkers or minerals. Spiders use it to obtain mechanical support structures and lever systems for locomotion, protection from adverse environmental influences, tools for piercing, cutting and interlocking, auxiliary structures for the transmission and filtering of sensory information, structural colours, transparent lenses for light manipulation and more. This paper illustrates the 'design space' of a single type of composite with varying internal architecture and its remarkable capability to serve a diversity of functions. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)'.

Melanins as Sustainable Resources for Advanced Biotechnological Applications

Melanins are a class of biopolymers that are widespread in nature and have diverse origins, chemical compositions, and functions. Their chemical, electrical, optical, and paramagnetic properties offer opportunities for applications in materials science, particularly for medical and technical uses. This review focuses on the application of analytical techniques to study melanins in multidisciplinary contexts with a view to their use as sustainable resources for advanced biotechnological applications, and how these may facilitate the achievement of the United Nations Sustainable Development Goals.

Comparative characterization of the hemocyanin-derived phenol oxidase activity from spiders inhabiting different thermal habitats

Enzymes adapted to cold temperatures are commonly characterized for having higher Michaelis-Menten constants (K_M) values and lower optimum and denaturation temperature, when compared to other meso or thermophilic enzymes. Phenoloxidase (PO) enzymes are ubiquitous in nature, however, they have not been reported in spiders. It is the oxygen carrier protein hemocyanin (Hc), found at high concentrations in their hemolymph, which displays an inducible PO activity. Hence, we hypothesize that Hc-derived PO activity could show features of cold adaptation in alpine species. We analyzed the Hc from two species of Theraphosidae from different thermal environments: Euathlus condorito (2400 m a.s.l.) and Grammostola rosea (500 m a.s.l.). Hc was purified from the hemolymph of both spiders and was characterized by identifying subunit composition and measuring the sodium dodecyl sulfate (SDS)-induced PO activity. The high-altitude spider Hc showed higher PO activity under all conditions and higher apparent Michaelis-Menten constant. Moreover, the optimum temperature for PO activity was lower for E. condorito Hc. These findings suggest a potential adaptation at the level of Hc-derived PO activity in Euathlus condorito, giving insights on possible mechanisms used by this mygalomorph spider to occupy extremes and variable thermal environments.

Structurally assisted super black in colourful peacock spiders

Male peacock spiders (Maratus, Salticidae) compete to attract female mates using elaborate, sexually selected displays. They evolved both brilliant colour and velvety black. Here, we use scanning electron microscopy, hyperspectral imaging and finite-difference time-domain optical modelling to investigate the deep black surfaces of peacock spiders. We found that super black regions reflect less than 0.5% of light (for a 30° collection angle) in Maratus speciosus (0.44%) and Maratus karrie (0.35%) owing to microscale structures. Both species evolved unusually high, tightly packed cuticular bumps (microlens arrays), and M. karrie has an additional dense covering of black brush-like scales atop the cuticle. Our optical models show that the radius and height of spider microlenses achieve a balance between (i) decreased surface reflectance and (ii) enhanced melanin absorption (through multiple scattering, diffraction out of the acceptance cone of female eyes and increased path length of light through absorbing melanin pigments). The birds of paradise (Paradiseidae), ecological analogues of peacock spiders, also evolved super black near bright colour patches. Super black locally eliminates white specular highlights, reference points used to calibrate colour perception, making nearby colours appear brighter, even luminous, to vertebrates. We propose that this pre-existing, qualitative sensory experience—‘sensory bias’—is also found in spiders, leading to the convergent evolution of super black for mating displays in jumping spiders.

Structure and pigment make the eyed elater's eyespots black

Surface structures that trap light leading to near complete structural absorption creates an appearance of "super black." Well known in the natural world from bird feathers and butterfly scales, super black has evolved independently from various anatomical structures. Due to an exceptional ability to reduce specular reflection, these biological materials have garnered interest from optical industries. Here we describe the false eyes of the eyed elater click beetle, which, while not classified as super black, still attains near complete absorption of light partly due to an array of vertically-aligned microtubules. These cone-shaped microtubules are modified hairs (setae) that are localized to eyespots on the dorsum of the beetle, and absorb 96.1% of incident light (at a 24.8° collection angle) in the spectrum between 300-700 nm. Filled with melanin, the setae combine structure and pigment to generate multiple reflections and refractions causing light to travel a greater distance. This light-capturing architecture leaves little light available to receivers and the false eyes appear as deep black making them appear more conspicuous to predators.

Spider Chitin: An Ultrafast Microwave-Assisted Method for Chitin Isolation from Caribena versicolor Spider Molt Cuticle

Chitin, as a fundamental polysaccharide in invertebrate skeletons, continues to be actively investigated, especially with respect to new sources and the development of effective methods for its extraction. Recent attention has been focused on marine crustaceans and sponges; however, the potential of spiders (order Araneae) as an alternative source of tubular chitin has been overlooked. In this work, we focused our attention on chitin from up to 12 cm-large Theraphosidae spiders, popularly known as tarantulas or bird-eating spiders. These organisms "lose" large quantities of cuticles during their molting cycle. Here, we present for the first time a highly effective method for the isolation of chitin from Caribena versicolor spider molt cuticle, as well as its identification and characterization using modern analytical methods. We suggest that the tube-like molt cuticle of this spider can serve as a naturally prefabricated and renewable source of tubular chitin with high potential for application in technology and biomedicine.

Elucidation of the hierarchical structure of natural eumelanins

Eumelanin is one of the most ubiquitous pigments in living organisms and plays an important role in coloration and UV protection. Because eumelanin is highly cross-linked and insoluble in solvents, the chemical structure is still not completely known. In this study, we used atomic force microscopy, X-ray photoelectron spectroscopy and solid-state nuclear magnetic resonance (NMR) to compare intact eumelanosomes (pigment granules mostly made of eumelanin) from four phylogentically distant species: cuttlefish (Sepia officinalis) inks, black fish crow (Corvus ossifragus) feathers, iridescent wild turkey (Melleagris gallopavo) feathers and black human hair. We found that eumelanosomes from all four species are composed of subunit nanoparticles with a length of 10-60 nm, consistent with earlier observations in eumelanosomes from the sepia ink and human hair. The solid-state NMR results indicate the presence of quinone methide tautomers in all four eumelanins. We also found clear differences in the UV absorbance, the ratio of 5,6-dihydroxyindole-2-carboxylic acid/5,6-dihydroxyindole and protonated aryl carbon ratios in sepia eumelanin relative to the other three. This comparison of natural eumelanin across a phylogenetically broad group of organisms provides insights into the change in the eumelanin structure over the evolutionary history and enables the production of synthetic eumelanin with properties that are similar to natural eumelanin.

Structural origins of coloration in the spider Phoroncidia rubroargentea Berland, 1913 (Araneae: Theridiidae) from Madagascar

This study investigates the structural basis for the red, silver and black coloration of the theridiid spider, Phoroncidia rubroargentea (Berland, 1913) from Madagascar. Specimens of this species can retain their colour after storage in ethanol for decades, whereas most other brightly pigmented spider specimens fade under identical preservation conditions. Using correlative optical, structural and chemical analysis, we identify the colour-generating structural elements and characterize their optical properties. The prominent silvery appearance of the spider's abdomen results from regularly arranged guanine microplatelets, similar to those found in other spiders and fish. The microplatelets are composed of a doublet structure twinned about the [[Formula: see text]] axis, as suggested by electron diffraction. The red coloration originates from chambered microspheres (approx. 1 µm in diameter), which contain structured fluorescent material. Co-localization of the red microparticles on top of the reflective guanine microplatelets appears to enhance the red coloration. The spider's thick cuticular layer, which encases its abdomen, varies in its optical properties, being transparent in regions where only guanine reflectors are present, and tanned, exhibiting light absorption where the red microspheres are found. Moreover, colour degradation in some preserved spider specimens that had suffered damage to the cuticular layer suggests that this region of the exoskeleton may play an important role in the stabilization of the red coloration.

Spiders have rich pigmentary and structural colour palettes

Elucidating the mechanisms of colour production in organisms is important for understanding how selection acts upon a variety of behaviours. Spiders provide many spectacular examples of colours used in courtship, predation, defence and thermoregulation, but are thought to lack many types of pigments common in other animals. Ommochromes, bilins and eumelanin have been identified in spiders, but not carotenoids or melanosomes. Here, we combined optical microscopy, refractive index matching, confocal Raman microspectroscopy and electron microscopy to investigate the basis of several types of colourful patches in spiders. We obtained four major results. First, we show that spiders use carotenoids to produce yellow, suggesting that such colours may be used for condition-dependent courtship signalling. Second, we established the Raman signature spectrum for ommochromes, facilitating the identification of ommochromes in a variety of organisms in the future. Third, we describe a potential new pigmentary-structural colour interaction that is unusual because of the use of long wavelength structural colour in combination with a slightly shorter wavelength pigment in the production of red. Finally, we present the first evidence for the presence of melanosomes in arthropods, using both scanning and transmission electron microscopy, overturning the assumption that melanosomes are a synapomorphy of vertebrates. Our research shows that spiders have a much richer colour production palette than previously thought, and this has implications for colour diversification and function in spiders and other arthropods.

Interactions between colour-producing mechanisms and their effects on the integumentary colour palette

Animal integumentary coloration plays a crucial role in visual communication and camouflage, and varies extensively among and within species and populations. To understand the pressures underlying such diversity, it is essential to elucidate the mechanisms by which animals have created novel integumentary coloration. Colours can be produced by selective absorption of light by skin pigments, through light scattering by structured or unstructured tissues, or by a combination of pigments and nanostructures. In this review, we highlight our current understanding of the interactions between pigments and structural integumentary tissues and molecules. We analyse the available evidence suggesting that these combined mechanisms are capable of creating colours and optical properties unachievable by either mechanism alone, thereby effectively expanding the animal colour palette. Moreover, structural and pigmentary colour mechanisms frequently interact in unexpected and overlooked ways, suggesting that classification of colours as being of any particular type may be difficult. Finally, we discuss how these mixtures are useful for investigating the largely unknown genetic, developmental and physical processes generating phenotypic diversity.This article is part of the themed issue 'Animal coloration: production, perception, function and application'.

Rainbow peacock spiders inspire miniature super-iridescent optics

Colour produced by wavelength-dependent light scattering is a key component of visual communication in nature and acts particularly strongly in visual signalling by structurally-coloured animals during courtship. Two miniature peacock spiders (Maratus robinsoni and M. chrysomelas) court females using tiny structured scales (~ 40 × 10 μm²) that reflect the full visual spectrum. Using TEM and optical modelling, we show that the spiders' scales have 2D nanogratings on microscale 3D convex surfaces with at least twice the resolving power of a conventional 2D diffraction grating of the same period. Whereas the long optical path lengths required for light-dispersive components to resolve individual wavelengths constrain current spectrometers to bulky sizes, our nano-3D printed prototypes demonstrate that the design principle of the peacock spiders' scales could inspire novel, miniature light-dispersive components.

Splendid coloration of the peacock spider Maratus splendens

Jumping spiders are well known for their acute vision and often bright colours. The male peacock spider Maratus splendens is richly coloured by scales that cover the body. The colours of the white, cream and red scales, which have an elaborate shape with numerous spines, are pigmentary. Blue scales are unpigmented and have a structural colour, created by an intricate photonic system consisting of two chitinous layers with ridges, separated by an air gap, with on the inner sides of the chitin layers an array of filaments. We have characterized the optical properties of the scales by microspectrophotometry, imaging scatterometry and light and scanning electron microscopy. Optical modelling revealed that the filament array constitutes a novel structural coloration system, which subtly fine tunes the scale reflectance to the observed blue coloration.

Eumelanin and pheomelanin are predominant pigments in bumblebee (Apidae: Bombus) pubescence

BACKGROUND

Bumblebees (Hymenoptera: Apidae: Bombus) are well known for their important inter- and intra-specific variation in hair (or pubescence) color patterns, but the chemical nature of the pigments associated with these patterns is not fully understood. For example, though melanization is believed to provide darker colors, it still unknown which types of melanin are responsible for each color, and no conclusive data are available for the lighter colors, including white.

METHODS

By using dispersive Raman spectroscopy analysis on 12 species/subspecies of bumblebees from seven subgenera, we tested the hypothesis that eumelanin and pheomelanin, the two main melanin types occurring in animals, are largely responsible for bumblebee pubescence coloration.

RESULTS

Eumelanin and pheomelanin occur in bumblebee pubescence. Black pigmentation is due to prevalent eumelanin, with visible signals of additional pheomelanin, while the yellow, orange, red and brown hairs clearly include pheomelanin. On the other hand, white hairs reward very weak Raman signals, suggesting that they are depigmented. Additional non-melanic pigments in yellow hair cannot be excluded but need other techniques to be detected. Raman spectra were more similar across similarly colored hairs, with no apparent effect of phylogeny and both melanin types appeared to be already used at the beginning of bumblebee radiation.

DISCUSSION

We suggest that the two main melanin forms, at variable amounts and/or vibrational states, are sufficient in giving almost the whole color range of bumblebee pubescence, allowing these insects to use a single precursor instead of synthesizing a variety of chemically different pigments. This would agree with commonly seen color interchanges between body segments across Bombus species.

Pheomelanin in the secondary sexual characters of male parasitoid wasps (Hymenoptera: Pteromalidae)

The occurrence and distribution of eumelanin and pheomelanin, the most prevalent biological pigments, has been rarely investigated in insects. Particularly yellowish to brownish body parts, which in many vertebrates are associated with pheomelanin, are visible in many insects but their chemical nature was rarely examined to a similar detail. Here, by using Dispersive Raman spectroscopy analysis, we found both eumelanin and pheomelanin in different body parts of male parasitoid wasps of three species of the genus Mesopolobus (Hymenoptera: Pteromalidae), which are known to have species-specific spots and coloured stripes on the legs and/or antennae which are displayed to females during courtship. We found a strong eumelanin signal in the antennal clava of all studied Mesopolobus species and in the circular black spot or callosity and the triangular black projection on the outer apical angle of the typically expanded middle tibia of Mesopolobus tibialis and Mesopolobus xanthocerus. Eumelanin was also the predominant pigment in the black thorax of Mesopolobus and other members of the family. Pheomelanin, on the other hand, was detected as predominant only in certain body parts of M. tibialis and M. xanthocerus, precisely in a very narrow, longitudinal brownish stripe on the middle femur and, only in M. tibialis, in a brownish oval-longitudinal stripe on the middle tibia. The two melanin types co-occurred in most pigmented areas, but more often one is clearly predominant relative to the other, according to the variation of Raman signal intensity of their signature peaks. A further tibial yellowish-orange stripe present in both these species did not include melanins of any type. Pheomelanin, could be more widespread than previously known in insects. A convergent evolution of melanin-based male sexual ornaments between vertebrates (e.g. bird feathers) and wasps can be suggested, opening to a new line of comparative evolutionary studies.

A dark cuticle allows higher investment in immunity, longevity and fecundity in a beetle upon a simulated parasite attack

Cuticle melanism in insects is linked to a number of life history traits: a positive relationship is hypothesized between melanism, immune function, fecundity and lifespan. However, it is not clear how activation of the immune system affects trade-offs between life history traits in female mealworm beetles (Tenebrio molitor) differing in cuticle melanization. The females with tan, brown and black cuticles examined in the present study did not differ in the intensity of encapsulation response, fecundity and longevity when their immune system was not activated. However, we found that immune activation and cuticle melanization have a significant effect on life history traits. Offspring number and lifespan decreased in females with tan and brown cuticles, while the fecundity and lifespan of black females were not affected. Importantly, we inserted the implants again and found a significant decrease in the strength of encapsulation response in females with tan and brown cuticles. In contrast, black females increased melanotic reactions against the nylon implant, suggesting immunological priming. The results show that cuticle melanization plays an important adaptive role under the risk of being infected, while the lack of these benefits before the insertion of nylon monofilaments suggests that there are costs associated with an activated immunity system.

The ocelli of Archaeognatha (Hexapoda): Functional morphology, pigment migration and chemical nature of the reflective tapetum

The ocelli of Archaeognatha, or jumping bristletails, differ from typical insect ocelli in shape and field of view. While the shape of the lateral ocelli is highly variable among species, most Machiloidea have sole shaped lateral ocelli beneath the compound eyes and a median ocellus that is oriented downward. This study investigated morphological and physiological aspects of the ocelli of Machilis hrabei and Lepismachilis spp.

The light reflecting ocellar tapetum in Machilis hrabei is made up by xanthine nanocrystals, as demonstrated by confocal Raman spectroscopy. Pigment granules in the photoreceptor cells move behind the tapetum in the dark adapted state. Such a vertical pigment migration in combination with a tapetum has not been described for any insect ocellus so far. The pigment migration has a dynamic range of around 4 log units and is maximally sensitive to green light. Adaptation from darkness to bright light lasts over an hour, which is slow compared to the radial pupil mechanism in some dragonflies and locusts.