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Arthropod exoskeleton

The arthropod is a rigid, external that envelops the segmented body and appendages of arthropods, serving as a defining structural feature that enables their diverse ecological adaptations. Composed primarily of microfibrils intertwined with proteins, it forms a lightweight yet durable that supports locomotion, protects internal organs from mechanical damage and predation, and prevents in terrestrial . This , secreted by a single-layered epidermal , requires periodic molting () to accommodate growth, a process conserved across the phylum's over one million extant spanning more than 500 million years of . Structurally, the exoskeleton consists of three main layers: the thin epicuticle, which provides a waxy permeability barrier against loss and pathogens through and proteins; the exocuticle, a hardened outer procuticle layer reinforced by sclerotization (cross-linking of proteins); and the thicker endocuticle, offering flexibility via unhardened chitin-protein fibers arranged in a helicoidal "Bouligand" for isotropic strength. In crustaceans, these layers often incorporate mineralization with (as or amorphous forms) or phosphates, enhancing rigidity for aquatic or burrowing lifestyles, while rely more on agents like quinones for hardening without heavy . Pore canals and sensory structures embedded within the further integrate functions like mechanoreception and immune defense, with the acting as a physical barrier augmented by proteins. Functionally, the facilitates muscle attachment via specialized cells and myotendinous junctions, allowing efficient transmission for in diverse habitats from to air. Its mechanical properties—high strength-to-weight ratio, impact resistance, and adaptability to specific loads (e.g., parallel orientations in claws for puncture strength)—have inspired biomimetic applications in materials . However, its inflexibility imposes constraints, necessitating energy-intensive molting cycles that expose vulnerabilities to predation and environmental stress during the soft-bodied post-molt phase. Across lineages, including , crustaceans, arachnids, and myriapods, variations in composition and layering reflect evolutionary divergences tailored to niches, underscoring the exoskeleton's role in the phylum's unparalleled .

Introduction

Definition and characteristics

The arthropod exoskeleton is a rigid, external covering primarily composed of , a that forms a hardened structure providing , against predators and environmental hazards, and points of attachment for muscles. This exoskeleton encases the entire body, functioning as a multifunctional that replaces internal skeletal elements found in other animal phyla. A defining characteristic is that the exoskeleton is acellular and non-living, consisting of a secreted by a single layer of underlying epidermal cells known as the hypodermis. It exhibits significant variation in thickness and rigidity across body regions to accommodate diverse functions; for instance, it is typically thin and flexible at joints to enable and movement, while being thicker and more sclerotized on dorsal shields or sclerites for robust protection. The integrates closely with the via tagmosis, the evolutionary fusion of serial segments into specialized tagmata such as the head, , and , where exoskeletal plates and sutures delineate these functional units and support region-specific adaptations. Compared to endoskeletons in phyla like Chordata, the arthropod exoskeleton provides an advantage in offering immediate protection post-embryogenesis, as larvae or juveniles emerge with a pre-formed external barrier that safeguards soft tissues from the outset of independent life.

Biological significance

The arthropod exoskeleton serves as a primary protective barrier, shielding against predators through its rigid, often sclerotized structure that deters attacks and provides mechanical resistance. In addition, it prevents in terrestrial species by forming an impermeable layer that minimizes loss, particularly via waxy epicuticle components adapted for arid environments. This barrier also inhibits invasion, such as fungi, through melanization and surface modifications that resist penetration. For aquatic arthropods, the exoskeleton maintains osmotic balance against fluctuations. Beyond protection, the provides structural support essential for , functioning as an external to which muscles attach via apodemes and cells, enabling leverage and efficient movement across diverse terrains. It integrates sensory functions by serving as a substrate for setae, chemoreceptors, and mechanoreceptors, allowing arthropods to detect environmental stimuli like touch, vibration, and chemicals for and . Ecologically, the facilitates to varied habitats, from terrestrial to environments, underpinning their dominance as over 85% of known animal and enabling exploitation of niches through enhanced mobility and resilience. However, it imposes trade-offs, including size limitations due to increasing weight relative to body volume, which constrains growth in larger forms, and the energy-intensive process of molting, which temporarily increases vulnerability while renewing the structure.

Chemical Composition

Organic components

The organic components of the arthropod exoskeleton primarily consist of , proteins, and , which form a composite matrix providing structural integrity and functional adaptability. , the predominant , is a linear of β-1,4-linked N-acetyl-D-glucosamine units, synthesized by chitin synthase enzymes in epidermal cells and assembled into microfibrils that impart tensile strength to the exoskeleton. These microfibrils, typically 10-25 in diameter, are oriented in a plywood-like structure within the procuticle, enhancing resistance to mechanical stress while allowing flexibility. Proteins constitute 20-60% of the organic matrix, depending on the species and body region, and include scleroproteins such as cuticulins and specialized elastomers like . Cuticulins, complexes, form a thin, impervious layer in the epicuticle, contributing to surface , while structural proteins known as arthropodins—water-soluble polypeptides rich in and —provide the foundational matrix that binds fibrils. These proteins undergo sclerotization through tanning, where phenolic compounds like N-β-alanyl-dopamine cross- the matrix via oxidative reactions catalyzed by phenoloxidases, resulting in hardened, insoluble sclerotins that increase rigidity. , a cross-linked polypeptide with high and content, exhibits rubber-like elasticity and is particularly abundant in dynamic regions such as hinges, where it enables rapid, reversible deformation with minimal energy loss; its is approximately 1 MPa, allowing extension up to 300% of its original length. Lipids and waxes, comprising 1-5% of the exoskeleton's organics, are concentrated in the epicuticle as a of hydrocarbons, fatty acids, and esters secreted by oenocytes. These components form a hydrophobic barrier that prevents in terrestrial arthropods, such as and myriapods, by reducing cuticular water permeability to less than 0.1 μg/cm²/h under dry conditions. In species like crustaceans, also aid in resistance, though their role is secondary to the primary structural organics.

Inorganic components

The inorganic components of the arthropod primarily consist of minerals that provide rigidity and mechanical support by reinforcing the organic matrix of and proteins. In crustaceans, the dominant mineral is , occurring as , , or magnesium-substituted variants like Mg-, which imparts exceptional hardness to the . This mineral can comprise up to 80% by weight in heavily calcified regions, such as the of species like the ( americanus), enabling resistance to predation and environmental stresses. Calcium phosphate, often as fluorapatite or amorphous forms, is also incorporated in crustacean structures such as mandibles, providing high hardness for feeding tools. Trace elements further modulate exoskeletal properties in crustaceans. Magnesium substitutes into calcite lattices in crustacean cuticles to fine-tune crystallinity and solubility. Biomineralization of these inorganic elements occurs post-moulting, when epidermal cells (hypodermis) actively deposit calcium and other ions into the newly formed chitin matrix, precipitating minerals along fiber networks and pore canals to build layered reinforcement. In decapods, this process is regulated by the enzyme carbonic anhydrase, which accelerates CO₂ hydration to supply bicarbonate ions for calcium carbonate formation, ensuring efficient calcification during the vulnerable post-moult phase. In contrast to crustaceans, other groups such as , arachnids, and myriapods exhibit minimal inorganic mineralization, depending instead on organic hardening mechanisms.

Microscopic Structure

Cuticle layers

The arthropod exoskeleton's is a hierarchical, non-cellular structure primarily composed of two main layers: the thin outer epicuticle and the thicker inner procuticle. These layers provide , , and selective permeability, with the epicuticle serving as the primary barrier to environmental factors. The epicuticle forms the outermost layer, typically 0.1-1 μm thick, and is divided into three sublayers that collectively ensure impermeability to and gases. The outermost cement layer, composed of tanned proteins and , acts as a protective over the underlying waxes. Beneath it lies the wax layer, approximately 0.25 μm thick, which consists of crystalline hydrocarbons that prevent . The innermost cuticulin sublayer is a thin, tanned membrane that provides the foundational barrier and is the first to be deposited during formation. The procuticle constitutes the bulk of the cuticle's thickness and is subdivided into the exocuticle and endocuticle. The exocuticle is the outer, hardened portion formed through sclerotization, involving cross-linking of proteins with to create rigidity in and other arthropods; in crustaceans, this layer is further reinforced by with . In contrast, the endocuticle is the inner, flexible layer with a lamellate organization, allowing for articulation and growth accommodation. A distinctive feature of the procuticle is the helicoidal arrangement of microfibrils, forming a twisted plywood-like Bouligand structure that enhances isotropic strength and resists crack propagation. Traversing the procuticle are pore canals, vertical channels with diameters ranging from 0.1 to 5 μm depending on the arthropod species and region, which facilitate the transport of nutrients, waste products, and pigments from the underlying epidermal cells to the surface. These canals, containing filamentous extensions, also connect to wax canals in the epicuticle for lipid distribution. The entire cuticle is secreted by epidermal cells, with total thickness varying from 1 to 100 μm across body regions; for instance, beetle elytra exhibit thicker cuticles, often exceeding 200 μm, to provide enhanced armor.

Cellular basis

The epidermal cells constitute a of positioned immediately beneath the , typically exhibiting columnar or cuboidal morphology that varies with developmental stage and location. These cells function as the primary secretory apparatus for formation, synthesizing and releasing microfibrils and structural proteins through apical microvilli, with key processes involving the Golgi apparatus for packaging into secretory vesicles. synthesis occurs within specialized compartments derived from the Golgi, followed by extrusion and assembly into the procuticle matrix alongside proteins that provide rigidity and flexibility. Anchoring the to the underlying hemocoel is the , a thin, acellular layer approximately 0.5 μm thick composed of a of amorphous mucopolysaccharides and a reticular layer rich in fibers. This structure not only provides mechanical support and a for epidermal attachment but also acts as a selective barrier regulating exchange with the in the hemocoel. Specialized derivatives of epidermal cells form dermal glands, which are unicellular or multicellular exocrine structures dedicated to producing localized secretions such as lubricants, adhesives, or defensive compounds. In scorpions, for instance, paired glands embedded in the represent highly modified dermal glands that synthesize and store potent neurotoxic proteins and peptides for delivery via the aculeus. A critical process in exoskeleton maintenance is apolysis, the initial stage of moulting preparation wherein the epidermis detaches from the overlying old cuticle through enzymatic dissolution of attachment sites, creating an apolytic space that allows for new cuticle deposition beneath the obsolete layer. This separation, observable in histological sections across arthropod taxa, ensures orderly renewal without disrupting underlying tissues. In insects, epidermal cells demonstrate remarkable plasticity during inter-moult phases, capable of dedifferentiating—reverting from a specialized secretory state to a more proliferative, stem-like condition—and undergoing to repair localized damage such as wounds or abrasions to the . This regenerative capacity enables minor structural maintenance without triggering a full moult cycle, highlighting the dynamic role of these cells in sustaining exoskeleton integrity over time.

Mechanical Properties

Strength and flexibility

The sclerotized exocuticle of the arthropod exhibits high , with a typically ranging from 1 to 20 GPa, providing rigidity comparable to that of (which has a modulus of 10–20 GPa). This stiffness arises primarily from the cross-linking of chitin-protein fibers during sclerotization and is often quantified using techniques, which reveal local variations in elastic properties across the layered structure. In contrast, the underlying endocuticle demonstrates greater flexibility, with a lower of approximately 2–6 GPa in hydrated states, enabling the exoskeleton to bend without fracturing under dynamic loads. The outer exocuticle layers excel in compressive resistance, achieving hardness up to 150–270 in mineralized regions, as seen in examples where measures values in this range; compressive strengths are typically lower, around 20–60 , but correlate with . Conversely, the inner endocuticle layers are better suited to tensile stresses, contributing to overall durability through their more compliant matrix. is enhanced by the helicoidal arrangement of fibrils in a Bouligand , which deflects propagating cracks via twisting and bridging mechanisms, yielding work-of-fracture values around 5–6 kJ/m² in insect cuticles. A notable example of these properties in action occurs in the claws of s (Homarus americanus), where graded mineralization increases hardness from 130 at the surface to 270 deeper in the exocuticle, allowing the structure to exert closing forces up to 250 (with material stresses around 25 ) without failure. This gradient optimizes load distribution during high-impact activities like prey capture. Environmental factors significantly influence these mechanical attributes; elevated humidity raises water content in the cuticle (up to 30%), reducing by orders of magnitude (e.g., from 250 to 10 in some insect cuticles) and softening the material for flexibility but compromising strength. Similarly, prolonged UV exposure induces photooxidative degradation of cuticular proteins and , lowering and by disrupting molecular cross-links, as observed in exposed specimens.

Adaptations for function

The of arthropods exhibits specialized adaptations that enable efficient , primarily through the incorporation of flexible arthrodial membranes at joints. These membranes connect rigid sclerites and permit multi-axis and extension, allowing coordinated movement essential for walking, running, and . In spiders, for instance, the arthrodial membrane in joints supports rapid, agile by providing deformability without compromising structural integrity. Additionally, pads integrated into the of jumping insects, such as locusts and froghoppers, absorb shock and store during takeoff, enabling explosive propulsion with minimal energy loss upon recoil. For protection, the exoskeleton features regionally thickened sclerites that enhance resistance to physical impacts, particularly in predatory or vulnerable species. In brachyuran crabs, the comprises layered, mineralized sclerites that deflect and absorb forces from predator strikes or environmental hazards, outperforming many synthetic composites in impact tolerance. This adaptation leverages the inherent mechanical properties of the , such as its high , to safeguard vital organs without excessive weight. The also functions as a sensory platform, embedding trichoid sensilla that detect environmental stimuli. These hair-like structures, anchored in cuticular sockets, respond to mechanical deflection from air currents, aiding and predator evasion in like damselflies. Certain trichoid sensilla further serve chemosensory roles, with porous tips facilitating the detection of pheromones and volatile chemicals, thus integrating tactile and olfactory cues for and mating behaviors. In aquatic arthropods such as , the exoskeleton's transparency represents a key for , minimizing light scattering to reduce visibility to predators in open water columns. This optical property, combined with the lightweight chitinous structure, supports , allowing efficient vertical migration without additional energy expenditure. Regional specialization is evident in developmental stages, where soft, compliant cuticles in larvae—such as those of —facilitate burrowing by enabling body elongation and substrate penetration during soil-dwelling phases.

Segmentation and Articulation

Segmental organization

The exoskeleton of arthropods is fundamentally organized into repeating segments, each typically comprising hardened sclerites that provide structural support while allowing for articulation. Dorsally, each segment features a tergite, while ventrally it includes a sternite, with lateral pleurites often present to complete the ring-like structure around the body. These sclerites are secreted by the underlying epidermis and form a protective cuticle that encases the soft tissues, enabling the segmented body plan characteristic of the phylum. Through a process known as tagmosis, these individual segments fuse or specialize into larger functional units called tagmata, adapting the body for diverse ecological roles. In crustaceans, for example, the head and thorax often merge into a , consolidating sensory and locomotor functions under a single . Similarly, in arachnids, the anterior segments form the prosoma, which bears appendages for feeding and locomotion, while the posterior opisthosoma houses digestive and reproductive organs. This regionalization enhances efficiency by grouping homologous structures without altering the underlying segmentation. Arthropod appendages are integrated into this segmental framework, with the exoskeleton extending to encase jointed limbs arising from specific body segments. Each appendage typically begins with a proximal coxa attached to the body wall, followed by distal segments such as the trochanter, femur, tibia, and tarsus, which allow precise movement. In insects, this organization is particularly evident, as the embryonic body initially develops 20 segments—comprising 6 in the head, 3 in the thorax, and 11 in the abdomen—that reduce through fusion to 11 visible adult segments, with the exoskeleton clearly delineating the fused head, three-segmented thorax, and multi-segmented abdomen. Intersegmental flexibility is maintained by unsclerotized membranous areas between sclerites, which permit bending and expansion essential for behaviors like feeding. These arthrodial membranes stretch to accommodate ingested material, particularly in the of , without compromising overall structural integrity.

Joints and sclerites

Sclerites are the individual hardened plates that form the rigid components of the exoskeleton, resulting from the sclerotization process that stiffens the . These plates vary in shape and size, providing structural support while allowing segmentation for flexibility; for example, the serves as a sclerotized shield on the abdomen of certain spiders. Sclerites are interconnected by sutures—thin lines of flexible —or arthrodial membranes, which permit bending and between segments without compromising overall integrity. Arthropod joints include hinge and pivot types that enable uniaxial or limited multiaxial motion. joints, often dicondylic with two condyles fitting into sockets, predominate in legs and appendages for flexion-extension. The arthrodial membrane, a thin, flexible region of unsclerotized at joint boundaries, facilitates smooth movement by lacking reinforcement and allowing elastic deformation. Exoskeletal invaginations known as apodemes extend inward from sclerites, providing anchor points for muscle insertion and enhancing leverage during contraction. Movement in arthropods relies on sclerite to create systems, where muscles attached to apodemes pull on rigid plates to amplify force and produce locomotion. These levers transform antagonistic muscle contractions into coordinated actions, such as walking or , with the distributing loads across segments. In soft-bodied transitions, like post-molt stages or in juvenile forms, hydrostatic generated by body fluids aids extension and shape change at flexible joints before full sclerotization. A striking example of specialized mechanics occurs in the (), where a saddle-shaped exoskeletal in the raptorial appendage enables ultrafast strikes through a system coupled with spring-like energy storage. This structure propels the dactyl club at speeds up to 23 m/s, generating peak forces up to 1500 N—equivalent to approximately 5000 times the animal's body weight (for a ~30 g individual)—via rapid release of stored . Wear on joints from repeated leads to microfractures in the arthrodial membrane or adjacent sclerites, which arthropods repair through localized secretion by underlying epidermal cells. This process deposits targeted layers of endocuticle beneath the damage site, thickening the area by up to 1.6 µm/day and restoring approximately 66% of original mechanical strength (to about 114 MPa) while doubling .

Growth and Renewal

Moulting process

The moulting process in arthropods, known as , is a hormonally regulated sequence essential for growth and development, allowing the animal to shed its rigid and expand. , such as , initiate the process by triggering apolysis, the detachment of the old from the underlying ; in , they are secreted by the prothoracic glands, while in crustaceans by Y-organs. Modulating hormones vary by group: in , produced by the corpora allata prevents premature in immature stages and ensures successive larval or nymphal moults before the final adult form; in crustaceans, molt-inhibiting hormone from the eyestalks regulates timing. The process unfolds in distinct stages. During apolysis, the epidermis separates from the old exoskeleton, creating an exuvial space beneath the cuticle. The epidermal cells then secrete a new, soft cuticle composed initially of proteins and , which begins to form the structural framework for the next . Subsequently, moulting fluid—containing enzymes such as chitinases and proteases—is released into the exuvial space to digest and recycle components of the old cuticle, breaking down its and proteins for reabsorption by the epidermis. The final stage, ecdysis proper, involves the physical shedding of the digested old (exuvia) through splits or tears, often along predetermined lines, as the animal wriggles free, facilitated by further hormonal signals like ecdysis-triggering . Variations in ecdysis occur between aquatic and terrestrial arthropods to accommodate environmental differences. In aquatic larvae, such as those of many , the old cuticle often splits in a characteristic Y-shaped pattern along the midline and head, allowing emergence into water without disrupting . Terrestrial forms, in contrast, typically undergo air-mediated emergence, where the splits enable the animal to pull away from the exuvia in a drier setting, often involving behavioral maneuvers to avoid . A notable in certain crustaceans, like , involves premoult storage of in gastroliths—calcified structures in the —that are resorbed and recycled post-moult to rapidly harden the new . This entire process demands a substantial , often consuming a significant portion of the animal's reserves for production, secretion, and physical exertion. Post-moult, the soft-bodied phase heightens vulnerability to predation, as the new provides minimal protection until hardening begins. Failed moults, due to interruptions in hormonal signaling or environmental stress, can lead to entrapment in the old and death.

Sclerotization and hardening

Sclerotization, also known as , is the primary biochemical process that rigidifies the newly formed in arthropods following , primarily affecting the exocuticle layer. This involves the oxidative cross-linking of structural cuticular proteins with phenolic compounds derived from catechols, such as N-acetyldopamine () and N-β-alanyldopamine (NBAD), which are oxidized to reactive quinones by phenoloxidases like laccase-2. These quinones then form covalent bonds with , , and other residues in the proteins, as well as with microfibrils, thereby increasing mechanical rigidity and stability while reducing plasticity. The process is catalyzed enzymatically in the after , with precursors incorporated during the final stages of epidermal secretion. Two main types of sclerotization occur in : β-sclerotization, which produces stable, dark through irreversible quinone-mediated cross-links, and α-sclerotization, which involves reversible adducts that can lead to less rigid structures. β-sclerotization predominates in hard cuticles, contributing to their darkened appearance and durability, while α-sclerotization is less common and associated with transitional properties. In regions requiring elasticity, such as joints and certain membranes, full sclerotization is avoided; for instance, the elastic protein in these areas resists extensive cross-linking, maintaining flexibility without significant . This selective inhibition ensures functional specialization within the . In addition to sclerotization, mineralization contributes to hardening in many arthropods, particularly crustaceans, by incorporating ions from the into the cuticular matrix. predominates in the general exoskeleton, while calcium (Ca²⁺) and (PO₄³⁻) ions are actively transported across the post-moult and precipitate as crystals, often in amorphous or crystalline forms like , in specialized structures such as mandibles. This process rapidly strengthens affected regions, with ion levels peaking during early post-moult to support and . In , post-moult hardening via sclerotization typically completes within 24-48 hours after , during which significant water loss occurs, concentrating cuticular solids and resulting in approximately 50% reduction in overall volume. Water content in the fresh drops from around 75% to 31%, facilitating and compaction that enhances and rigidity. Flexible regions, such as wing veins, exhibit incomplete hardening, retaining higher and minimal cross-linking to preserve and prevent brittleness.

Evolutionary Aspects

Origins and development

The of arthropods first appears in the fossil record during the early period, approximately 520 million years ago (Ma), with well-preserved examples in trilobites such as those from the Chengjiang biota. These early forms exhibit a sclerotized composed primarily of , providing structural support and protection, and likely evolved from a softer, annelid-like ancestral that was permeable and less rigid. Phylogenetic analyses indicate that the fully arthrodized , characterized by segmental tergal and sternal plates, arose as an autapomorphy within Euarthropoda, the crown group encompassing modern arthropods and their relatives. Developmentally, the arthropod exoskeleton originates from the embryonic epidermis, where the cuticle begins to form during late embryogenesis through the secretion of chitin-protein matrices. This process involves epidermal invaginations that establish segmental boundaries and facilitate the deposition of cuticular layers, as observed in millipedes where invaginations coincide with segment addition and muscle reorganization. Genetically, Hox genes play a central role in regulating segment-specific sclerotization, directing the differential hardening and patterning of the exoskeleton along the anterior-posterior axis. Complementing this, segment-polarity genes such as engrailed and wingless define boundaries between sclerites by expressing in complementary ectodermal stripes, ensuring precise articulation and compartmentalization of the exoskeleton. Recent phylogenomic studies, leveraging genomic data from diverse euarthropod lineages, support a single evolutionary origin of the chitinous within Euarthropoda during the , with subsequent modifications in mineralization. As of 2025, advanced phylogenomic analyses continue to affirm this monophyletic origin and highlight genetic mechanisms underlying diversification. In particular, post-2020 analyses reveal that of the evolved independently in crustaceans through the recruitment of novel proteins for deposition, distinct from the unmineralized cuticles of chelicerates and myriapods. By the period, around 485–443 Ma, arthropod exoskeletons diversified further, with the evolution of tagmosis—the fusion of segments into functional units—enhancing escape responses to predation pressures from emerging predators. This adaptation is evidenced by fossilized malformations and repaired exoskeletal damage in trilobites, indicating intensified predator-prey interactions that drove sclerite reinforcement and regional specialization.

Diversity across arthropods

The exoskeleton of exhibits remarkable diversity across major taxonomic groups, reflecting adaptations to distinct ecological niches, lifestyles, and physiological demands. While all arthropod exoskeletons are fundamentally composed of and proteins, variations in sclerotization, mineralization, and flexibility enable specialized functions such as , , and . This diversity underscores the evolutionary divergence within the , with each group optimizing exoskeletal properties for terrestrial, , or semi-aquatic environments. In , the is heavily sclerotized through cross-linking of proteins, providing rigidity and strength essential for flight and terrestrial mobility; this is particularly evident in the hardened forewings (elytra) of , which serve as protective covers for hindwings and are formed by a thickened, sclerotized secreted by the wing epidermis. dominates the composition, forming microfibrils that reinforce the protein matrix and contribute to the overall toughness of the structure. Unlike larval stages, adult exhibit reduced or absent , as growth ceases post-metamorphosis, allowing the to persist without renewal and conserving energy for . Crustaceans, in contrast, feature a calcified exoskeleton enriched with (primarily or amorphous forms), which imparts exceptional hardness and supports their often aquatic or semi-aquatic habits; in decapods like and lobsters, this mineralized shell enhances structural integrity against predators and environmental pressures. The branchial chamber, housing the gills, is lined with a thin, non-mineralized deposited before to facilitate and without compromising flexibility in this respiratory region. Chelicerates, including arachnids such as spiders and scorpions, possess an uncalcified reliant on chitin-protein sclerotization for support, differing markedly from the mineralized crustacean form and allowing greater flexibility suited to terrestrial predation. Book lungs, integrated into the ventral as stacked lamellae, enable efficient oxygen uptake through direct , with the surrounding remaining pliable to accommodate respiratory movements. In ticks, the demonstrates exceptional flexibility, expanding dramatically during blood meals to increase body weight by up to 100-fold without , facilitated by unfolding of the pre-formed . Myriapods, encompassing centipedes and millipedes, have a relatively soft with minimal sclerotization and no , providing flexibility for burrowing and rapid movement in habitats compared to the rigid structures of . Their legs, while jointed, benefit from this softer chitinous covering, enabling agile without the brittleness seen in more hardened forms. occurs periodically throughout life, allowing continuous addition of body segments and legs, which supports in these elongate species. Recent studies have drawn inspiration from these variations, such as hybrid systems combining proteins with exoskeletal-like structures to create mm-scale soft exoskeletons for ; these designs mimic the and charge transport in , enabling reversible bending and compliant interactions with delicate environments like or webs. Additionally, poses emerging threats, with warming and reducing in aquatic arthropods—such as juvenile spiny lobsters—leading to softer, less mineralized s (e.g., 6-13% lower calcium content) that compromise predator defenses. In comparison, the of onychophorans ( worms), considered a softer precursor to arthropods, lacks rigidity and relies on a flexible, chitinous but non-sclerotized supported by a , highlighting the evolutionary transition toward hardened, jointed forms in arthropods.