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Exoskeleton

An exoskeleton is a hard, external covering that both supports and protects the body of certain animals, most notably arthropods such as insects, crustaceans, and arachnids, where it is composed primarily of chitin and provides structural integrity, muscle attachment sites, a barrier against desiccation, and sensory functions.^[1]^[2] In these organisms, the exoskeleton encases the body like a rigid armor, enabling efficient movement while constraining growth, which necessitates periodic molting to allow for expansion.^[3]

Definition and Characteristics

Definition

An exoskeleton is a rigid external covering that provides structural support, protection, and shape to the body of certain animals, primarily invertebrates, formed from materials secreted by the underlying epidermal cells. This structure contrasts with flexible integuments found in other organisms, offering a hardened framework that defines the organism's external form. In arthropods, the exoskeleton exhibits a layered architecture, consisting of an outer epicuticle, a middle exocuticle, and an inner endocuticle, which together form the procuticle beneath the epicuticle. These layers provide varying degrees of rigidity and flexibility, with the epicuticle serving as a thin, protective barrier and the procuticle contributing to the overall mechanical strength. The term "exoskeleton" derives from the Greek roots exo- ("outside") and skeletos ("dried up" or "mummified"), referring to a skeleton positioned externally. It was first introduced in a biological context during the 19th century, around 1841, by the English anatomist Richard Owen to describe hardened external structures in animals such as crustaceans.

Chemical Composition

Exoskeletons in arthropods are primarily composed of chitin, a long-chain polysaccharide derived from N-acetylglucosamine units, combined with sclerotized proteins that provide structural rigidity through phenolic cross-linking. In contrast, exoskeletons in mollusks and brachiopods consist mainly of calcium carbonate in the form of calcite or aragonite, often as low-magnesium calcite in brachiopods, with minor organic components including proteins and polysaccharides that facilitate biomineralization. Some diatoms possess silica-based frustules, which are exoskeletons composed of amorphous silica. The arthropod exoskeleton exhibits a layered structure that contributes to its material properties. The outermost epicuticle is a thin, acellular layer rich in waxes and lipids, serving to prevent water loss without containing chitin. Beneath it lies the exocuticle, where chitin microfibrils are embedded in a matrix of sclerotized proteins hardened via quinone tanning, enhancing mechanical strength. The innermost endocuticle provides flexibility through unhardened layers of parallel chitin fibers and proteins arranged in stacked laminae. Variations in exoskeleton composition arise from mineralization processes, such as the deposition of calcium carbonate crystals onto chitin scaffolds in crustaceans, often forming magnesian calcite or amorphous calcium phosphate phases that integrate with the organic matrix. In mollusks, the calcium carbonate is organized into distinct microstructural layers like nacre, influenced by acidic proteins that nucleate and orient crystal growth. These mineral-organic composites allow for tailored hardness and toughness across taxa.

Occurrence and Types

In Arthropods

Exoskeletons are a defining characteristic of the phylum Arthropoda, which encompasses over a million described species and represents the most diverse animal group on Earth, including the classes Insecta (insects), Crustacea (crustaceans), Arachnida (arachnids), and Myriapoda (myriapods).^[4] In all these groups, the exoskeleton forms the external covering that delineates body segments, jointed appendages, and specialized sensory structures such as antennae and setae, providing a rigid framework essential for their morphology and locomotion. This chitin-based structure, often reinforced with proteins and minerals, is secreted by the underlying epidermis and covers the entire body, enabling arthropods to thrive in diverse terrestrial, freshwater, and marine environments.^[5] A key adaptation of the arthropod exoskeleton is its jointed nature, which allows for flexible movement through articulated segments at the bases of appendages and between body regions, facilitating behaviors like walking, swimming, and grasping.^[1] Another prominent adaptation is tagmosis, the evolutionary fusion of primitive segments into functional body regions known as tagmata, which optimizes specialization; for instance, in insects, the head (cephalo) integrates sensory and feeding structures, the thorax supports locomotion, and the abdomen houses reproductive and digestive organs.^[6] In arachnids like spiders, tagmosis results in a prosoma (cephalothorax) for sensing and chelicerae, and an opisthosoma for visceral functions, enhancing efficiency in predation and web-building. Representative examples illustrate these adaptations' versatility. In insects such as dragonflies, the lightweight, sclerotized cuticle of the exoskeleton forms rigid wing bases that articulate with the thorax, enabling powered flight through rapid oscillations while maintaining structural integrity against aerodynamic forces.^[7] Among crustaceans, the carapace—a dorsal shield of the exoskeleton in species like crabs and lobsters—often incorporates calcium carbonate for added hardness, providing streamlined protection and buoyancy support in aquatic habitats where it shields gills and reduces drag during swimming.^[4] Molting frequency varies by life stage and species; juvenile lobsters may molt up to 10 times in their first year, while adults typically molt annually to accommodate growth in marine conditions.^[8]

In Other Invertebrates

Exoskeletons in non-arthropod invertebrates are generally less common and simpler in structure compared to the segmented, chitin-based full-body coverings typical of arthropods, often manifesting as protective shells or minimal cuticular layers rather than comprehensive skeletal supports.^[9] These structures primarily serve defensive roles, with mineralized compositions like calcium carbonate in many cases providing rigidity against predation and environmental stresses.^[10] In mollusks, exoskeletons take the form of hard shells secreted by the mantle, composed mainly of calcium carbonate in crystalline forms such as calcite or aragonite. Bivalves, for instance, possess two-valved shells that enclose the soft body, offering robust protection while allowing filter-feeding.^[10] Chitons exhibit a distinctive shell arrangement of eight overlapping calcareous plates, which provide flexibility for navigating rocky substrates and can be used to clamp tightly against surfaces for defense.^[10] Gastropods often feature a single coiled shell, supplemented in many species by an operculum—a chitinous or calcified plate attached to the foot that seals the shell aperture when the animal retracts, preventing desiccation and deterring predators.^[11] Brachiopods, marine invertebrates unrelated to mollusks, develop bivalved shells that function similarly as exoskeletons, with inarticulate forms typically phosphatic and articulate forms calcareous. These shells, secreted by the mantle, encase the lophophore feeding structure and provide anchorage in benthic environments.^[12]^[13] Annelids possess a thin chitinous cuticle covering the body, but their exoskeletal elements are limited to chitinous setae or chaetae—bristle-like projections from each segment that aid in locomotion and anchoring without forming a rigid framework.^[9] Tardigrades, microscopic extremophiles, are covered by a thin, flexible cuticle composed of chitin and proteins, serving as a lightweight exoskeleton that accommodates their ability to enter cryptobiosis under harsh conditions.^[14]

Functions

Mechanical Support

The exoskeleton in arthropods serves as a rigid framework that facilitates mechanical support by providing attachment sites for muscles, enabling efficient force transmission and leverage during movement. Muscles attach directly to the inner surface of the exoskeleton or to internal invaginations known as apodemes, which act as tendon-like structures to amplify mechanical advantage. This arrangement allows arthropods to generate substantial forces relative to their body size; for instance, in ants, the exoskeleton's rigid chitinous structure combined with apodeme attachments enables lifting capacities up to 50 times their body weight through optimized limb geometry and joint design.^[15]^[16] In locomotion, the segmented and jointed nature of the exoskeleton permits articulated movement, particularly in insects where flexible joints in legs allow for rapid, precise stepping and jumping. The exoskeleton's sclerotized regions provide the necessary rigidity to bear body weight and transmit propulsive forces, with biomechanical properties varying by species—soft cuticles exhibit a Young's modulus of about 1 kPa for flexibility, while heavily sclerotized parts reach up to 20 GPa for load-bearing stability. In larger crustaceans, such as crabs, high mineralization with calcium carbonate enhances exoskeletal rigidity, supporting weight-bearing during terrestrial or aquatic locomotion without deformation under compressive strengths of approximately 50-60 MPa in claw regions.^[17]^[18] Biomechanical principles underlying this support involve stress distribution across exoskeletal segments to prevent localized failure. In arthropods, the exoskeleton's multi-layered structure—comprising epicuticle, exocuticle, and endocuticle—distributes tensile and compressive stresses effectively, with specialized organs like struts at joints reducing peak stresses by approximately 95% as demonstrated in finite element models of crab merus-carpus articulations. This segmental design acts like a series of lever arms at joints, optimizing torque for posture and gait without requiring internal hydrostatic reinforcement in rigid phases. In soft-bodied contexts, such as post-molt crustaceans, hydrostatic pressure within fluid-filled body cavities supplements exoskeletal support, providing temporary rigidity with stiffness values in the range of 100-300 MPa to maintain posture until sclerotization completes.^[19]^[20]

Protection

Exoskeletons serve as a primary physical barrier against predators by forming a rigid, chitin-based armor that resists penetration and crushing forces. In arthropods, this structure effectively deters attacks from vertebrates and invertebrates, as the tough cuticle prevents many predators from accessing internal tissues. For instance, the exoskeleton of insects and crustaceans provides robust defense, often rendering bites or stings ineffective due to its layered composition.^[21]^[22]^[23] This barrier also shields against environmental stressors, including desiccation and ultraviolet (UV) radiation. The waxy epicuticle layer in terrestrial arthropods minimizes water loss, enabling survival in arid conditions by reducing transpiration through the cuticle. Additionally, the exoskeleton absorbs or blocks UV rays, protecting underlying tissues from radiation damage, as seen in planktonic crustaceans like Daphnia where the shell blocks up to 35% of UV light.^[24]^[25]^[26] Camouflage through coloration in insect cuticles further enhances protection by reducing visibility to predators. Pigments and structural colors in the cuticle, such as those in leafhoppers, provide adaptive blending with backgrounds, while reversible color changes in some arthropods allow quick adjustments to environmental cues for concealment.^[27] Armor-like features amplify defensive capabilities, including spines in crustaceans that deter grasping or biting by predators. In species like the crab Pachygrapsus crassipes, needle-like spines on the exoskeleton create a hazardous surface, complicating attacks. Mollusks such as chitons exhibit thickened shells composed of aragonite plates, offering enhanced resistance to abrasion and predation through their multilayered, flexible design.^[28]^[29] Following damage, exoskeletons demonstrate regenerative potential, restoring structural integrity post-injury. In insects like locusts, wound healing processes nearly double the mechanical strength of the cuticle after incisions, achieving up to 66% of original resilience. Immature arthropods can regenerate lost appendages over multiple molts, minimizing long-term vulnerability.^[30]^[22] Environmental adaptations tailor exoskeletons for specific habitats, such as waterproofing in terrestrial forms via lipid-rich epicuticles that prevent desiccation in low-humidity environments. In deep-sea arthropods, like hydrothermal vent crustaceans, the mineralized exoskeleton withstands extreme hydrostatic pressures, maintaining integrity under conditions exceeding 100 atmospheres. Chemical hardening processes, involving sclerotization of the procuticle, contribute to these adaptive strengths.^[31]^[32]

Sensory and Other Roles

In arthropods, the exoskeleton provides attachment sites for sensory appendages such as antennae, which are equipped with mechanoreceptors and chemosensors to detect environmental stimuli like touch, chemicals, and air currents.^[33] Additionally, the exoskeleton is embedded with setae, hair-like structures that function as mechanoreceptors; these innervated sensilla detect vibrations, airflow, and tactile inputs, enabling rapid sensory feedback for navigation and predator avoidance.^[34] In some mollusks with calcareous shells serving as exoskeletons, statocysts act as gravity-sensing organs; these spherical structures contain statoliths that stimulate ciliated mechanoreceptor cells in the cyst wall, facilitating balance and orientation in aquatic environments.^[35] The exoskeleton also contributes to physiological regulation, particularly osmoregulation, through the low permeability of its outer epicuticle layer, which impedes water and ion loss in terrestrial and semi-terrestrial arthropods, maintaining internal homeostasis in varying salinities.^[36] Furthermore, pigmentation in the exoskeleton plays roles in thermoregulation and signaling; darker melanized cuticles in insects like beetles absorb solar radiation to elevate body temperature for enhanced metabolic activity in cool conditions, while iridescent or aposematic colors serve as visual signals for mate attraction or warning predators of toxicity.^[37] Metabolically, the exoskeleton supports enzyme-mediated processes, notably through phenoloxidases such as tyrosinase and laccase located beneath the epicuticle; these enzymes catalyze sclerotization by oxidizing phenols to form cross-links in the procuticle, while also contributing to detoxification via melanin production that encapsulates pathogens or neutralizes toxins during immune responses.^[38]

Growth and Maintenance

Molting Process

The molting process, or ecdysis, enables arthropods to renew their exoskeleton for growth, consisting of three primary stages: pre-molt (proecdysis or apolysis), ecdysis (shedding), and post-molt (metecdysis or sclerotization).^[39]^[40] During the pre-molt stage, the epidermis detaches from the old cuticle through apolysis, triggered by rising levels of ecdysteroids such as ecdysone, which stimulate epidermal cells to secrete a new, soft cuticle beneath the old one while enzymes digest and recycle components of the existing exoskeleton.^[41]^[42] This stage prepares the animal for expansion and typically lasts days to weeks, depending on species and environmental factors.^[43] Ecdysis marks the active shedding of the old exoskeleton, coordinated by a cascade of neuropeptides including ecdysis-triggering hormone (ETH) released from Inka cells, which initiates pre-ecdysic behaviors like air swallowing for body expansion and peristaltic movements to break the weakened seams of the old cuticle.^[42]^[40] Juvenile hormone, secreted by the corpora allata, modulates the nature of the upcoming molt—promoting larval-to-larval transitions in juveniles while suppressing metamorphic changes until the final instar—but does not directly control the shedding mechanics.^[44]^[41] The entire hormonal regulation ensures precise timing, with ecdysteroid peaks driving apolysis and subsequent declines signaling ETH release for ecdysis.^[42] In the post-molt stage, the newly expanded soft cuticle undergoes sclerotization, a tanning process involving phenolic compounds that cross-link proteins and chitin, restoring rigidity and impermeability over hours to days.^[39]^[45] This phase briefly references chitin remodeling, where the polysaccharide is reorganized into the layered structure of the new exoskeleton. Molting imposes substantial energy costs, as arthropods often cease feeding and draw on lipid and protein reserves to fuel the high metabolic demands of cuticle synthesis and resorption, potentially elevating oxygen consumption by approximately 40% in some insect species during pre-molt.^[46] The post-molt vulnerability is acute, with the pliable new exoskeleton offering minimal protection against predators or mechanical damage until full hardening occurs, making this a high-risk period that influences behavioral strategies like burrowing or nocturnal activity.^[42]^[43] Molting frequency varies widely across arthropods; for instance, insect larvae undergo frequent molts—typically 4 to 8 times during development to accommodate rapid growth—while adults generally cease molting after reaching maturity.^[47] In contrast, crustaceans like crabs exhibit higher frequency in juveniles (often 10 or more molts) that declines in adults to once per year or less, balancing growth with reproductive demands.^[48]^[49]

Size Limitations

The primary physical constraint on the size of organisms with exoskeletons arises from the square-cube law, which dictates that as linear dimensions scale up, surface area increases with the square of the scaling factor while volume—and thus mass—increases with the cube.^[50] This disparity means that the exoskeleton's structural strength, dependent on its cross-sectional area, cannot adequately support the disproportionately heavier body weight in larger terrestrial forms, leading to mechanical failure risks such as buckling under compressive loads.^[51] For arthropods, this scaling issue is exacerbated by the rigid, chitin-based exoskeleton, which must bear the full load without internal skeletal reinforcement, effectively capping maximum body sizes for land-dwellers.^[52] Historical evidence illustrates these limits starkly: the Carboniferous millipede Arthropleura, the largest known arthropod, reached lengths of approximately 2.63 meters, but such giants were exceptional and tied to environmental factors like elevated atmospheric oxygen levels that mitigated respiratory constraints alongside mechanical ones.^[53] In contrast, modern terrestrial arthropods rarely exceed much smaller dimensions; the coconut crab (Birgus latro), the largest extant land arthropod, achieves a leg span of up to 1 meter and a body weight of about 4 kilograms, representing a practical upper bound for exoskeleton-supported terrestrial locomotion under current conditions.^[54] These size disparities highlight how exoskeletal rigidity, combined with scaling physics, prevents arthropods from evolving to the scale of large vertebrates. Certain adaptations partially circumvent these limitations in specific habitats. In aquatic environments, buoyancy counteracts gravitational forces, reducing the effective weight borne by the exoskeleton and enabling larger body sizes, as seen in extinct eurypterids like Jaekelopterus that approached 2.5 meters in length.^[52] For flying insects, the exoskeleton incorporates lightweight, porous chitin structures optimized for minimal mass while maintaining rigidity, allowing species like the Goliath beetle to reach 15 centimeters in length without compromising aerial mobility.^[51] Growth via periodic molting further enables incremental size increases within these bounds, though it becomes increasingly risky at larger scales due to vulnerability during the soft-bodied phase.^[55]

Evolutionary Perspectives

Origins and Development

The exoskeleton first emerged during the Cambrian explosion approximately 540 million years ago, marking a pivotal transition from soft-bodied ancestors to armored forms among early metazoans.^[56] This rapid diversification is associated with the evolution of segmented body plans, where Hox genes played a crucial role in specifying anterior-posterior identities and promoting tagmosis, the fusion of segments into functional units.^[57] These genetic mechanisms, conserved across arthropods, facilitated the development of a modular architecture that supported the integration of protective external structures.^[58] In arthropods, the exoskeleton develops through secretion by underlying epidermal cells, which deposit layers of chitinous cuticle that harden to form the rigid outer framework.^[59] Genetic regulation of this process involves segment-polarity genes, such as engrailed, which define boundaries between segments by expressing in posterior compartments, ensuring precise patterning of the cuticle along the body axis.^[60] This developmental pathway allows for coordinated growth and differentiation, with engrailed expression marking sites where limb primordia and cuticular folds initiate.^[58] Early metazoan exoskeletons transitioned from flexible, organic cuticles—primarily composed of chitin—to mineralized shells through the incorporation of calcium carbonate or phosphate within the protein-chitin matrix, enhancing mechanical strength in response to ecological pressures.^[61] This mineralization likely evolved independently in multiple lineages, building upon the foundational flexible cuticle to support larger body sizes and more complex locomotion in aquatic environments.^[62]

Paleontological Record

The paleontological record of exoskeletons is exceptionally rich due to the durability of their hard, often mineralized structures, which resist decay and facilitate preservation in sedimentary rocks.^[63] Arthropod exoskeletons, composed primarily of chitin reinforced with calcium carbonate in many cases, biomineralize to form calcite or other minerals that enhance fossilization potential.^[64] For instance, trilobite exoskeletons from the Ordovician period (approximately 485–443 million years ago) are among the most abundant and well-preserved fossils, providing detailed insights into early arthropod morphology and diversity.^[65] Exceptional fossil deposits reveal the early diversification of exoskeleton-bearing arthropods during the Cambrian period. The Burgess Shale Formation in Canada, dating to about 508 million years ago, preserves a wide array of arthropods with intact exoskeletons, including trilobites and stem-group forms like Marrella, illustrating the rapid evolutionary radiation of euarthropods shortly after their Cambrian origins.^[56] In the Devonian period (419–359 million years ago), crustacean fossils such as Oxyuropoda from Irish floodplains demonstrate early adaptations of exoskeletons to freshwater environments, with calcified structures aiding preservation.^[66] The fossil record of exoskeletons underscores their evolutionary significance in response to environmental changes. Rising oxygenation levels during the Ediacaran-Cambrian transition (around 541–521 million years ago) likely facilitated the synthesis of chitin-based exoskeletons by enabling aerobic metabolism in larger, more active arthropods.^[67] Mass extinctions profoundly affected exoskeleton bearers; for example, the end-Permian event (252 million years ago) wiped out trilobites entirely, eliminating over 90% of marine arthropod species and reshaping Paleozoic ecosystems.^[68]

Comparisons and Biomechanics

Versus Endoskeleton

Exoskeletons and endoskeletons represent two fundamental designs for skeletal support in animals, differing primarily in their positioning and structural coverage. An exoskeleton is an external, rigid framework that encases the entire body, often composed of chitin or calcium-based materials, providing a continuous outer shell that interfaces directly with the environment.^[69] In contrast, an endoskeleton is an internal system of localized, hard elements such as bone or cartilage, embedded within soft tissues to form a supportive core that allows for muscle attachment and organ protection without full-body encasement.^[69] These designs reflect adaptations to diverse body plans, with exoskeletons enabling precise segmentation for jointed appendages in many invertebrates, while endoskeletons facilitate flexible, growth-accommodating frameworks in vertebrates.^[70] In terms of occurrence, exoskeletons predominate among invertebrates, particularly in the phylum Arthropoda, where they characterize over 85% of all known animal species, encompassing vast numbers of small-bodied organisms like insects, spiders, and crustaceans.^[4] Endoskeletons, however, are primarily found in chordates, especially vertebrates, which represent a much smaller fraction of animal diversity, with around 66,000 described species relying on internal bony or cartilaginous structures for support.^[71] This distribution underscores the prevalence of external skeletal designs in the majority of animal life, driven by the evolutionary success of arthropod-like body plans in terrestrial and aquatic niches.^[72] Certain animal groups exhibit hybrid skeletal features, blending elements of both systems. For instance, echinoderms such as sea stars and sea urchins possess an endoskeleton composed of calcareous ossicles embedded just beneath the thin epidermal layer, creating a structure that is internal yet superficially positioned, akin to a subdermal exoskeleton in function and proximity to the surface.^[73] This arrangement provides rigid support while maintaining a flexible outer covering, distinguishing it from the fully external exoskeletons of arthropods or the deeply internalized bones of vertebrates.^[74]

Advantages and Disadvantages

Exoskeletons offer several key advantages, particularly for smaller organisms. Upon formation following molting, the new exoskeleton provides immediate and robust protection against physical damage, predators, and environmental stressors, as the freshly secreted chitinous layers harden rapidly to form a durable barrier.^[21] This structure is also lightweight due to its composition primarily of chitin, a polymer that delivers high strength relative to weight, enabling efficient locomotion and muscle leverage in small-bodied arthropods without excessive energetic costs.^[75] Additionally, the molting process facilitates repair by allowing the complete replacement of damaged or worn cuticle, regenerating lost appendages or healing injuries through the secretion of a new integument.^[76] Despite these benefits, exoskeletons present notable disadvantages related to growth and structural constraints. Expansion requires periodic shedding of the rigid outer layer, a process known as molting or ecdysis, during which the animal becomes soft-bodied and highly vulnerable to predation and injury for several hours or days until the new exoskeleton sclerotizes.^[77] This intermittency inherently limits body size, as the exoskeleton's thickness must scale disproportionately with mass to provide adequate support, eventually rendering it impractical for larger forms due to prohibitive weight and metabolic demands.^[78] For instance, terrestrial arthropods rarely exceed a few kilograms; the largest, the coconut crab (Birgus latro), weighs up to 4 kg.^[79] In evolutionary contexts, these traits yield specific trade-offs suited to certain lifestyles. Exoskeletons excel in facilitating rapid reproduction among insects, where small size and quick molting cycles support short generation times and high fecundity, contributing to their ecological dominance.^[80] Conversely, the rigidity and molting demands reduce adaptability for achieving the mobility and endurance seen in larger vertebrates, favoring exoskeletal designs primarily in compact, high-turnover taxa rather than expansive, agile forms.^[21]

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Sep 2, 2020 · Insect colours assist in body protection, signalling, and physiological adaptations. Colours also convey multiple channels of information.
[39]
[PDF] Design of a Multifunctional Biomineralized Armor System: The Shell ...
This thesis explores multiscale material and morphological design principles of the shells of chitons (Mollusca: Polyplacophora). The chiton shell consists of ...
[40]
Bioinspired design of flexible armor based on chiton scales - PMC
Dec 10, 2019 · In addition to the eight primary shell plates, protection is provided by the scaled girdle, which runs along the perimeter of the entire body ( ...Missing: thickening | Show results with:thickening
[41]
Bridging the gap: wound healing in insects restores mechanical ...
We show that after an incision, a healing process occurred which almost doubled the mechanical strength of locust tibial cuticle, restoring it to 66% of the ...
[42]
Water-Proofing Properties of Cuticular Lipids - Academia.edu
Epicuticular lipids play a critical role in allowing arthropods to thrive in terrestrial environments, by reducing transpiration of water through the cuticle.
[43]
Thermal Resistance in Deep-Sea Hydrothermal Vent Crustaceans
Nov 21, 2024 · was shown to have an exoskeleton highly resistant to temperature and pressure [10]. It suggests that the exoskeletons of hydrothermal vent ...
[44]
Arthropod - Nerves, Sensory Organs - Britannica
The sensory nerve endings are lodged in cuticular hairs (setae), peglike projections, cones, pits, or slits, which may occur in large numbers on antennae, ...
[45]
Biomechanics in Soft Mechanical Sensing: From Natural Case ...
Oct 24, 2018 · Mechanosensory sensilla of arthropods can be subdivided into tactile hairs and trichobothria, which function as medium flow sensors [45] (Figure ...
[46]
Function of Molluscan Statocysts - SpringerLink
The gravity sensors of most molluscs are spherical organs called statocysts. The wall of the sphere contains mechanosensory cells whose sensory cilia project ...Missing: exoskeleton | Show results with:exoskeleton
[47]
Advantages of being a dark beetle – Insect colouration ... - Ecography
Sep 2, 2016 · The thermal benefit of melanism is based on the fact, that darker individuals heat up faster and reach higher body temperatures than light ...Missing: exoskeleton | Show results with:exoskeleton
[48]
[PDF] The immune role of the arthropod exoskeleton
Nov 17, 2012 · Sclerotisation is allowed by the presence, beneath the epicuticle, of enzymes including phenoloxidases such as laccase and tyrosinase. These ...
[49]
Molting - an overview | ScienceDirect Topics
Molting is a multistage process which can be divided into four major stages, postmolt (postecdysis), intermolt, premolt (proecdysis) and molt (ecdysis) (Dall et ...<|separator|>
[50]
Ancient origins of arthropod moulting pathway components - PMC
Arthropods shed their exoskeletons as they grow, a process called ecdysis or moulting, and this behaviour is controlled by a set of hormones and small protein- ...
[51]
Evolution of Ecdysis and Metamorphosis in Arthropods
Jun 22, 2015 · Ecdysteroids, such as ecdysone in insects, are commonly known as the molting hormones that control the timing of ecdysis.
[52]
The ecdysis triggering hormone system is essential for successful ...
Apr 18, 2017 · Moulting entails the production of a new exoskeleton and shedding of the old one and is induced by a pulse in ecdysteroids, which activates a ...
[53]
Ecdysis - an overview | ScienceDirect Topics
The life of a decapod consists of alternating periods of premolt, molt, postmolt, and intermolt; all of these phases are controlled by hormones (endocrines).
[54]
Hormonal Control of Molting & Metamorphosis - ENT 425 - NC State
In immature insects, juvenile hormone is secreted by the corpora allata prior to each molt. This hormone inhibits the genes that promote development of adult ...
[55]
The moulting arthropod: a complete genetic toolkit review
Jul 22, 2024 · Exoskeletons are a defining character of all arthropods that provide physical support for their segmented bodies and appendages as well as ...
[56]
Vital Carbohydrate and Lipid Metabolites in Serum Involved ... - MDPI
Nov 10, 1989 · Since fasting occurs during molting, which requires a large supply of energy, internal energy reserves are critical. However, the dynamics of ...Missing: costs vulnerability
[57]
New Insight Into the Molting and Growth in Crustaceans - Frontiers
Molting, a critical biological process in crustaceans that requires a large amount of energy to absorb water and swell the exoskeleton, is highly related to ...
[58]
Morphogenesis – ENT 425 – General Entomology
The insect sheds its old exoskeleton (ecdysis) and continues to fully expand the new one. Over the next few hours, sclerites will harden and darken as quinone ...Molting · Metamorphosis · Larval Types
[59]
Arthropod - Exoskeleton, Segmentation, Jointed Appendages
Oct 1, 2025 · The exoskeleton is composed of a thin, outer protein layer, the epicuticle, and a thick, inner, chitin–protein layer, the procuticle.
[60]
Growth, molting duration and carapace hardness of blue swimming ...
In portunid crabs, larvae molt four to six times from zoea to megalopa stage (Z to M stage) and juveniles molt more than ten times from instar stage (C stage), ...Growth, Molting Duration And... · 2. Material And Methods · 3. ResultsMissing: insects | Show results with:insects
[61]
Exoskeleton strength: The physics of failure - Understanding Evolution
Exoskeleton strength is discussed in the context of how scaling up arthropods affects the exoskeleton and muscles, and how molting may limit size.Missing: square cube<|control11|><|separator|>
[62]
Shape optimization in exoskeletons and endoskeletons - NIH
Sep 12, 2012 · This paper addresses the question of strength and mechanical failure in exoskeletons and endoskeletons. We developed a new, more sophisticated model to predict ...Missing: leverage | Show results with:leverage
[63]
Aquatic versus terrestrial crab skeletal support: morphology ...
Oct 29, 2018 · Constraints for arthropod body size and locomotion on land are typically perceived as mechanical limitations of a rigid exoskeleton; however ...
[64]
The largest arthropod in Earth history: insights from newly ...
Dec 21, 2021 · The remains represent 12–14 anterior Arthropleura tergites in the form of a partially sand-filled dorsal exoskeleton. The original organism is ...
[65]
Coconut crabs: the bird-eating behemoths thriving on isolated ...
When scaled up for a fully grown, four-kilogram coconut crab, the maximum force would be around 3,300 newtons. Coconut crabs can also lift up to about 30 ...
[66]
An evolutionary constraint: Small size
### Summary of Constraints on Arthropod Size Due to Exoskeleton
[67]
Early fossil record of Euarthropoda and the Cambrian Explosion
May 21, 2018 · Fossil Konservat-Lagerstätten, such as Burgess Shale-type deposits (BSTs), show the evolution of the euarthropod stem lineage during the Cambrian from 518 ...Early Fossil Record Of... · Microscopic Fossils Of... · Trilobites Appear Earlier...
[68]
Hox genes and the evolution of the arthropod body plan - PubMed
Hox genes' expression patterns are analyzed to understand the mechanisms in the evolution of the arthropod body plan. The review summarizes data on 10 ...
[69]
Arthropods: Developmental diversity within a (super) phylum - PNAS
It is not yet clear when in arthropod evolution the Hox3 and ftz-related genes acquired the new functions in embryonic patterning seen in higher insects, or ...Arthropods: Developmental... · Hox Genes, Tagmosis, And... · Hox Genes And Segment...
[70]
How the Ecdysozoan Changed Its Coat - PMC - NIH
Oct 11, 2005 · A new exoskeleton is then secreted by the epidermis, but is soft until the remains of the overlying old cuticle are shed at ecdysis. The new ...
[71]
Arthropod segmentation | Development | The Company of Biologists
Sep 25, 2019 · Parasegment boundaries are established during embryogenesis by 'segment-polarity' genes, such as engrailed and wingless, which are expressed in ...
[72]
Escalation and ecological selectively of mineralogy in the Cambrian ...
Biomineralised hard parts appeared in protists at ~ 750–812 Ma, metazoans at ~ 550 Ma, and macroalgae by ~ 515 Ma, but the major radiation of skeletons is ...
[73]
Fossil Arthropods - Trilobites, Insects and Spiders, and Others (U.S. ...
Oct 25, 2024 · Trilobites are perhaps the most famous form of fossil arthropod. These marine animals had a hard exoskeleton that was prone to fossilization, ...
[74]
The Lifestyles of the Trilobites | American Scientist
We have a good fossil record of them because they were the first arthropods to secrete a hard exoskeleton made of the mineral calcite. Not that the whole ...
[75]
Trilobites - British Geological Survey
Trilobites appeared in the Cambrian Period and became extinct at the end of the Permian Period. In Britain, trilobites occur in rocks of Cambrian, Ordovician ...
[76]
The oldest peracarid crustacean reveals a Late Devonian freshwater ...
Here, we provide a complete re-analysis of a fossil arthropod—Oxyuropoda—reported in 1908 from the Late Devonian floodplains of Ireland, and left with ...Missing: exoskeletons | Show results with:exoskeletons<|control11|><|separator|>
[77]
Oxygen, ecology, and the Cambrian radiation of animals - PMC
Jul 29, 2013 · We explore the effect of low oxygen levels on the feeding ecology of polychaetes, the dominant macrofaunal animals in deep-sea sediments.
[78]
What were trilobites? | Oxford University Museum of Natural History
Trilobites are a group of extinct marine arthropods that first appeared around 521 million years ago, shortly after the beginning of the Cambrian period.
[79]
Motor units and skeletal systems | Organismal Biology
An exoskeleton is an external skeleton that consists of a hard encasement on the surface of an organism. Exoskeletons provide defense against predators, support ...
[80]
Types of Skeletal Systems - OpenEd CUNY
An exoskeleton is a hard external skeleton that protects the outer surface of an organism and enables movement through muscles attached on the inside. An ...Missing: definition | Show results with:definition
[81]
Problem of Estimating Exact Numbers of Arthropods Bugs ...
“One estimate indicates that arthropods have 1,170,000 described species and account for over 80% of all known living animal species. Another study ...
[82]
Echinodermata: Morphology
Echinoderm skeletons are made up of interlocking calcium carbonate plates and spines. This skeleton is enclosed by the epidermis and is thus an endoskeleton.
[83]
Phylum Echinodermata | manoa.hawaii.edu/ExploringOurFluidEarth
The sea star's skeleton, like the sea urchin's, is an endoskeleton consisting of small plates of calcium carbonate embedded in the epidermis. These plates, ...
[84]
[PDF] Teaching Guide to Exoskeletons
Exoskeletons are more efficient with a smaller body size. From an engineer's POV an insect is a set of hollow tubes. For the weight, a tube is more resistant ...Missing: lightweight advantages
[85]
Damage, repair and regeneration in insect cuticle: The story so far ...
Insects have to molt their exoskeleton to grow. The mechanical properties of the exoskeleton material change during this process (e.g., Wigglesworth, 1948; ...<|separator|>
[86]
Phylum Arthropoda | CK-12 Foundation
Their exoskeleton provides protection, structural support, and reduces water loss, but requires moulting (ecdysis) for growth. Arthropods have jointed ...
[87]
Reasons for Success - ENT 425 - NC State University
As a “suit of armor”, the exoskeleton can resist both physical and chemical attack. It is covered by an impervious layer of wax that prevents desiccation. Much ...