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58-439: Examples of exoskeletons in animals include the cuticle skeletons shared by arthropods ( insects , chelicerates , myriapods and crustaceans ) and tardigrades , as well as the skeletal cups formed by hardened secretion of stony corals , the test /tunic of sea squirts and sea urchins , and the prominent mollusc shell shared by snails , clams , tusk shells , chitons and nautilus . Some vertebrate animals, such as

116-500: A furan ring and the molecule is oxidized further to form azadirone and azadiradione . The third ring is then opened and oxidized to form the C-seco-limonoids such as nimbin , nimbidinin and salannin , which has been esterified with a molecule of tiglic acid , which is derived from L-isoleucine . It is currently proposed that the target molecule is arrived at by biosynthetically converting azadirone into salanin, which

174-401: A C 27 steroid. Tirucallol undergoes an allylic isomerization to form butyrospermol , which is then oxidized. The oxidized butyrospermol subsequently rearranges via a Wagner-Meerwein 1,2-methyl shift to form apotirucallol . Apotirucallol becomes a tetranortriterpenoid when the four terminal carbons from the side chain are cleaved off. The remaining carbons on the side chain cyclize to form

232-484: A calcified exoskeleton, but mineralized skeletons did not become common until the beginning of the Cambrian period, with the rise of the " small shelly fauna ". Just after the base of the Cambrian, these miniature fossils become diverse and abundant – this abruptness may be an illusion since the chemical conditions which preserved the small shells appeared at the same time. Most other shell-forming organisms appeared during

290-430: A calcified exoskeleton. Some Cloudina shells even show evidence of predation, in the form of borings. The fossil record primarily contains mineralized exoskeletons, since these are by far the most durable. Since most lineages with exoskeletons are thought to have started with a non-mineralized exoskeleton which they later mineralized, it is difficult to comment on the very early evolution of each lineage's exoskeleton. It

348-404: A common misconception, echinoderms do not possess an exoskeleton and their test is always contained within a layer of living tissue. Exoskeletons have evolved independently many times; 18 lineages evolved calcified exoskeletons alone. Further, other lineages have produced tough outer coatings, such as some mammals, that are analogous to an exoskeleton. This coating is constructed from bone in

406-481: A commonly used biological pesticide, Bacillus thuringiensis . Azadirachtin fulfills many of the criteria needed for a good insecticide . Azadirachtin is biodegradable (it degrades within 100 hours when exposed to light and water) and shows very low toxicity to mammals (the LD 50 in rats is > 3,540 mg/kg making it practically non-toxic). This compound is found in the seeds (0.2 to 0.8 percent by weight) of

464-479: A complex development cycle of metamorphosis in which young animals may be totally different from older phases, such as the nauplius larvae of crustaceans, the nymphs of say, the Odonata , or the larvae of Endopterygota , such as maggots of flies. Such larval stages commonly have ecological and life cycle roles totally different from those of the mature animals. Secondly, often a major injury in one phase, such as

522-538: Is a secondary metabolite present in neem seeds. It is a highly oxidized tetranortriterpenoid which boasts a plethora of oxygen-bearing functional groups, including an enol ether , acetal, hemiacetal , tetra-substituted epoxide and a variety of carboxylic esters . Azadirachtin has a complex molecular structure; it presents both secondary and tertiary hydroxyl groups and a tetrahydrofuran ether in its molecular structure , alongside 16 stereogenic centres, 7 of which are tetrasubstituted. These characteristics explain

580-502: Is a component of a complex matrix of materials. It practically always is associated with protein molecules that often are in a more or less sclerotised state, stiffened or hardened by cross-linking and by linkage to other molecules in the matrix. In some groups of animals, most conspicuously the Crustacea , the matrix is greatly enriched with, or even dominated by, hard minerals, usually calcite or similar carbonates that form much of

638-400: Is a laminated structure of layers of interwoven fibrous chitin and protein molecules, while the exocuticle is the layer in which any major thickening, armouring and biomineralization occurs. Biomineralization with calcite is particularly common in Crustacea , whereas sclerotization particularly occurs in insects . The exocuticle is greatly reduced in many soft-bodied insects, especially in

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696-405: Is a multi-layered structure with four functional regions: epicuticle , procuticle , epidermis and basement membrane . Of these, the epicuticle is a multi-layered external barrier that, especially in terrestrial arthropods, acts as a barrier against desiccation . The strength of the exoskeleton is provided by the underlying procuticle , which is in turn secreted by the epithelial cells in

754-446: Is called the sternum , which commonly bears sternites . The two lateral regions are called the pleura (singular pleurum) and any sclerites they bear are called pleurites. The arthropod exoskeleton is divided into different functional units, each comprising a series of grouped segments; such a group is called a tagma , and the tagmata are adapted to different functions in a given arthropod body. For example, tagmata of insects include

812-486: Is eventually hardened. In contrast, moulting reptiles shed only the outer layer of skin and often exhibit indeterminate growth. These animals produce new skin and integuments throughout their life, replacing them according to growth. Arthropod growth, however, is limited by the space within its current exoskeleton. Failure to shed the exoskeleton once outgrown can result in the animal's death or prevent subadults from reaching maturity, thus preventing them from reproducing. This

870-458: Is found in some of the earliest fossil molluscs; but it also has armour plates on the sides of its foot, and these are mineralised with the iron sulfides pyrite and greigite , which had never previously been found in any metazoan but whose ingredients are emitted in large quantities by the vents. Arthropod exoskeleton Arthropods are covered with a tough, resilient integument , cuticle or exoskeleton of chitin . Generally

928-420: Is generally mixed at a rate of 1 US fluid ounce per US gallon (7.8 mL/L) of water when used as a pesticide. Azadirachtin is the active ingredient in many pesticides including TreeAzin, AzaMax, BioNEEM, AzaGuard, and AzaSol, Terramera Proof and Terramera Cirkil. Azadirachtin has a synergistic effect with the biocontrol agent Beauveria . Nimbecidine is a natural product insecticide mix which

986-542: Is in creating the proteins and polysaccharides required for the shell's composite structure , not in the precipitation of the mineral components. Skeletonization also appeared at almost the same time that animals started burrowing to avoid predation, and one of the earliest exoskeletons was made of glued-together mineral flakes, suggesting that skeletonization was likewise a response to increased pressure from predators. Ocean chemistry may also control which mineral shells are constructed of. Calcium carbonate has two forms,

1044-599: Is known, however, that in a very short course of time, just before the Cambrian period, exoskeletons made of various materials – silica, calcium phosphate , calcite , aragonite , and even glued-together mineral flakes – sprang up in a range of different environments. Most lineages adopted the form of calcium carbonate which was stable in the ocean at the time they first mineralized, and did not change from this mineral morph - even when it became less favourable. Some Precambrian (Ediacaran) organisms produced tough but non-mineralized outer shells, while others, such as Cloudina , had

1102-429: Is liable to failure under compression. This combination is especially effective in resisting predation, as predators tend to exert compression on the outer layer, and tension on the inner. Its degree of sclerotisation or mineralisation determines how the cuticle responds to deformation . Below a certain degree of deformation changes of shape or dimension of the cuticle are elastic and the original shape returns after

1160-440: Is mostly azadirachtin, with some other limonoids . Azadirachtin interferes with a wide variety of insect pathways. Azadirachtin is formed via an elaborate biosynthetic pathway, but is believed that the steroid tirucallol is the precursor to the neem triterpenoid secondary metabolites. Tirucallol is formed from two units of farnesyl diphosphate (FPP) to form a C 30 triterpene, but then loses three methyl groups to become

1218-405: Is periodic and concentrated into a period of time when the exoskeleton is shed, called moulting or ecdysis , which is under the control of a hormone called ecdysone . Moulting is a complex process that is invariably dangerous for the arthropod involved. Before the old exoskeleton is shed, the cuticle separates from the epidermis through a process called apolysis . Early in the process of apolysis

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1276-401: Is the mechanism behind some insect pesticides, such as Azadirachtin . Exoskeletons, as hard parts of organisms, are greatly useful in assisting the preservation of organisms, whose soft parts usually rot before they can be fossilized. Mineralized exoskeletons can be preserved as shell fragments. The possession of an exoskeleton permits a couple of other routes to fossilization . For instance,

1334-430: Is used to refer to those parts of an insect's body lacking in setae (bristles) or scales . Chemically, chitin is a long-chain polymer of a N-acetylglucosamine , which is a derivative of glucose. The polymer bonds between the glucose units are β(1→4) links, the same as in cellulose . In its unmodified form, chitin is translucent, pliable, resilient and tough. In arthropods and other organisms however, it generally

1392-515: The armadillo , and hair in the pangolin . The armour of reptiles like turtles and dinosaurs like Ankylosaurs is constructed of bone; crocodiles have bony scutes and horny scales. Since exoskeletons are rigid, they present some limits to growth. Organisms with open shells can grow by adding new material to the aperture of their shell, as is the case in gastropods , bivalves , and other molluscans . A true exoskeleton, like that found in panarthropods, must be shed via moulting ( ecdysis ) when

1450-419: The exoskeleton . In some organisms the mineral content may exceed 95%. The role of the chitin and proteins in such structures is more than just holding the crystals together; the crystal structure itself is so affected as to prevent the propagation of cracks under stress, leading to remarkable strength. The process of formation of such mineral-rich matrices is called biomineralization . The difference between

1508-429: The larval stages such as caterpillars and the larvae of parasitoidal Hymenoptera . In addition to the chitinous-proteinaceous composite of the cuticle, many crustaceans , some myriapods and the extinct trilobites further impregnate the cuticle with mineral salts, above all calcium carbonate, which can make up to 40% of the cuticle. The armoured product commonly has great mechanical strength. The two layers of

1566-647: The neem tree, Azadirachta indica (hence the prefix aza does not imply an aza compound , but refers to the scientific species name ). Many more compounds, related to azadirachtin, are present in the seeds as well as in the leaves and the bark of the neem tree which also show strong biological activities among various pest insects Effects of these preparations on beneficial arthropods are generally considered to be minimal . Some laboratory and field studies have found neem extracts to be compatible with biological control. Because pure neem oil contains other insecticidal and fungicidal compounds in addition to azadirachtin, it

1624-543: The turtle , have both an endoskeleton and a protective exoskeleton . Exoskeletons contain rigid and resistant components that fulfil a set of functional roles in addition to structural support in many animals, including protection, respiration, excretion, sensation, feeding and courtship display , and as an osmotic barrier against desiccation in terrestrial organisms. Exoskeletons have roles in defence from parasites and predators and in providing attachment points for musculature . Arthropod exoskeletons contain chitin ;

1682-515: The Cambrian period, with the Bryozoans being the only calcifying phylum to appear later, in the Ordovician . The sudden appearance of shells has been linked to a change in ocean chemistry which made the calcium compounds of which the shells are constructed stable enough to be precipitated into a shell. However, this is unlikely to be a sufficient cause, as the main construction cost of shells

1740-463: The addition of calcium carbonate makes them harder and stronger, at the price of increased weight. Ingrowths of the arthropod exoskeleton known as apodemes serve as attachment sites for muscles. These structures are composed of chitin and are approximately six times stronger and twice the stiffness of vertebrate tendons . Similar to tendons, apodemes can stretch to store elastic energy for jumping, notably in locusts . Calcium carbonates constitute

1798-446: The animal starts to outgrow it. A new exoskeleton is produced beneath the old one, and the new skeleton is soft and pliable before shedding the old one. The animal will typically stay in a den or burrow during moulting, as it is quite vulnerable to trauma during this period. Once at least partially set, the organism will plump itself up to try to expand the exoskeleton. The new exoskeleton is still capable of growing to some degree before it

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1856-422: The cuticle have different properties. The outer layer is where most of the thickening, biomineralization and sclerotisation takes place, and its material tends to be strong under compressive stresses , though weaker under tension. When a rigid region fails under stress , it does so by cracking. The inner layer is not as highly sclerotised, and is correspondingly softer but tougher; it resists tensile stresses but

1914-435: The cuticle then takes place. The new integument still is soft and usually is pale, and it is said to be teneral or callow . It then undergoes a hardening and pigmentation process that might take anything from several minutes to several days, depending on the nature of the animal and the circumstances. Although the process of ecdysis is metabolically risky and expensive, it does have some advantages. For one thing it permits

1972-439: The epidermis, which begins as a tough, flexible layer of chitin . Arthropod cuticle is a biological composite material , consisting of two main portions: fibrous chains of alpha- chitin within a matrix of silk-like and globular proteins, of which the best-known is the rubbery protein called resilin . The relative abundance of these two main components varies from approximately 50/50 to 80/20 chitin protein, with softer parts of

2030-441: The epithelial cells release enzymatic moulting fluid between the old cuticle and the epidermis. The enzymes partly digest the endocuticle and the epidermis absorbs the digested material for the animal to assimilate. Much of that digested material is re-used to build the new cuticle. Once the new cuticle has formed sufficiently, the animal splits the remaining parts of the old integument along built-in lines of weakness and sheds them in

2088-409: The exoskeleton are called sclerites. Sclerites may be simple protective armour, but also may form mechanical components of the exoskeleton , such as in the legs, joints, fins or wings. In the typical body segment of an insect or many other Arthropoda, there are four principal regions. The dorsal region is the tergum ; if the tergum bears any sclerites, those are called tergites . The ventral region

2146-426: The exoskeleton having a higher proportion of chitin. The cuticle is soft when first secreted, but it soon hardens as required, in a process of sclerotization . The process is poorly understood, but it involves forms of tanning in which phenolic chemicals crosslink protein molecules or anchor them to surrounding molecules such as chitins. Part of the effect is to make the tanned material hydrophobic . By varying

2204-445: The exoskeleton will have thickened areas in which the chitin is reinforced or stiffened by materials such as minerals or hardened proteins. This happens in parts of the body where there is a need for rigidity or elasticity. Typically the mineral crystals, mainly calcium carbonate , are deposited among the chitin and protein molecules in a process called biomineralization . The crystals and fibres interpenetrate and reinforce each other,

2262-467: The fossil record shortly before the base of the Cambrian period , 550  million years ago . The evolution of a mineralised exoskeleton is considered a possible driving force of the Cambrian explosion of animal life, resulting in a diversification of predatory and defensive tactics. However, some Precambrian ( Ediacaran ) organisms produced tough outer shells while others, such as Cloudina , had

2320-464: The great difficulty encountered when trying to prepare this compound from simple precursors, using methods of synthetic organic chemistry . Hence, the first total synthesis was published over 22 years after the compound's discovery: this first synthesis was completed by the research group of Steven Ley at the University of Cambridge in 2007. The described synthesis was a relay approach , with

2378-472: The head, which is a fused capsule, the thorax as nearly a fixed capsule, and the abdomen usually divided into a series of articulating segments. Each segment has sclerites according to its requirements for external rigidity; for example, in the larva of some flies, there are none at all and the exoskeleton is effectively all membranous ; the abdomen of an adult fly is covered with light sclerites connected by joints of membranous cuticle. In some beetles most of

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2436-406: The integument of a heavily armoured crab, in which there is a very high degree of modification by biomineralization. The chemical and physical nature of the arthropod exoskeleton limits its ability to stretch or change shape as the animal grows. In some special cases, such as the abdomens of termite queens and honeypot ants means that continuous growth of arthropods is not possible. Therefore, growth

2494-486: The joints are so tightly connected, that the body is practically in an armoured, rigid box. However, in most Arthropoda the bodily tagmata are so connected and jointed with flexible cuticle and muscles that they have at least some freedom of movement, and many such animals, such as the Chilopoda or the larvae of mosquitoes are very mobile indeed. In addition, the limbs of arthropods are jointed, so characteristically that

2552-430: The lateral pleura form the hardened plates or sclerites of a typical body segment. In either case, in contrast to the carapace of a tortoise or the cranium of a vertebrate, the exoskeleton has little ability to grow or change its form once it has matured. Except in special cases, whenever the animal needs to grow, it moults, shedding the old skin after growing a new skin from beneath. A typical arthropod exoskeleton

2610-478: The loss of a leg from an insect nymph, or a claw from a young crab, can be repaired after one or two stages of ecdysis. Similarly, delicate parts that need periodic replacement, such as the outer surfaces of the eye lenses of spiders, or the urticating hairs of caterpillars, can be shed, making way for new structures. Azadirachtin Azadirachtin , a chemical compound belonging to the limonoid group,

2668-419: The magnesium/calcium ratio of the oceans appears to have a negligible impact on organisms' success, which is instead controlled mainly by how well they recover from mass extinctions. A recently discovered modern gastropod Chrysomallon squamiferum that lives near deep-sea hydrothermal vents illustrates the influence of both ancient and modern local chemical environments: its shell is made of aragonite, which

2726-408: The minerals supplying the hardness and resistance to compression, while the chitin supplies the tensile strength. Biomineralization occurs mainly in crustaceans. In insects and arachnids , the main reinforcing materials are various proteins hardened by linking the fibres in processes called sclerotisation and the hardened proteins are called sclerotin . The dorsal tergum , ventral sternum , and

2784-425: The molluscs, whose shells often comprise both forms, most lineages use just one form of the mineral. The form used appears to reflect the seawater chemistry – thus which form was more easily precipitated – at the time that the lineage first evolved a calcified skeleton, and does not change thereafter. However, the relative abundance of calcite- and aragonite-using lineages does not reflect subsequent seawater chemistry –

2842-612: The parts of organisms that were already mineralised are usually preserved, such as the shells of molluscs. It helps that exoskeletons often contain "muscle scars", marks where muscles have been attached to the exoskeleton, which may allow the reconstruction of much of an organism's internal parts from its exoskeleton alone. The most significant limitation is that, although there are 30-plus phyla of living animals, two-thirds of these phyla have never been found as fossils, because most animal species are soft-bodied and decay before they can become fossilised. Mineralized skeletons first appear in

2900-575: The required, heavily functionalized decalin intermediate being made by total synthesis on a small scale, but being derived from the natural product itself for the gram-scale operations required to complete the synthesis. Initially found to be active as a feeding inhibitor towards the desert locust ( Schistocerca gregaria ), it is now known to affect over 200 species of insects , by acting mainly as an antifeedant and growth disruptor. Azadirachtin exhibits considerable toxicity towards African cotton leafworm ( Spodoptera littoralis ), which are resistant to

2958-403: The shells of molluscs, brachiopods , and some tube-building polychaete worms. Silica forms the exoskeleton in the microscopic diatoms and radiolaria . One mollusc species, the scaly-foot gastropod , even uses the iron sulfides greigite and pyrite . Some organisms, such as some foraminifera , agglutinate exoskeletons by sticking grains of sand and shell to their exterior. Contrary to

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3016-436: The stable calcite and the metastable aragonite, which is stable within a reasonable range of chemical environments but rapidly becomes unstable outside this range. When the oceans contain a relatively high proportion of magnesium compared to calcium, aragonite is more stable, but as the magnesium concentration drops, it becomes less stable, hence harder to incorporate into an exoskeleton, as it will tend to dissolve. Except for

3074-401: The stress is removed. Beyond that level of deformation, non-reversible, plastic deformation occurs until finally the cuticle cracks or splits. Generally, the less sclerotised the cuticle, the greater the deformation required to damage the cuticle irreversibly. On the other hand, the more heavily the cuticle is armoured, the greater the stress required to deform it harmfully. Hardened plates in

3132-509: The strong layer can resist compaction, allowing a mould of the organism to be formed underneath the skeleton, which may later decay. Alternatively, exceptional preservation may result in chitin being mineralised, as in the Burgess Shale , or transformed to the resistant polymer keratin , which can resist decay and be recovered. However, our dependence on fossilised skeletons also significantly limits our understanding of evolution. Only

3190-468: The types of interaction between the proteins and chitins, the insect metabolism produces regions of exoskeleton that differ in their wet and dry behaviour, their colour and their mechanical properties. The chitinous procuticle is formed of an outer exocuticle and the inner endocuticle , and between the exocuticle and endocuticle there may be another layer called mesocuticle which has distinctive staining properties. The tough and flexible endocuticle

3248-443: The unmodified and modified forms of chitinous arthropodan exoskeletons can be seen by comparing the body wall of say a bee larva, in which modification is minimal, to any armoured species of beetle , or the fangs of a spider. In both those examples there is heavy modification by sclerotisation. Again, contrasting strongly with both unmodified organic material such as largely pure chitin, and with sclerotised chitin and proteins, consider

3306-400: The very name "Arthropoda" literally means "jointed legs" in reflection of the fact. The internal surface of the exoskeleton is often infolded, forming a set of structures called apodemes that serve for the attachment of muscles, and functionally amounting to endoskeletal components. They are highly complex in some groups, particularly in Crustacea . Within entomology , the term glabrous

3364-430: The visible process of ecdysis, generally shedding and discarding the epicuticle and the reduced exocuticle, though some species carry them along for camouflage or protection. The shed portions are called the exuviae . After the old cuticle is shed, the arthropod typically pumps up its body (for example, by air or water intake) to allow the new cuticle to expand to a larger size: the process of hardening by dehydration of

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