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Arthropods are invertebrates with segmented bodies and jointed limbs. They are the largest phylum of invertebrates, comprising crustaceans, insects, arachnids, and other classes. Arthropods have a hard, protective exoskeleton, which provides a large surface area for the attachment of muscles. Interestingly, arthropods have striated muscles, similar to the skeletal muscles of humans. This means they have a faster contraction rate than smooth muscles, which likely enabled the development of flight in many insects.
| Characteristics | Values |
|---|---|
| Type of muscles | Striated |
| Muscle structure | Not obliquely striated or smooth |
| Sarcomere structure | Varying lengths |
| Sarcomere length in locusts' wing muscles | 3.9 micrometres |
| Sarcomere length in locusts' leg muscles | 8.5 micrometres |
| Sarcomere length in other insects' wing muscles | 2.5 micrometres |
| Force exerted by muscles | Controlled by frequency of action potentials in axons |
| Muscles for movement | Attached to the inner surface of the exoskeleton |
| Muscles and exoskeleton | Muscles attached to the exoskeleton to flex limbs |
| Muscles and hydraulic pressure | Some arthropods use hydraulic pressure to extend limbs |
| Muscles and skeleton | Muscles work antagonistically with the skeleton |
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What You'll Learn
- Arthropods have a nervous system with a large ventral nerve cord
- Arthropods have a hard, protective exoskeleton
- Arthropods have a wide variety of respiratory systems
- Arthropods have a coelom, a membrane-lined cavity between the gut and body wall
- Arthropods use muscles attached to the inside of the exoskeleton to flex their limbs
Arthropods have a nervous system with a large ventral nerve cord
Arthropods are invertebrates with segmented bodies and jointed limbs. They are the largest phylum of invertebrates, comprising crustaceans, insects, arachnids (spiders and scorpions), and other classes. They have a nervous system with a large ventral nerve cord that branches into many smaller nerve fibres that innervate the body. The ventral nerve cord is connected to the brain by the circumesophageal connectives and is composed of a double row of ganglia connected longitudinally by connectives and transversely by commissures.
The ventral nerve cord leads to a small brain in the cephalic or head segment of the arthropod's body. The arthropodan brain consists of three main regions: the protocerebrum, deutocerebrum, and tritocerebrum. The anterior protocerebrum, which receives the nerves of the eyes and other organs, contains optic centres and bodies known as corpora pedunculata. The neuropils in the protocerebrum function as integrative systems for the anterior sense organs, especially the eyes, and in control of movement. They are also responsible for the initiation of complex behaviours. The deutocerebrum contains association centres for the first antennae, while the posterior tritocerebrum contains association neuropils for the second antennae and gives rise to nerves that innervate the mouthparts and the anterior digestive canal.
Different groups of arthropods exhibit varying degrees of fusion of the ganglia. Insects, for example, have a subesophageal ganglion formed by the fusion of three pairs of ganglia, which sends nerves to the mouthparts and salivary glands. The segmental ganglia in the thorax and abdomen provide nerves to the appendages, dorsal muscles, sense organs, and heart. Insects have three pairs of thoracic ganglia and up to ten abdominal ganglia. The most common sensory receptors in arthropods are the cuticular hairs, which are sensitive to touch, vibration, water currents, or sound waves. Some hairs are chemoreceptors, detecting odours or chemicals in the water.
The quick evasion of predators has likely influenced the evolution of giant-fibre systems in arthropods. These giant fibres conduct impulses at much higher velocities than smaller axons, and they are responsible for survival responses to environmental stimuli. Neurosecretory cells, which are found in all major invertebrate groups, reach their highest degree of development in arthropods. The principal system of insects consists of neurosecretory cells in the protocerebrum of the brain, with axons that innervate structures called corpora cardiaca.
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Arthropods have a hard, protective exoskeleton
Arthropods are invertebrates with segmented bodies and jointed limbs. They have a hard, protective exoskeleton, which is the outer shell of the animal. This exoskeleton is made of chitin, a polymer of N-Acetylglucosamine. The exoskeleton is a multi-layered structure with four functional regions: the epicuticle, procuticle, epidermis, and basement membrane. The epicuticle is a multi-layered external barrier that acts as a barrier against desiccation, especially in terrestrial arthropods. The strength of the exoskeleton is provided by the underlying procuticle, which is secreted by the epithelial cells in the epidermis. The procuticle begins as a tough, flexible layer of chitin.
The exoskeleton has thickened areas where the chitin is reinforced or stiffened by materials such as minerals or hardened proteins. This occurs in parts of the body where there is a need for rigidity or elasticity. The mineral crystals, mainly calcium carbonate, are deposited among the chitin and protein molecules in a process called biomineralization. This process occurs mainly in crustaceans, and the crystals and fibres reinforce each other. The minerals supply hardness and resistance to compression, while the chitin provides tensile strength.
In insects and arachnids, the main reinforcing materials are various proteins that are hardened by linking the fibres in a process called sclerotisation. The hardened proteins are called sclerotin. The hardened plates in the exoskeleton are called sclerites, and they may be simple protective armour or form mechanical components of the exoskeleton, such as in the legs, joints, fins, or wings. The dorsal region is the tergum, and if the tergum bears any sclerites, they are called tergites. The ventral region is called the sternum, which commonly bears sternites. The two lateral regions are called the pleura, and any sclerites they bear are called pleurites.
Arthropods moult, or shed their exoskeletons periodically, to enable growth. During the moult, they form a larger exoskeleton to allow for expansion. The underlying cells release enzymes that digest the base of the old exoskeleton and then secrete a new exoskeleton beneath the old one. The old skeleton is usually abandoned, but in some species, it is eaten or carried along for camouflage or protection. The new exoskeleton is soft and flexible, and it is stretched and tanned to harden it within a few hours or days of moulting.
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Arthropods have a wide variety of respiratory systems
The respiratory system is responsible for delivering sufficient oxygen to all cells of the body and for removing carbon dioxide (CO2) produced as a waste product of cellular respiration. The respiratory system of insects (and many other arthropods) is separate from the circulatory system. It is a complex network of tubes (called a tracheal system) that delivers oxygen-containing air to every cell of the body. Air enters the insect's body through valve-like openings in the exoskeleton. These openings (called spiracles) are located laterally along the thorax and abdomen of most insects — usually one pair of spiracles per body segment. Air flow is regulated by small muscles that operate one or two flap-like valves within each spiracle — contracting to close the spiracle, or relaxing to open it. After passing through a spiracle, air enters a longitudinal tracheal trunk, eventually diffusing throughout a complex, branching network of tracheal tubes that subdivides into smaller tubes.
At the end of each tracheal branch, a special cell (the tracheole) provides a thin, moist interface for the exchange of gases between atmospheric air and a living cell. Oxygen in the tracheal tube first dissolves in the liquid of the tracheole and then diffuses into the cytoplasm of an adjacent cell. At the same time, carbon dioxide diffuses out of the cell and, eventually, out of the body through the tracheal system. Each tracheal tube develops as an invagination of the ectoderm during embryonic development. To prevent its collapse under pressure, a thin, reinforcing “wire” of cuticle (the taenidia) winds spirally through the membranous wall. This design (similar in structure to a heater hose on an automobile or an exhaust duct on a clothes dryer) gives tracheal tubes the ability to flex and stretch without developing kinks that might restrict airflow.
The absence of taenidia in certain parts of the tracheal system allows the formation of collapsible air sacs, balloon-like structures that may store a reserve of air. In dry terrestrial environments, this temporary air supply allows an insect to conserve water by closing its spiracles during periods of high evaporative stress. Aquatic insects consume the stored air while underwater or use it to regulate buoyancy. During a molt, air sacs fill and enlarge as the insect breaks free of the old exoskeleton and expands a new one.
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Arthropods have a coelom, a membrane-lined cavity between the gut and body wall
Arthropods are invertebrates with segmented bodies and jointed limbs. They have a coelom, a membrane-lined cavity between the gut and body wall that accommodates the internal organs. The coelom is a characteristic feature of metazoans and is formed during embryo development from the three germinal layers. It is lined by mesodermal epithelium cells, or mesothelium, and is filled with fluid. This fluid acts as a hydrostatic skeleton, allowing soft-bodied animals to move and giving the body a definite shape. It also provides protection from mechanical shock and supports the immune system.
The coelom is present in arthropods and is known as a haemocoel. It is filled with blood and runs most of the length of the body, with blood flowing through it. The heart, a muscular tube, contracts in ripples, pushing blood forwards. Arthropods have a wide variety of respiratory systems, with small species often relying on simple diffusion through the body surface for oxygen. Crustacea usually have gills, while many arachnids have book lungs.
In arthropods, the coelom is reduced to small areas around the reproductive and excretory systems. This is because the strong, segmented limbs of arthropods eliminate the need for one of the coelom's main ancestral functions, which is to act as a hydrostatic skeleton. The coelom is replaced by a hemocoel, a cavity that runs most of the length of the body. Arthropods have an open circulatory system, with blood returning to the heart through small pores.
Arthropods have a hard, protective exoskeleton, with muscles attached to the inner surface for movement. The exoskeleton is made of chitin, a polymer of N-Acetylglucosamine, and is periodically shed and replaced with a larger version to allow for growth. Some groups, such as crabs and barnacles, also secrete calcium carbonate into the exoskeleton, making it thick and hard. While most invertebrate musculature is of the smooth type, arthropod muscles are primarily striated, allowing for a faster contraction rate, which probably enabled the development of flight in many insects.
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Arthropods use muscles attached to the inside of the exoskeleton to flex their limbs
Arthropods are invertebrates with segmented bodies and jointed limbs. They are the largest phylum of invertebrate animals, comprising crustaceans, insects, arachnids (spiders and scorpions), and other classes. The bodies of arthropods are segmented internally, and their nervous, muscular, circulatory, and excretory systems have repeated components.
Arthropods have a hard, protective exoskeleton (outer shell) made of chitin, a polymer of N-Acetylglucosamine. The exoskeleton provides a large surface area for the attachment of muscles. Arthropods use muscles attached to the inside of the exoskeleton to flex their limbs. The exoskeleton functions in support, movement, and protection from the external environment. The cylindrical design of the exoskeleton resists bending, and only a small amount of skeletal material is needed to prevent buckling.
The muscles of arthropods are primarily striated, similar to the skeletal muscles of humans. Striated muscle has a faster contraction rate than smooth muscle, which is the type of muscle found in most invertebrates. The fast contraction rate of striated muscle probably enabled the development of flight in many insects. Arthropods and vertebrates share the common problem of controlling a rigid, articulated skeleton using neurally controlled, striated muscle. However, arthropods and vertebrates appear to use fundamentally different motor control tactics.
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Frequently asked questions
Yes, arthropods have striated muscles.
Striated muscle is similar to the skeletal muscles of humans. They are neurally controlled and are responsible for the fast contraction rate in arthropods.
Arthropods include insects, spiders, scorpions, centipedes, millipedes, crabs, crayfish, shrimp, lobsters, and barnacles.
Arthropod muscles are attached to the inside of the exoskeleton to flex their limbs. The exoskeleton provides a large surface area for the attachment of muscles and provides support, movement, and protection.
Arthropods use peripheral neuromodulation to alter the properties of both neuromuscular junctions and muscle fibers. They also use a small number of heterogeneous motoneurons to control their muscles.
