Logan Steele
Arthropods are invertebrates with segmented bodies and jointed limbs. They are the largest phylum of invertebrates and include crustaceans, insects, arachnids, and other classes. Arthropods have an exoskeleton, which is a skeleton that is external to the body, unlike the internal skeleton of vertebrates. The exoskeleton provides a large surface area for the attachment of muscles and assists in support, movement, and protection from the environment. Arthropods have a wide variety of respiratory systems, with small species often relying on simple diffusion through the body surface for oxygen. The muscles of arthropods are designed to exert force against their rigid exoskeleton, and they use these muscles to flex their limbs.
| Characteristics | Values |
|---|---|
| Definition | Arthropods are invertebrates with segmented bodies and jointed limbs |
| Largest phylum of invertebrates | Arthropods are the largest animal phylum, accounting for over 80% of all known living animal species |
| Sub-groups | Crustaceans, insects, arachnids (spiders and scorpions), and other classes |
| Muscles | All arthropod muscles are striated and have sarcomeres of varying lengths |
| Heart | The heart is a muscular tube that runs just under the back and for most of the length of the hemocoel |
| Exoskeleton | The exoskeleton provides a large surface area for the attachment of muscles and protection from the external environment |
| Movement | Arthropods use muscles attached to the inside of the exoskeleton to flex their limbs |
| Wing muscles | Wing muscles in many insects have shorter sarcomeres, often about the same length as those in mammalian muscle |
| Size | The largest arthropods live in the sea, where they gain support from the buoyancy of seawater |
| Molting | Arthropods must molt (shed) their exoskeletons periodically to grow |
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What You'll Learn
Arthropods have muscles attached to the inside of their exoskeletons
Arthropods are invertebrates with segmented bodies and jointed limbs. They are the largest phylum of invertebrate animals and include crustaceans, insects, arachnids (spiders and scorpions), and other classes. Arthropods have an external skeleton, known as an exoskeleton, which is made of chitin, a polymer of N-Acetylglucosamine. This exoskeleton provides a large surface area for the attachment of muscles and is divided into separate plates to assist in movement. The muscles for movement are attached to the inner surface of the exoskeleton, and arthropods use these muscles to flex their limbs.
The exoskeleton also serves to support the weight of the arthropod and resist the stresses of locomotion. It provides protection from the external environment, with its cylindrical design resisting bending and requiring only a small amount of skeletal material to prevent buckling. The exoskeleton does, however, impose limits on the maximum size of an arthropod, especially those that live on land. This is because an excessive amount of skeleton would be needed to support a large bulk, and there is a risk of collapse following a molt when the skeleton is still soft.
Arthropods have a wide variety of respiratory systems. Small species often do not have any respiratory systems, as their high ratio of surface area to volume enables simple diffusion through the body surface to supply enough oxygen. Crustaceans usually have gills that are modified appendages, while many arachnids have book lungs. Insects, myriapods, and arachnids have tracheae, or systems of branching tunnels, that deliver oxygen directly to individual cells.
Arthropods have a coelom, a membrane-lined cavity between the gut and the body wall that accommodates the internal organs. The coelom of arthropods is reduced to small areas around the reproductive and excretory systems, with most of the cavity being taken up by a hemocoel, through which blood flows. Arthropods have open circulatory systems, and in chelicerates and crustaceans, the blood carries oxygen to the tissues.
Arthropods have evolved more compact bodies and a smaller number of legs, with the number of pairs of legs used in walking being no more than seven in pill bugs, four or five in shrimps and crabs, four in arachnids, and three in insects. This reduces the problem of mechanical interference and leg interference during movement.
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Arthropods have striated muscles
Arthropods are invertebrates with segmented bodies and jointed limbs. They are the largest phylum of invertebrate animals and include crustaceans, insects, arachnids (spiders and scorpions), and other classes. Arthropods have an exoskeleton, which is a skeleton that is external to the body. This exoskeleton provides a large surface area for the attachment of muscles and, in addition to functioning in support and movement, also provides protection from the external environment.
The exoskeleton of arthropods works as a system of levers, which is largely responsible for making muscles antagonistic. The muscles of arthropods are designed to exert force against this rigid exoskeleton. Arthropods use muscles attached to the inside of the exoskeleton to flex their limbs, although some still use hydraulic pressure to extend them, a system inherited from their pre-arthropod ancestors. For example, all spiders extend their legs hydraulically and can generate pressures up to eight times their resting level.
The principal wing muscles of arthropods are the dorsoventral muscles, which run vertically from the sternum to the tergum, and the longitudinal muscles, which run lengthwise along the segment. The contraction of these muscles causes the wings to move up and down. The wings have joints connecting them to the tergum and to the sternum.
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Arthropods have a heart that is a muscular tube
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. They have a skeleton with jointed appendages formed from a stiff cuticle that is divided into separate plates to assist in movement. This skeleton works as a system of levers and is largely responsible for making muscles antagonistic.
The heart is divided segmentally into chambers that are separated by valves (ostia) to ensure a one-way flow of hemolymph. These valves are made up of several myofibers and protrude into the lumen. The ostia allow blood to enter the heart and prevent it from leaving before it reaches the front. The heart's contraction rate varies from species to species and is typically in the range of 30 to 200 beats per minute.
The circulatory system of arthropods is open, which means that blood (usually called hemolymph) spends much of its time flowing freely within body cavities where it makes direct contact with all internal tissues and organs. In contrast, vertebrates have a closed circulatory system where blood is always contained within vessels such as arteries, veins, capillaries, or the heart itself.
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Arthropods have a nervous system
The arthropod nervous system consists of a dorsal brain and a ventral, ganglionated longitudinal nerve cord from which lateral nerves extend in each segment. This system is similar to that of annelid worms, from which arthropods may have evolved. The neuromuscular organization of arthropods differs from that of vertebrates. In arthropods, the state of contraction of a muscle is determined by which of several different types of neurons supply one muscle cell are fired. In contrast, the vertebrate system has one neuron supplying a number of muscle cells, together forming a functional motor unit.
Arthropods possess eyes, but in most species, these can only detect the intensity and direction of the light source. Some arthropods with more sophisticated visual systems, such as the predaceous mantis shrimp, some crabs, and many insects, possess compound eyes that are extremely effective in detecting motion. Arthropods also have other types of sensors, such as setae, which can be used to detect air or water currents, contact with objects, or to increase the surface area of swimming appendages.
Arthropods have an open circulatory system, with a few short, open-ended arteries. The blood delivers oxygen to the tissues in chelicerates and crustaceans, while hexapods use a separate system of tracheae. The heart is a muscular tube that contracts in ripples to push blood forward, and it is connected to the body wall by elastic ligaments or small muscles.
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Arthropods have a wide variety of respiratory systems
Aquatic arthropods, such as crustaceans and chelicerate horseshoe crabs, possess gills for respiration. These vary in structure and location but are always outgrowths of the integument (skin) and are covered by the exoskeleton. Crustacea usually have gills that are modified appendages. Gills are situated within the gill chamber, which is located on each lateral side of the cephalothorax and covered by the gill cover or branchiostegite. Gills originate as out-pushings of the body wall. In Amphipoda, the gills are outgrowths of the thoracic limbs, and in Isopods, the endopodites of the second and fifth pleopods are modified as gills.
Terrestrial arthropods possess tracheae and book lungs as respiratory organs. Tracheae are a system of tiny tubes that permit the passage of gases into the interior of the body. In some arthropods, the tracheal tubes are bathed by blood, but in insects, the minute terminal endings (tracheoles) are embedded in the tissues, even within muscle cells. The tracheae originate as the invagination of the body wall. The wall of the trachea is composed of three layers: the internal layer, called intima, a middle layer of epithelium, and an outer layer of basement membrane. Tracheae deliver oxygen directly to individual cells in many insects, myriapods, and arachnids. Many arachnids have book lungs.
The respiratory system of insects (and many other arthropods) is separate from the circulatory system. Insects have spiracles on their exoskeletons to allow air to enter the trachea. Spiracles act as muscular valves in some insects. The immature Odonates (insects) have their rectum modified into a branchial basket. Its wall is contractile and richly supplied with tracheae branches. This kind of respiration is often referred to as anal respiration.
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Frequently asked questions
Yes, arthropods have muscles. They use muscles attached to the inside of the exoskeleton to flex their limbs.
Arthropod muscles are used for movement and exerting force against a rigid skeleton. They also have muscles that control the heart, which contracts in ripples to push blood forwards.
Arthropod muscles are striated, with sarcomeres of varying lengths. They have a faster contraction rate than smooth muscles, which probably enabled the development of flight in many insects.
The exoskeleton provides a large surface area for the attachment of muscles. The exoskeleton also provides support, movement, and protection from the external environment.
