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An invertebrate animal with a chitinous exoskeleton and numerous jointed limbs that belong to the Arthropoda phylum. The Greek word árthron, which means “joint,” is the source of the word arthropod.
What is Arthropod?
The presence of numerous joints, a chitinous exoskeleton, segmentation, and an open circulatory system distinguishes an arthropod from other invertebrates.
The interior organs of the creatures are protected by the chitinous exoskeleton. It also serves as an attachment point for creatures’ muscles, particularly for arthropods with moving appendages.
The exoskeleton of an arthropod is made up of layers of chitin and proteins. Arthropods use segmentation to account for the specialisation of their structures and organs. Segments also serve as a location for arthropods’ numerous limbs or appendages to emerge. These appendages provide a variety of tasks, including feeling, movement, copulation, and eating, among others.
The heart and short arteries make up the open circulatory system of arthropods, with the heart delivering hemolymph to sinuses around the internal organs through the shirt arteries.
The phylum Arthropoda is further split into sub-phylum groupings, each of which is defined by the characteristics of the creatures inside it, such as the subphylum Trilobitomorpha. Crustacea comprises lobsters, shrimp, and crabs, all of which are distinguished by their extinct trilobites. Members of the Chelicerata family have numerous limbs. Scorpions and spiders are among them. The Tracheata subphylum includes myriapods and insects.
Insects, shrimp, millipedes, spiders, and crabs are all members of the Arthropoda phylum, which contains roughly 85 percent of all creatures. There are over 30 million species of arthropods recognised and described. Insects make up about a million of these species.
Arthropod species may be found in a variety of environments, including deep oceans and high heights. As a result, members of the phylum Arthropoda have a wide range of forms and history.
The biomass of arthropods is thought to be higher than that of all other creatures. They have characteristics that separate them from other invertebrates. Exoskeletons and jointed limbs are examples of these features.
Arthropod exoskeletons are formed from a hard cuticle covering. Arthropods’ muscles are linked to the cuticle that runs between their body segments and limbs, allowing them to move freely. The cuticle is mostly made up of chitin, which is plentiful on the planet.
Arthropods have segmented bodies that seem bilaterally symmetric, and the number of segments in their bodies varies, as do their appendages, which serve a variety of purposes depending on the area from which they arise.
Arthropods have a dorsal heart and a vascular space that makes up a hemocoel in their circulatory system. A brain or an oesophageal centre is located anteriorly, while a nerve cord with ganglions is located ventrally. The muscles of arthropods are mostly striated fibres lacking epithelial cells or cilia.
Arthropods have been around for millions of years. The dominating arthropods, trilobites, for example, radiated approximately 550 million years ago in the early Palaeozoic oceans. Furthermore, it is thought that arthropods evolved from their early annelid predecessors throughout the times of development. In the Devonian period, approximately 350 million years ago, the first arachnids, as well as insects, marine spiders, and millipedes, arrived around 400 million years ago.
The biggest phylum in the Kingdom of Animalia is Arthropoda. Insects, arachnids, myriapods, and crustaceans are among them. They are distinguished by the following characteristics: A strong chitinous exoskeleton, segmentation, multiple jointed (paired) limbs, and an open circulatory system are all present.
Structure of Arthropods
Arthropod exoskeletons are critical to both success and limits. Exoskeletons serve a variety of purposes, including providing protection from the outside world, assisting in movement, supporting the body, reducing water loss, and storing energy. The exoskeleton is made up of two layers: the epicuticle, which is non-chitinous and impermeable, and the endocuticle, which is elastic and porous.
Chitin and protein make up the majority of the endocuticle. Calcium carbonate has the ability to harden the outer layer of the endocuticle. A variety of marine crustaceans, as well as arachnids and insects that sclerotize cuticle proteins, have this feature. Because arthropods have such a diverse range of morphologies, their mouthparts are toughened, allowing them to consume a wide range of food sources. In addition, the exoskeleton of insects and myriapods lines the trachea, or breathing passages, and reaches to the stomach. In addition to its flexibility, this remarkable mix of soft membranes and hard materials gives rigidity and strength.
Because the solid endoskeletons of vertebrates are approximately three times weaker than the same area of an exoskeleton, the natural composition of the arthropod provides particular benefits over the endoskeletons of vertebrates, such as higher bending resistance. As a result, in order for the endoskeleton to be as strong as the exoskeleton, it must be thicker, leaving less room for muscles.
Arthropods’ endoskeletons are mechanically efficient due to their flexibility, and, as a result, they create a broad range of body shapes and morphologies, resulting in a huge diversity in the composition of various arthropods. This variant isn’t seen anywhere else in the Animalia kingdom. However, exoskeletons may result in some restrictions, such as the organism’s physical size being limited as it grows. As a result, as the organism becomes larger, the outer exoskeleton covering will be shed. The exoskeleton must be pierced with sensilla in order to observe the outside environment.
Despite their size limits, some ancient aquatic arthropods grew to be 1.8 metres long, and a few crustaceans, such as gigantic spider-crabs, may weigh up to 6.4 kilogrammes, but most arthropods are currently very tiny.
Arthropods in the sea can grow to be much larger than those on land. The biggest land spiders and insects weigh less than 100 grams. The Goliath beetles, the world’s biggest bugs, are 10 cm wide and 15 cm long. The tiniest arthropods are mites and parasitic wasps, measuring less than 0.23 mm in length. Even though they are tiny (less than the weight of a cell’s nucleus), they have complex activities and structures.
The image above depicts the variety among different species of the phylum Arthropoda; arthropods’ length and size range from 1 millimetre to 4 metres. They do, however, share several distinguishing characteristics, such as the existence of an outer exoskeleton in all Arthropoda. They also have a segmented body and appendages that are jointed.
Respiration in Arthropods
Arthropod respiration changes depending on the species. Trachea are tiny tubes that allow oxygen to diffuse into all regions of the body in myriapods and insects. Physical limitations regulate the diffusion process in arthropods, so that an organism’s growth has no effect on the amount of oxygen supplied to its tissues. Tracheal respiration, on the other hand, is extremely efficient, allowing for breathing with only a tiny partial pressure difference between the outside environment and the pressure in the trachea.
The trachea may provide oxygen to the insect’s muscles through respiration. Diffusion through the cuticle or gills is used by marine arthropods (such as in the Crustacea). Arachnids have gills that are enclosed (book lung).
Even while exoskeletons constrain arthropods, they have numerous additional adaptations that have allowed them to live and prosper for millions of years.
Life Cycle of Arthropods
Sexual reproduction is the primary mode of reproduction for arthropods. Some arthropods, such as barnacles, are hermaphroditic, meaning they have organs from both sexes. Parthenogenesis is only used by a few crustaceans and insects to reproduce. When the conditions become more suitable, they resume sexual reproduction.
Arthropods that dwell in watery environments frequently reproduce sexually, usually by external fertilisation. Crustaceans and Opiliones use specialised appendages known as penises or gonopods to procreate sexually.
Internal fertilisation is used by all terrestrial arthropods to reproduce. Males achieve this by generating packets of sperm called spermatophores, which females then accept into their bodies.
Arthropods are known for laying eggs. Gravid scorpions, on the other hand, lay eggs that hatch inside their bodies.
Arthropods can be fully formed adults when they are born. Others, on the other hand, are powerless until they moult for the first time.
Insects without cuticles hatch as caterpillars or grubs in order to develop their adult tissues.
Molting is a process that controls the growth of arthropods with an outer shell. Molting is controlled by numerous hormones that work together to promote the growth of the arthropod.
The enzymes breakdown and tear down the old cuticle’s inner layers, allowing the new cuticle to be detached. The arthropod consumes air or water to solidify the new flexible cuticle.
Without hatching, marine crustaceans develop immediately. After being given water, large eggs hatch. During its life cycle, the Chinese mitten crab migrates to the surface and to the bottom to reproduce. As a result, it has an impermeable cuticle that can survive large variations in osmotic pressure.
Types of Arthropods
Arthropods may survive in both freshwater and on land. Freshwater is home to the majority of crayfish and brachiopods. Amphipods, copepods, crabs, shrimp, isopods, and ostracods are examples of additional taxa that live in freshwater.
Freshwater creatures adapt to their environment by incorporating systems that prevent water from entering their bodies. They resist the clearance of ions from their bodies into the surrounding environment, despite the fact that the fluids inside their bodies are more diluted than the fluids in their surroundings.
The cuticle of crustaceans is virtually impermeable, preventing water from accessing their bodies. The only permeable component that allows gas exchange is the gills. As a result, in crustaceans, gills are in charge of osmoregulation.
To fight the dilution of water inside their bodies, crustaceans release amino acids from their muscles into their blood. The excretory organs help them get rid of surplus water in their bodies.
Semi-terrestrial animals, like the fiddler crab, live in the transition zone between air and water. They have fluids in their bodies that cause their osmotic pressure to be relatively high. This discovery implies that semi-terrestrial life may have originated from saltwater. Land crabs (a whole family of amphipods) have stiffened gills that function as lungs and allow some gaseous exchange, allowing them to survive in both water and on land. Despite the fact that they spend most of their time on land, they reproduce in the water and leave their eggs to grow directly.
Classification of Arthropod
The phylum is divided into many categories. Arthropoda have been postulated since arthropods were first discovered. As a result, there is no standard system for classifying arthropods. The main structural characteristics of arthropods, such as the genital opening location and the mandible’s articulation, distinguish various schemes. These characteristics are also used to categorise arthropods in a hierarchical order.
This categorization is divided into two categories. According to one theory, arthropods are monophyletic because they evolved from common ancestors. This is the most often used arthropod categorization in the literature. The alternative theory proposes that arthropods evolved from separate progenitors and subsequently became architecturally similar to one another via evolution, implying that arthropods are polyphyletic.
For arthropods, each perspective has its own subdivisions. The monophyletic group believes that all arthropods descended from an ancestor with unique body segmentations. Crustaceans, insects, arachnids, pentastomids, trilobites, myriapods, and other monophyletic arthropods are divided into numerous categories.
The Trilobitomorpha, which contains extinct trilobites, the Mandibulata (having articulated mandibles split into seven groups, with antenna), and the Chelicerata are three classification schemes for arthropods (without antenna but with a head and with appendages used for walking). The unclear link between Pentastomida, Tardigrada, and Onychophora, as well as the other arthropods, is one of the scheme’s fundamental flaws.
(5) Onychophora, and
(6) Crustacea, according to another classification.
There are three recognised categories of arthropods based on polyphyletic categorization. These are the organisations.
1. Myriapoda, Onychophora, and Hexapods are all members of the Uniramia family.
2. The Crustacea are the second category.
3. The Chelicerata are the third category.
Because each group represents a different phylum, the phylum Arthropoda is not recognised. Some groupings are unclear, such as trilobites, which are thought to constitute a different phylum due to their unique appearance. Uniramia (containing the Tardigrada) and Pentastomida are also difficult to classify.
Arthropods are divided into five subphyla according to the most current arthropod classification:
1. Aquatic Crustacea (e.g. shrimp, crab species, and lobsters)
2. Chelicerata (e.g. marine horseshoe crabs, mites, and scorpions)
3. Myriapods are a kind of myriapod (a subphylum containing centipedes and millipedes).
4. Hexapods or Insecta, which are related to crustaceans and hexapods.
5. Trilobites, a group of aquatic creatures.
There are three subphyla of arthropods: Trilobita, Chelicerata, and Mandibulata.
Chelicerata (spiders, scorpions, etc.), Crustacea (shrimps, lobsters, crabs, etc.), Tracheata (insects and myriapods), and Trilobitomorpha are the primary sub-phylum groupings of Arthropoda (extinct trilobites).
Evolution of Arthropods
The first arthropods were most likely trilobites. About 500 million years ago, their fossils were unearthed. Trilobites are arthropods with several segments and walking legs that live in the sea. More arthropods appeared and developed after trilobites. Fused segments are one of its distinguishing characteristics. They also lost their appendages. Arthropods eventually developed into having a three-segmented body.
Trilobites, the first known arthropod species, were discovered around 500 million years ago and thrived for around 200 million years before becoming extinct, leaving behind a large quantity of remains.
Arthropods were most likely the first creatures to exist on the planet. Millipedes were the first arthropods to move on land. They created a trachea, or lungs, for breathing, as well as an exoskeleton to shield arthropods from drying up and give structural support.
Arthropods have a similar segmented anatomy that gives birth to numerous structurally diverse groups. Despite the fact that all arthropods have exoskeletons, their thorax, head, and abdomen are morphologically distinct. Many of them, for example, lack segmented appendages, which are typical in many other arthropod families. The genetic diversity among these species determines the structural differences between them.
The Onychophora are a group of creatures that are believed to be transitional between arthropods and annelids. Their bodies are segmented, and they share many characteristics with arthropods, such as tracheal breathing, claws at the ends of paired limbs, and the structure of their reproductive system. Their repeating segments of excretory organs, the shape of the head and eyes, and their unstriated muscles, on the other hand, are comparable to annelids. Onychophora and arthropods are thought to have shared ancestors, the aquatic lobopods, according to various data.
Crustaceans and insects are thought to share a common ancestor. The order of the mitochondrial genes, which is the same in both groups, supports this theory. Both classes should be classified together in such findings. Crustacea, on the other hand, has an extra set of antennae on the second segment, which is the primary distinction between the two families.
Chewing mandibles are seen in a variety of crustaceans and insects. These mandibles in the third segment developed via truncation of the whole limb, according to fossils. Insects do not have the same level of diversity as crustaceans. This explains the thorax’s continuous configuration of three pairs of limbs with no limbs in the abdomen. The variety among crustaceans is due to the many genetic ways in which isopods (feeding appendages) are produced in place of walking legs. The head is located in the first section of the thorax.
Ecological Importance of Arthropods
Arthropods are an important part of the environment because they play a role in food chains. Decomposers, pollinators, herbivores, parasites, seed dispersers, and predators are all examples of arthropods. They have many features that enable them to play an important part in the ecological system.
They have a high reproductive capacity, several adaptations for diverse environmental circumstances, a tiny size, and a wide range of biodiversity. Arthropods are employed as biological markers to determine the ecosystem’s health. Biological indicators are primarily used in the study of ecosystem fluctuations and conditions by giving data that aids in the management of ecosystem plans.
Due to their vast geographical spread, large patch size, fast turnover, large population number, and patch dynamics, arthropods have more temporal and explicit scales than birds and vertebrates.
Carnivorous, parasitic, and other consumers rely on arthropods as a source of food. They also function as eliminators, acting as detritivores and decomposers of deceased organisms. They also aid in the dispersal of seeds, pollination, and the creation of microhabitats. As a result, the presence of arthropods in an environment reflects the ecosystem’s health.
Arthropods in the soil breakdown the forest environment, while winged arthropods pollinate it. As a result, the loss of pollinators will eventually lead to a reduction in seed production, while the loss of soil arthropods will likely result in an ecological imbalance.
Medical and Economic Use of Arthropods
Arthropods play a significant role in human life. Humans, for example, utilise a variety of crustacean species as a food source (e.g. lobsters, crabs, etc.), Others, on the other hand, can be hazardous to people if they have recently consumed neurotoxins-producing plants. Furthermore, some of them are capable of transmitting parasites to people. Bees, wax, silk, and dye are all made with the help of insects and other animals. They can also help with flower pollination.
The presence of a pair of mandibles, two pairs of antennae, two pairs of maxillae, and five or more pairs of legs distinguishes crustaceans. The body is split into a segmented abdomen and a cephalothorax in more evolved species like lobsters and decapods. Their appendages have been adapted to fulfil particular tasks. The majority of Crustacea species are found in the sea. Only 3% of species live entirely on land, whereas the remainder dwell in freshwater.
Crustaceans breathe through their cuticles, which perform gas exchange all over their bodies, or through their gills. The length, evolution, and habitat of Crustacea species differ. Without an antenna, the chelicerate body is generally split into an opisthosoma and a prosoma. Unlike insects and crustaceans, chelicerates use their appendages as legs rather than mouths.
Crabs are one of the most common crustaceans on the planet. They are members of the Decapoda order. Crabs may colonise nearly any habitat, including both terrestrial and marine environments. They may dwell in deep waters with depths of up to 6,000 metres or on mountains with a height of up to 2,000 metres above sea level. Crabs may also live in estuarine environments with daily temperature and salinity fluctuations. Crabs have adapted to survive in a land environment. They return to the water on occasion. Other spices require no water at all, while others simply require a temporary source of water, such as tiny water bodies in tree holes.
In the world, there are around 6,793 crab species. Crabs of the genus Brachyura communicate by waving or drumming, and they are generally aggressive, fighting over a house or a female crab. They work together to defend and feed their families. To conceal, mate, or relax, some crabs burrow in mud or sand. Crabs often eat algae, but depending on their species, they may also ingest worms, debris, worms, bacteria, or mollusks. Some crab species change their appearance over time. Their development is marked by a decrease in the size of the abdomen or an increase in the robustness of the body.
Crabs mate for life. They are attracted to one another for auditory, chemical, vibratory, or optical reasons. Chemical attractants are used by most aquatic crabs, while visual attractants are used by terrestrial crabs. Internal fertilisation is seen in the majority of crabs. Eggs are elaborated into the female’s abdomen below the tail for protection after fertilisation.
In contrast to crustaceans and insects, myriapods have antennae in their anterior segment and large mandibles in their second segment. Because the cuticle of myriapods includes a small amount of wax, it is not as impenetrable as that of arachnids. In myriapods, such as insects, excretion is not developed. In myriapods, malpighian tubules are responsible for excretion. The tracheal respiratory system, which myriapods use to breathe, distributes oxygen to the blood rather than the cells. The spiracles in their trunks can not be closed to reduce water conservation. To summarise, myriapods have adapted to the terrestrial environment even better than spiders.
Insects are found in over a million different species. They are, in fact, the most well-known animal order. Insects have developed an adaptive connection with blooming plants on land. Nevertheless, unlike crustaceans, many young insects exist in freshwater, whereas just a few species of insects dwell in the sea. As a result of their adaptation to specific features such as their capacity to fly, flexibility in their life cycle, and water conservation, insects are classified as terrestrial creatures.
Insects can escape competing with big animals due to their tiny size. Insects’ reproductive capacity also boosts the size of their colony. It also adds to the fast development of genetics. Because of the tiny size of the organism, oxygen diffuses via branching tracheal tubes that end in a cell or between two cells. Their respiratory tracheal system may directly give oxygen to all cells. Due to the thickness of the cuticle, the tubes are always open.
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