Arthropod

An arthropod is an invertebrate animal that has an exoskeleton, a segmented body, and paired jointed appendages. Arthropods are the largest phylum in animal kingdom as they are found everywhere. Arthropods are responsible for forming the phylum of Euarthropoda. A few of the examples of the families of organisms that are an example of arthropods are insects such as ants, bees, and dragonflies. Arachnids such as the scorpions and the spiders, Myriapods such as the centipedes and the millipedes.Other crustaceans such as crabs, lobsters, and shrimp. 

The term “arthropod” is derived from a Greek word that means “jointed foot.” It's most likely an arthropod if the organism has an exoskeleton of joints between its feet and body. Arthropods are a group of animals that evolved skeletons on the outside, in the form of hard shells composed of a substance called "chitin," rather than on the inside, to provide structural protection. Arthropods’ bodies also differ from other vertebrates in a big way as their skeletal system is simpler and less efficient and that is the reason most of the Arthropods cannot outgrow their size. 

The arthropod whole-body plan consists of segments and each segment has a pair of appendages. Moulting is the process in which the arthropods shed their skins to grow.

Arthropods have a body and an external skeleton that is bilaterally symmetrical. Wings are present in some insects. With up to 10 million species, they are a very diverse group.

The internal cavity of an arthropod is called the hemocoel which helps in gathering all the interior organs and through the help of hemolymph, blood is circulated all over the body. The arthropods have an open circulatory system.

Arthropods' internal organs, including their exteriors, are made up of recurring segments. Their nervous system is "ladder-like," with paired ventral nerve cords that extend along each segment and form paired ganglia. Their brains are formed by the fusion of the ganglia of these segments and encircle the esophagus. Their heads are formed by the fusion of different numbers of segments, and their brains are formed by the fusion of the ganglia of these segments. Arthropods' respiratory and excretory systems differ based on their habitat as well as which subphylum they belong to.

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For vision, arthropods use a mixture of compound eyes and pigment-pit ocelli. Spiders' primary eyes are ocelli, which can create images and, in some cases, swivel to detect prey. Arthropods have a variety of chemical and mechanical sensors, most of which are dependent on modifications to the multiple setae bristles that protrude from their cuticles. Internal fertilization is used for all terrestrial animals, although the semen is often transferred indirectly through an appendage or the field.

All the arthropods are believed to have evolved from a single common ancestor though there are few scientists who still debate and are not sure of who the common ancestor was. Arthropods now contribute to human food production in two ways which are active as food and, more specifically, indirectly as seed pollinators. People, cattle, and crops have also been known to be infected by certain insects. 

In this article, we are going to discuss and raise a few questions such as what is an arthropod? which is the largest phylum of the animal kingdom?  the classification of phylum Arthropoda and also a few important frequently asked questions will also be answered. 

What is an Arthropod?

• An arthropod is an invertebrate that is usually characterized by the presence of multiple joints, a chitinous exoskeleton, various segmentation, and an open circulatory system. 

• The internal organs of the species are protected by the chitinous exoskeleton. It also serves as an attachment point for organisms' muscles, especially for arthropods with moving appendages.

• The exoskeleton of arthropods is made up of layers of chitin and proteins. The specialization of structures and organs in arthropods is accounted for by segmentation. Segments also serve as a location for arthropods' various limbs or appendages to emerge. These appendages serve a variety of roles, including feeling, driving, copulation, and feeding, among others.

• The arthropods have an open circulatory system which consists of the main heart and short arteries where the heart is able to deliver the hemolymph to the sinuses which surround the internal organs through shirt arteries.

• The phylum Arthropoda is further classified into sub-phylum classes, each of which is described by the characteristics of the species within it, such as the subphylum Trilobitomorpha. Crustacea includes lobsters, shrimps, and crabs, all of which are distinguished by their extinct trilobites. Members of the Chelicerata family have several bodies. Scorpions and spiders are among them. Myriapods and insects are members of the Tracheata subphylum.

• The biological arthropod definition is that “An invertebrate animal which is identifiable by its chitinous exoskeleton and multiple jointed appendages and it also comes under the phylum Arthropoda.”

Diversity of Arthropods 

• The actual number of arthropod species is unknown and it can’t be determined the actual precise number of them but the estimated number of arthropod species is between 2 million to about 30 million all around the world. This estimated number is enough, to say the least, that the arthropod species are all around the world and it accounts for more than eighty percent of all the known animal species. 

• It is also impossible to estimate the number of arthropod species around the world. This is due to census simulation predictions that were projected into other areas in order to scale up from local counts to global counts. According to a survey published in 1992, Costa Rica has 500,000 animal and plant species, with 365,000 of them being arthropods.

• The arthropods are very important members of the marine, land, freshwater, and air ecosystem. The arthropods are one of the two major animal groups that have successfully adapted to live in a dry environment. Other animals that have successfully adapted to live in dry conditions are the amniotes whose living members include animals such as reptiles, birds, and mammals 

• One of the subspecies of the arthropods is the insects which are considered to be the most species-rich member of all ecological guilds in the land and the freshwater environments. The heaviest insects weigh up to 70 grams whereas the lightest insects weigh up to 25 micrograms which is a millionth of a gram. 

• Some live crustaceans are much larger; for example, the legs of the Japanese spider crab can reach up to 4 meters or 13 feet, and the American lobster is the heaviest of all living arthropods, weighing over 20 kilograms

Characteristics of Arthropoda 

• The Arthropoda phylum is the largest phylum in animal kingdom which includes over 85% of the animals which are distributed all around the world. A few of the arthropods examples are insects, crabs, crabs and millipedes, and many more. 

• The number of discovered, known, and described arthropods are about 30 million species and over one million of these species are of insects. 

• Arthropod species are found to live in various habitats as they are found in deep oceans and on higher altitudes and that is the reason that the members of the phylum Arthropoda vary greatly in their structure and history.

• The biomass of arthropods is thought to be higher than that of all other species. They have distinguishing features that set them apart from other invertebrates. The jointed limbs and exoskeletons are examples of these characteristics.

• Arthropod exoskeletons are formed from the rough cuticle coating. Arthropods' muscles are bound to the cuticle that runs through their body segments and arms, allowing them to walk freely. The cuticle is mostly made up of chitin starch, which is plentiful on the planet.

• Another important feature of the arthropods is their segmented bodies that are deemed to be bilaterally symmetric in nature and the number of different segments in their bodies with their appendages have been found to have several functions to the region from where they originate. 

• Arthropods have a dorsal heart and a blood cavity that makes up a haemocoel in their circulatory system. The central nervous system of the arthropod consists of a brain or the oesophageal centre anteriorly and a nerve cord with ganglions ventrally. The arthropod muscles are striated fibres but do not have epithelial cells with cilia. 

• Arthropods have been around for millions of years. The prevalent arthropods, trilobites, for example, were radiated around 550 million years ago in the early Palaeozoic oceans. Furthermore, it is thought that arthropods evolved from their early annelid ancestors during times of evolution. In the Devonian epoch, around 350 million years ago, the first arachnid, as well as butterflies, sea spiders, and millipedes, emerged about 400 million years ago.

Arthropods Structure

• Arthropod exoskeletons are critical to their performance and shortcomings. Exoskeletons have a variety of purposes, including providing protection from the outside world, assisting in locomotion, supporting the body, reducing water leakage, and storing electricity. The exoskeleton is made up of two layers: the epicuticle, which is non-chitinous and impermeable, and the endocuticle, which is porous and permeable.

• Chitin and protein are what the endocuticle is composed of. The endocuticle outer layer is very hard due to the presence and action of calcium carbonate on it. This trait has been observed in a variety of marine crustaceans, as well as arachnids and insects that sclerotize cuticle proteins. Since arthropods have such a diverse range of mechanisms, their mouthparts are hardened, allowing them to consume a wide range of food sources. In addition, the exoskeleton lines the trachea or respiratory tubes of insects and myriapods, and it extends to the gut. In addition to their versatility, this perfect mix of soft membranes and hard materials provides rigidity and strength.

• The natural composition of the arthropods gives them a special advantage over the endoskeleton of other vertebrates such as the solid endoskeleton of the arthropods is three times stronger than the other vertebrates in the same area of an exoskeleton. 

• Arthropods' endoskeletons are physically efficient due to their versatility, and as a result, they shape a broad variety of body structures and shapes, resulting in a wide variance in the morphology of different arthropods. This difference isn't seen anywhere else in the Animalia kingdom. However, exoskeletons can result in certain restrictions, such as the organism's physical size being reduced as it grows. As a result, as the organism becomes larger, the outer exoskeleton coating will shed. The exoskeleton must be perforated with sensilla in order to track the outside world.

• Despite the fact that there is a limitation on how much the arthropods can grow, it is believed that few of the extinct arthropods were able to grow to a length of 1.8 meters and few crustaceans such as the famous giant spider crabs can weigh up to 7 kilograms and except few arthropods, many of the arthropods are relatively very small in size. 

• It is believed by many scientists that marine arthropods can grow bigger than land arthropods. The largest spiders and insects that can be found on the land can weigh up to 200 grams or less. The largest ground spiders and insects weigh less than 100 grams. The Goliath beetles, the world's largest insect, are 10 cm wide and 15 cm tall. Mites and parasitic wasps are the tiniest arthropods, measuring less than 0.23 mm in length. And if they are lightweight (less than the weight of a cell's nucleus), they have complex habits and structures.

• The figure given below shows the diversity between different species of the phylum Arthropoda. Different arthropods examples are used to show the length and size of the arthropods as they can be as small as one millimetre and they can also be as large as four meters. Though they may differ in the structures they do have common features such as the segmented bodies with appendages and the presence of an outer exoskeleton.

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Life Cycle and Reproduction in the Arthropods 

• Most of the Arthropods mainly reproduce by sexual reproduction. However, there are many arthropods that are found to be hermaphroditic which means they have the organs of both sexes. One of the examples of hermaphroditic arthropods is barnacles. There are also a few crustaceans and insects that can reproduce by the means of parthenogenesis. When the conditions are favourable. Many of the arthropods revert back to reproducing sexually. 

• Sexual reproduction is the most preferred option by the arthropods that live in the aquatic environment and it is done by means of external fertilization. Crustaceans and Opiliones use specialized appendages such as the gonopods to reproduce sexually.

• All the terrestrial arthropods are known to reproduce by the means of internal fertilization. This is done when the males produce packets of sperms known as spermatophores which are then taken into the bodies by the females. 

• Arthropods are known to lay eggs but in the case of gravid scorpions, they produce eggs that hatch inside the body. 

• Arthropods will be fully grown adults when they are born. Others, on the other hand, are helpless until they moult for the first time. Insects without cuticles hatch as caterpillars or grubs in order to grow their adult tissues.

• Molting is a mechanism that controls the growth of arthropods with an outer hardcover. Molting is regulated by various hormones that work together to stimulate the growth of the Arthropod.

• The enzymes digest and break down the old cuticle's inner layers, allowing the new cuticle to be separated. The arthropod swallows air or water to harden the new soft cuticle.

• Without hatching, marine crustaceans evolve directly. After being given water, large eggs hatch. During its life cycle, the Chinese mitten crab migrates to the top and to the bottom to breed. As a result, it has an impermeable cuticle that can tolerate large increases in osmotic strain.

Respiration in Arthropods

• Respiration in arthropods is the common process in all arthropod species and it differs significantly. Many insects and the Myriapods have trachea, which is generally small tubes that help in the diffusion of oxygen throughout the body. Physical restrictions regulate the diffusion mechanism in arthropods so that an organism's development has little effect on the amount of oxygen supplied to its tissues. 

• Tracheal respiration, on the other hand, is very effective and allows for breathing with just a slight partial pressure differential between the outer atmosphere and the pressure within the trachea.

• Respiration when done through the trachea can help to deliver oxygen to the muscles of the insects. The marine arthropods are known to respire through the trachea which helps in delivering oxygen to the muscles of the insects. 

• In conclusion, we can say that the respiratory systems of arthropods vary greatly. Small animals also don't have any and their high surface area to volume ratio allows for easy oxygen diffusion across the body surface. The gills of Crustacea have normally modified appendages. Book lungs are found in many arachnids. Many butterflies, myriapods, and arachnids have tracheae, which are networks of branching tunnels that pass from body wall openings and supply oxygen directly to individual cells.

Classification of Phylum Arthropoda

• Since the introduction of the arthropods, different classifications of the phylum Arthropoda has been proposed and therefore there is no specific scheme that has been developed for classifying the arthropods. All the arthropods belong to the phylum of Euarthropoda which is sometimes referred to as Arthropoda.

• Euarthropoda or Arthropods is basically subdivided into five subphyla and of all the five subphyla only one is extinct. The five subphyla are listed below. 

1. Trilobites are considered to be a group of formerly many marine animals that had disappeared in the Permian–Triassic extinction event. Though before the event happened they were in a decline stage and were reduced to an extensive amount that they are now reduced to one order in the Late Devonian extinction.

2. Chelicerates consist of arthropods such as spiders, mites, scorpions, horseshoe crabs, and other related organisms. Chelicerae, appendages just over or in front of the lips, are a distinguishing feature. Chelicerae are small claws that scorpions and horseshoe crabs use to eat, but they have evolved into fangs that deliver venom in spiders.

3. Myriapods consist of arthropods such as the centipedes, millipedes, and their other relatives. Myriapods have many body segments and each segment has two pairs of legs. Myriapods are sometimes grouped with hexapods.

4. Crustaceans are considered to be primarily aquatic and are basically characterized by having biramous appendages. A few of the arthropods examples that are included in the Crustaceans are crabs, lobsters, crayfish, shrimp, and many more 

5. Hexapods mainly consist of insects and three small orders of insects such as the animals with six thoracic legs. The Hexapods are sometimes grouped with the myriapods in a group known as the Uniramia

• Apart from these major groups, there are a variety of fossil types, mainly from the Early Cambrian, that are difficult to place, either because they have no apparent affiliation to any of the main groups or because they have a strong affinity to all of them. Marrella was the first to be described as being distinct from the other well-known groups.

Types of Arthropods 

• Have you ever raised a question or asked yourself about what are the different types of Arthropods based on habitat?. Well, Arthropods are found to live either on land or in freshwater. Most of the arthropods that live in freshwater are crayfish and brachiopods. There are also other members of the other group that lives in the freshwater such as copepods, crabs, shrimps, isopods, ostracods, and amphipods.

• Freshwater species respond to their environment by incorporating structures that block water from entering their bodies. They avoid the clearance of ions from their bodies to the outside atmosphere, despite the fact that the fluids within their bodies are more dilated than the fluids in their environment.

• Crustaceans have an almost impermeable cuticle that prevents water from penetrating their bodies. The only permeable component that allows gas exchange in the gills. As a result, in Crustaceans, gills are in control of osmoregulation.

• In order to resist the dilution of water inside their bodies, the Crustaceans are known to release amino acids from the muscles into their blood. With the help of the excretory organs, they are able to get rid of the excess water. 

• Semi-terrestrial animals, including 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 comparatively high. This discovery indicates that semi-terrestrial life may have originated from seawater. Ground crabs (a full family of amphipods) have stiffened gills that serve as lungs and enable some gaseous exchange, allowing them to survive in both water and on land. About the fact that they spend most of their time on land, they breed in the sea and leave their eggs to mature directly.

Ecological Importance of the Arthropods 

• Arthropods are considered to be the main component of the ecosystem as they have occupied an important place in the food chain. Almost 80% of the living creatures around the world are said to be Arthropods as they could be seen at the darkest places inside the oceans and can also be seen on the brightest mountains. Arthropods do not have one role as they could be decomposers, pollinators, parasites, seed dispersers, herbivores, and predators. 

• Arthropods have a variety of characteristics that allow them to play an important role in the ecological system. They have high reproductive ability, a wide range of adaptations for diverse environmental environments, a limited scale, and a wide range of biodiversity. Arthropods are used as biological markers to determine the ecosystem's health. Biological metrics are mostly used in the analysis of habitat fluctuations and status by supplying data that aids in the management of ecosystem plans.

• In comparison to the birds and vertebrates, the Arthropods have more temporal and explicit scales just because of their wide geographical distribution, quick turnovers, large size population, and patch dynamics.

• Arthropods are extensively consumed by many animals and they are basically the providers for the carnivorous, parasitoids, and other consumers. Arthropods also act as an elimination where they act as detritivores and decomposers that usually decompose the dead animals. They also act as an agent for dispersing seeds, pollination, and designing microhabitats. Without arthropods, the proper functioning of the ecosystem will be disturbed and that is the reason that the presence of arthropods in the ecosystem is very important. 

• Arthropods in the soil decompose the forest habitat while flying arthropods pollinate it. As a result, the lack of pollinators will potentially lead to a reduction in seed production, while the loss of soil arthropods will likely result in an ecological imbalance.