What Are Arthropods? An Introduction To Arthropod Characteristics, Examples, And Class

The phylum Arthropoda, which includes arthropods, is the biggest group of invertebrate organisms (creatures without a vertebral column), accounting for well over 80% of all animals.

Spiders, insects, shrimps, and crabs are among the many species found in this phylum, which can be found in a variety of settings ranging from the seafloor to the mountain summits.

Arthropods have a number of distinct traits, including:

• Exoskeleton  (a type of exoskeleton that protects the creature from harm)
• Body that is segmented
• Appendages that are joined together
• Symmetrical on both sides

This article will discuss arthropods – including examples of them, their unique characteristics and which classification they belong to.

Which Classification Do Arthropods Belong To?

Animalia – Arthropods are members of the Animal Kingdom.

Consequently, they are multicellular eukaryotes with the following traits for the most part:

• Sexual reproduction is asexual reproduction.
• Heterotrophic
• Cell walls are absent in cells.
• At some time in their lives, they should be able to travel from one location to another.

Arthropods constitute the phylum Arthropoda, which includes over a million identified varieties of invertebrate animals and is also known as Euarthropoda in some circles.

The Arthropoda phylum is subdivided into the following subphyla:

Trilobitomorpha is a subphylum of the Trilobitomorpha phylum.

Trilobites, inhabitants of subphylum Trilobitomorpha, are now completely extinct.

They were common during the Cambrian to Ordovician geologic eras, but they became extinct throughout the Cambrian geological years as a result of a Permo-Triassic extinction event, according to research (the Great Dying).

What Are The Distinct Characteristics Of Arthropods?

A Flattened Oval Body

The anatomy was divided into various pieces, including the Cephalon head shield, compound eyes on the upper half of the head area, a set of antenna, the thorax bearing biramous limbs, and a segmental terminal segment known as pygidium.

Measurements

They measured one millimetre to 76 centimetres in length. While lesser trilobite species measured one millimetre to ten centimetres in length, some species reached lengths of up to 76 centimetres.

Lobe System

Like the title suggests, trilobites have three longitudinal lobes on their bodies (central axial lobe)

Body Type

The bulk of trilobites had a broad body, however a few had a streamlined/narrow body. Some had a prickly surface, and some had a smooth one.

They were mostly free-living organisms that may be found in a variety of maritime ecologies.

Subphylum Trilobitomorpha is composed of a solitary Class Trilobita that includes groups like agnostids, which are usually mistaken for planktons.

Numerous trilobite organisms have been discovered from fossil records, adding to the sub-knowledge. phylum’s

Because trilobites were mostly marine animals, their food was likely to consist primarily of dead or decaying materials, as well as aquatic plankton and crustaceans.

Chelicerata Subphylum

Horseshoe crabs, scorpions, mites, ticks, and spiders are among the marine and terrestrial animals that make up the Chelicerata subphylum.

Chelicerata comprises a set of extinct creatures, such as eurypterids, in addition to several living animals.

Members of the subphylum Chelicerata could be found in a variety of environments in the world, with over 100,000 species described.

While some of these species can damage humans and animals, the vast bulk are harmless, free-living organisms, with some even being beneficial.

The Chelicerata subphylum is organised into four classes:

Arachnida Class

Scorpions, spiders, mites, and ticks are among the many species that belong to the Arachnida class, which is part of the Chelicerata subphylum.

Currently, approximately 90,000 species of the Arachnida class have been recognised, with over 8,000 of them found in North America.

Arachnids could be found in both aquatic and terrestrial habitats, depending on the species.

For example, some mites (Hydrachnidia) are among the most common species on the planet. Freshwater conditions are particularly hospitable to Hydrachnidia (water mites).

Other than Hydrachnidia, there are several additional forms of mites that can survive in water (semi-aquatic).

The majority of arachnids, on the other hand, are terrestrial animals that prey on smaller insects.

Arachnida gets its origins from the Greek term aráchn, which means spider. It’s also interesting to note that spiders account for the vast bulk of arachnids.

Arachnidae – Major Characteristics

• The abdomen (posterior section of the body) and the cephalothorax (front part of the body) are the two bodily parts. 
• The dorsal carapace as well as the ventral sternum make up the cephalothorax.
• The chelicerae (fangs) are the first set of appendages, followed by the pedipalps, which operate as sensory organs in certain creatures (araneae) or as claws in others (scorpions), and the rest, which can be used for moving (legs).
• Arachnid mouthparts are designed for consuming a liquid food instead of crunching.
• There are four pairs of legs.
• Antennae are not present.
• Arachnid venom glands differ from species to species.
• Have a dorsal heart, allowing for a more open circulatory system.
• Have a ventral nerve connection and fused ganglia in their nervous system
• Pedipalps are utilised to transmit spermatophores for fertilisation, which is the most common method of reproduction.

Merostomata Class

Horseshoe crabs (four species) and eurypterids are two forms of creatures in the Merostomata class (sea scorpions).

Sea scorpions have become extinct, whereas horseshoe crabs remain around.

Horseshoe crabs, the only surviving creatures in this class, are mostly marine organisms, in contrast to those other classes that include organisms from many settings.

As a result, with the exception of Carcinoscorpius rotundicauda, most species are marine organisms.

Merostomata – Major Characteristics

• Horseshoe crabs have a dorsal plate which resembles a horseshoe, hence the name.
• They were discovered on the seafloor, possibly buried under muck.
• There are six appendages in total. a chelicera for feeding and five for movement (four pairs for strolling and the last pair for pushing the animal).
• The cephalothorax (the section of the body covered by a hard shell) and the abdomen are the two primary body parts. The organisms’ tail spine is located at the back of their bodies. When crabs are overturned, the tail spines (caudal spine), also known as a telson, are utilised to turn them.
• Females lay eggs in shallow coastal waters, which the male fertilises externally.
• On their abdominal legs, they have book gills that are used for gas exchange.
• Circulatory system that is well developed
• A neurological system made composed of ganglia, a circum-esophageal brain, and a ventral nerve cord.
• The two compounded lateral eyes are made up of five pairs of eyes.

Pycnogonida Class

Pycnogonida is a group of marine creatures that includes over 1,000 species and is sometimes referred to as sea spiders.

Despite the fact that they are called sea spiders, they are not true spiders, but rather creatures that resemble spiders.

Pycnogonids – Major Characteristics

• Because of their long legs and little abdomen, they are also called Pantopoda, which means “all legs.”
• Claws can be found on long legs.
• Although the bulk of Pycnogonids appear small, some deep-sea Pycnogonids can grow to be 70 centimetres in diameter.
• They have a lengthy proboscis by which they suck the prey’s liquids
• They also have a pair of talons on their heads, as well as ovigers, which they utilise to transport their eggs.
• While the majority of Pycnogonids possess eight legs, others may have more.

Chelicerata Subphylum – Major Characteristics

• There are two primary body segments.
• Have a total of four legs
• Pedipalps are present, but there are no antennae.
• Have Chelicerae (food-holding cells)
• The bulk of creatures within the subphylum are terrestrial, with a few exceptions.

Myriapoda Subphylum

There are around 13,000 species in the subphylum Myriapoda, which are divided into four primary classes.

Myriapods possess a segmented body and multiple segmented legs, as do all arthropods.

They have a chitin-based exoskeleton. Myriapods range in size from a few millimetres to about 30 centimetres.

Some, on the other hand, are nearly tiny.

Myriapods decompose plant material in a variety of terrestrial habitats, including soil, wetlands, and moist forests.

The bulk of Myriapods are herbivores, which means they eat plant matter. Some, on the other hand, live as small-animal predators.

Myriapods comprise the following animals:

• symphlans
• millipedes
• pauropods
• centipedes

Subphylum Myriapoda has the following classes:

Chilopoda Class

Centipedes belong to the Chilopoda class. They are uniramian arthropods and have a flattened, elongated body with between 12 and 100 sections.

Centipedes’ first and last body segments, however, may differ in shape from the rest of the body.

Centipedes’ heads (first segment) have appendages that have been adapted to generate jaw-like, venomous claws/fangs with a set of jointed antennae. 

Although the name centipede refers to a creature with “one hundred legs,” members of the Chilopoda class have anywhere from 30 to 300 legs, depending on the variety.

Chilopoda members are terrestrial, meaning they dwell on land.

They do, however, require moist environments and can be found in microhabitats like under rocks, logs, litter, and dark, moist locations, among others. 

The bulk of centipedes survive in these ecosystems as hunters of other small mammals and arthropods.

Scolopendra gigantea located in South America, on the other hand, have been observed preying on much larger species including frogs and mice.

Internal fertilisation is used by members of the Chilopoda class.

The female lays eggs in damp habitats after fertilisation, and the nymphs hatch and go through a sequence of moults before reaching adulthood.

Diplopoda Class

There are roughly 10,000 millipede species in the Diplopoda class.

While the name millipede implies that they must have 1,000 limbs, the largest species only have about 350, with the majority having only about 30 pairs.

Millipedes, unlike centipedes, have diplosomite segments (two fused segments on each fused segment) with four legs on each fused segment.

The head lacks legs, although the first three parts each have only one pair.

One set of antennae, and also chewing mouthparts for eating on plants, are found on the head segment.

Diplopoda’s Other Characteristics

• A few creatures have simple eyes.
• Each section has two pairs of ganglion and cardiac arteries.
• The length should be between 2mm and 280mm.
• The bulk of millipedes have calcareous dorsal plates as an exoskeleton.
• Many of the species’ defence strategies include winding into a coil and producing a noxious fluid or gas.
• Male millipedes utilise specialised legs inside the seventh section to transport sperm cells to female millipedes. Female millipedes deposit eggs in damp habitats, which hatch into young millipedes who go through numerous moults before maturing.
• Millipedes eat decomposing plants and other plant matter.

Pauropoda Class

Pauropods are little animals that range in size from 0.5 to 1.5 mm in length when compared to members of the Chilopoda and Diplopoda classes.

When contrasted to the other groups, their bodies are lighter (white or light brown) and softer, with between 8 to 11 segments.

Pauropoda’s Other Characteristics

• So far, approximately 700 species are known.
• The majority of creatures have two sexes.
• It thrives in damp environments.
• They eat microscopic root material, including fungal root-like structures.
• Can move incredibly quickly
• Have tiny, weak chewing mouthparts and a poorly developed head with branched antennae.

Symphyla Class

Symphylans are centipede-like terrestrial creatures that belong to the Symphyla class.

They are, however, smaller and have a more transparent aspect.

Symphylans, in comparison to some other classes, are little understood, with around 160 species now known.

Symphylans Other Characteristics

• They have tomosvary organs, which allow them to detect moisture and chemical composition changes.
• They have a diffuse neural system and a pair of antennas on their heads.
• Their bodies are made up of a head and a segmented trunk that range in length from 2 to 30mm. They have a well-developed mouth with mandibles.
• They live in wet microhabitats and eat plants as well as some animal matter.
• Is there a distinction between the sexes?
• They can be found all around the world, although they are most plentiful in the tropics.
• Body is slender and delicate.
• A stiff spine (which assists mobility) and a specific sac that balances water and salts are situated at the base of every leg.

Crustaceae Subphylum

Subphylum Crustaceae is among the most well-known groupings of invertebrates, containing some of the most well-known animals (lobsters, crabs, and so on).

There are over 70,000 species of Crustaceae in the Subphylum Crustaceae, which are divided into six classes.

Crustaceans can be located in a range of habitats, from terrestrial to aquatic, depending on the species.

As a result, they differ widely in size, form, habit, and life cycle, among other traits.

Crustacean Classes and Characteristics

• Branchiopoda is a class of animals that belong to the branchiopod
• Branchiopods are organisms that belong to the class Branchiopoda, which is divided into three orders: Anostraca, Diplostraca and Notostraca.
• There are roughly 800 species in this category, which are divided into four major (living) groups: Anostraca, Cladocera, Conchostraca, and Notostraca.
• The bulk of these animals (including sea monkeys, tadpole shrimp, and clam shrimp) are aqueous and can be found in both freshwater and marine habitats.
• They come in a variety of sizes (0.25ml through 10cm in length) and body shapes, making it difficult to categorise them.

These species also have the following characteristics:

• The thorax and abdomen are joined together.
• The number of appendages differs between species.
• Eyes that are compound and/or simple
• In some species, parthenogenesis or sexual organization can be used to reproduce.
• Others hunt on tiny animals in their habitat, but the majority survive on suspended/settled organic debris.
• The cardiovascular and neurological systems, as well as the digestive and excretory systems, are all well developed.

Remipedia Class 

Remipedia members have a cephalon as well as an extended, segmented trunk separated into two areas on their bodies.

The following are some of the traits connected with these organisms:

• Antennae and limbs that are angled laterally
• Worm-like
• 10 through 32 body segments, ranging in length between 0.30 and 1.8 inches
• A set of mandibles and four maxillae make up the mouth parts.
• Can be spotted on the beaches of islands and in submerged marine caves.
• Consume a variety of shrimp and fish.
• Female and male reproductive systems are found in hermaphrodites.

Cephalocarida Class

This group’s members feature a head, as well as a divided thorax and abdomen.

The thorax (8 sections) possesses appendages on each segment, whereas the abdomen segments lack limbs. 

There are 12 benthic species in this group, all of which are small (2-4mm), have a big head portion, without eyes, two sets of antenna (at the front of the jaw), mandibles, and two sets of maxillae, and have a telson upon the abdomen.

They feed on marine waste and can be observed in the intertidal zones.

Maxillopoda Class

Due to the numerous species which include copepods and barnacles, the class Maxillopoda is poorly understood.

Apart from barnacles, these organisms share a few traits, such as their tiny size and short bodies, through the use of the feet for feeding.

They may, on the other side, have a comparable number of components on their segmented bodies: 5 segments in the cephalic section, 6 segments in the thoracic section, and 4 segments in the abdominal section.

Ostracoda Class 

About 30,000 species of tiny bivalve mollusks make up the Ostracode class.

They can be found in a variety of aquatic habitats around the world, with the bulk of them dwelling on the ocean floor. 

Some species, however, can be found in both freshwater ecosystems and moist terrestrial areas like woods. The bulk of ostracods reproduce sexually, with a few exceptions. 

Ostracods have been demonstrated to be filter feeders in their natural habitats (lagoons, deep ocean, and freshwater bodies, for example).

Some, on the other hand, utilise their claws to eat smaller creatures.

Ostracods have a number of other traits, including:

• Lengths ranging from 0.1mm to 32mm
• They resemble shrimp. Pores can be found on their shells.
• They have the fewest limbs compared to other classes.

Malacostraca Class

There are about 30,000 species in the Malacostraca class, which can be found in marine, freshwater, and terrestrial settings. They have a strong exoskeleton and a structure that is separated into three sections.

For the most part, Malacostracans are divided (five sections on each body part/section), with each segment possessing a pair of appendages.

Malacostracans have a number of other features, including:

• Gills on the inside of the body
• Larger members have a well-developed open circulatory system.
• A huge brain with ganglia attached to it
• The stomach is divided into two chambers.
• They are sexually dioecious.
• In nature, the bulk of these species are carnivorous.

Hexapoda Class

With over 700,000 species, this is among the most varied categories.

Numerous winged creatures and some flightless species like proturans, springtails, and bristletails are among them.

The bulk of the species in this group live on land (in soil), however a few and varied larval versions can be observed in freshwater.

The following are some of the sub-key phylum’s characteristics:

• There are three body components.
• The thorax is supported by six jointed legs.
• Their eyes are compound.

Arthropod Diet

Arthropods can feed in a variety of ways. There are specialisations within these major categories, such as herbivores, carnivores, detritus eaters, filter feeders, and parasites.

Usually, paired limbs from around their mouths are used to collect and handle food, and they are usually tailored according to the animal’s diet. 

The mouthparts of the insect family Aphididae, for example, are designed to pierce vegetation and extract plant fluids.

Fiddler crustaceans, which come from tunnels on sand flats during low tide, sweep up surface sands with their little claws and deposit it within the mouthparts, where fine hairs sift it. 

The mineral substance is discharged as a little “spitball,” while the material is digested.

Ejected debris from crabs may leave the top of a flat at the end of a low-tide period if there is a big population of crabs.

After reburying themselves, the crustacean mole crabs, also known as sand crabs, use their antenna to collect plankton out from receding waves. 

With particular limbs (maxillae) near the mouth, planktonic crustacean copepods as small as a few millimetres long can capture up to a few thousand diatoms over 24 hours.

A variety of carnivorous arthropods, including scorpions, pseudoscorpions, and centipedes, employ poison to catch food, which is normally administered with a pair of limbs; scorpions use a solitary stinger at the tip of their tail. 

The poison is delivered by a set of teeth (chelicerae) surrounding the mouth in spiders, and poison claws lie behind the head in centipedes.

Only a few of these species contain venom that can kill humans.

The foregut and hindgut (the front and back regions of the digestive tract) are coated with the same bones present around the outside of the body, which moults with the remainder of the skeleton. 

Only the little middle region (midgut) is devoid of chitinous coating.

The structure of the digestive tract changes substantially depending on the animal’s diet and feeding method. 

The midgut, on the other hand, is the primary site of enzymatic activity and processed food absorption.

In the instance of spiders, the enzymes may travel forwards into the front region of the guts or even outside through into the prey’s body.

Arthropod Research 

Biodiversity refers to the diversity of living things at all scales, from genetics to organisms to environments – it is the bedrock of the biological sciences.

There is now widespread scientific agreement that species diversity is vanishing at an unparalleled rate in human history.

We know that rising biodiversity loss is affecting biosphere-level processes that provide $3-33 trillion in annual environmental services. 

Biodiversity research is urgently needed because growing human populations and accompanying human activities are increasingly destroying natural areas that support biodiversity around the world.

Human influences on ecosystems (habitat destruction, fragmentation, and reorganisation) and creatures are among the causes of biodiversity loss. 

Biodiversity is critical to human health: the quality of our air, water, and food is dependent on renewable processes and healthy ecosystems, which are fueled by biodiversity.

By explaining the unique characteristics of the biota and partnering with local academics, government, and conservation groups, work in the Division of Entomology contributes to a better knowledge and conservation of biodiversity. 

Basic biological research must be hastened, otherwise we will be limited in our knowledge to fix looming scientific and human challenges.

Current Goals, Efforts, And Strategies

• Original research, student training, and public involvement through civic science and extension programmes are among major goals. 
• Using collection-based study, comparative morphology, comparative genomes, and other applicable data, enhance our knowledge of evolutionary processes, arthropod biodiversity, and systematics.
• Conduct world-class studies on the development, ecosystems, and preservation of insects, especially pollinating and native insects.

Climate Change Implications For Arthropods

According to several research projects, tropical arthropods are highly vulnerable to climate change.

If these projections come true, climate change could have a far greater influence on tropical forest functioning and variety than previously thought. 

Despite the fact that arthropods make up more than two-thirds of terrestrial animals, data on their prevalence and rates of extinction in tropical ecosystems is scarce.

The reaction of arthropods to climatic changes is of special importance due to their number, diversity, and essential functions as herbivores, pollination, predators, and prey. 

According to research, rates of rise for insects dwelling at mid-to-high latitudes should increase as the temperature warms, while insects in the tropics should fall by up to 20%. 

Reduced population growth paired with increased metabolic rates could result in lower abundances and higher extinction rates for arthropods. 

If these projections come true, climate change might have a far greater influence on the operations and diversity of the world’s tropical forests than previously thought.

Despite the fact that arthropods make up a substantial portion of all terrestrial animals and are critical to the Earth’s ecosystems’ ecological well-being, long-term statistics on population richness and extinction trends are scarce.

Climate change, together with ecological disturbance and insecticides, have been identified as main causative processes in insect decline studies focusing on temperate species. 

While climate change has been shown to reduce plant variety, modify species of plant composition, and increase trees, mortality, and biomass in tropical forests, very little data is available about the effects of climate change on rainforest arthropods.

Distribution Of Arthropods

We don’t know how the various families of arthropods are linked, despite the fact that they are everywhere.

Many competing theories have been suggested by scientists about how arthropods developed and diversified. 

The links among groups of arthropods have been studied by scientists.

They used modern technology to re-examine known data and research some freshly discovered Cambrian fossils in an attempt to solve arthropod phylogenetic concerns. 

DNA sequencing, electron microscopy, and machine phylogenetic analysis were among the novel technologies used by the researchers.

Because today’s animals have undergone a lot of evolution, it’s difficult to find knowledge about the arthropod phylogenetic tree’s deep branches. 

Researchers investigate the phylogenies of every arthropod family and then select only those species which are most likely to reveal knowledge about their ancestors to improve their odds of obtaining evidence to these early branches. 

Horseshoe crabs, scorpions, worms, leptostracan ‘shrimps,’ and wingless insects like silverfish are examples of such species.

Our comprehension of arthropod ancestry is far from complete, and finding definite answers to this topic will have far-reaching implications for biodiversity research.

Summary

Arthropods are classified as any member of the phylum Arthropoda, which includes such known types as spiders, millipedes, and lobsters, and is the biggest phylum in the animal kingdom. 

Arthropods have existed for a very long period.

Their progenitors lived 530 million years back in Cambrian waters, during a period that is still poorly understood.

Arthropods exist today as a result of their ability to adapt to changing settings over such a long period of time.

This phylum contains around 84 percent among all known animal species.

Arthropods can be found in every ecosystem on the planet and exhibit a wide range of adaptations.

Several varieties live in aquatic conditions, while others live on land; some even have the ability to fly.

Due to the increasing impact of climate change and human activity on the environments of arthropods, efforts must be made to reduce the negative effects of these factors on the arthropods ecosystem to decrease the rates of extinction among their populations.

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