Arthropods

Definition, Characteristics, Classification and Habitat


Definition: What are Arthropods?



Arthropods, which make up the phylum Arthropoda, is the largest group of invertebrates (animals without a vertebral column) consisting of well over 80 percent of all animals. The phylum includes such animals as spiders, insects, shrimps and craps among many others that can be found in a wide range of environments from the ocean floor to the mountain peaks.


Some of the main characteristics of arthropods include:


  • They have an exoskeleton
  • Segmented body
  • They have joined appendages
  • Bilaterally symmetrical


Kingdom: Animal - Arthropod belongs to the Animal Kingdom. As such, they are multicellular eukaryotic organisms that, for the most part, have the following characteristics:



  • Sexual reproduction
  • Heterotrophic
  • Cells lack cell walls
  • They have the ability to move from one point to another at least at some stage of their life

Phylum: Arthropoda - Also referred to as Euarthropoda in some spheres, arthropods make up the phylum Arthropoda consisting of well over a million described species of invertebrate animals.


Phylum Arthropoda is divided into the following sub-phyla:


Subphylum Trilobitomorpha


Members of the subphylum Trilobitomorpha, referred to as trilobites, are today wholly extinct. Although studies have shown them to have been abundant between the Cambrian and Ordovician geologic periods, they went extinct during the Permian geologic period as a result of the Permo-Triassic extinction event (the Great Dying).



Characteristics - Subphylum Trilobitomorpha


  • A flattened body that was oval in shape - The body was divided into several parts including the head shield known as Cephalon, compound eyes located on the upper part of the head section, a pair of antennae, the thorax with biramous limbs as well as a segmented terminal section known as pygidium.
  • They ranged from one millimeter to 76cm in length - Whereas smaller trilobite species ranged from 1mm to 10 cm in length, some of the species grew to lengths of up to 76cm.
  • Three longitudinal lobes - As the name (trilobites) suggests,  their bodies had three longitudinal lobes (central axial lobe)
  • A majority of trilobites had a wide body with a few having a streamlined/narrow body
  • Some were spiny while others had a smooth surface
  • They were largely free-living organisms that were found in a many marine ecological niches

Subphylum Trilobitomorpha consists of a single Class (Trilobita) that consisted of such groups as agnostids that are commonly interpreted as planktons. Many trilobite species have been identified from fossil records thus providing more details about the sub-phylum. 


Some of the species that have been identified so far include:

  • Kainella meridionalis,
  • Onychopyge cf. riojana
  • Ogygites collingwoodensis
  • Incaia bishopi
  • Triarthrus eatoni

* Given that trilobites were largely marine animals, their diet may have largely consisted of dead or decaying matter as well as marine plankton and some crustaceans.


Subphylum Chelicerata


Subphylum Chelicerata is composed of such marine and terrestrial animals as scorpions, horseshoe crabs, ticks, mites and spiders among others. Apart from various existing animals, Chelicerata also includes a number of extinct organisms such as eurypterids.

With well over 100,000 described species, members of subphylum Chelicerata can be found in different types of habitats in the environment. While a few of these organisms can cause harm to human beings and animals, a majority are harmless, free-living organisms while some are beneficial.


Subphylum Chelicerata is divided into the following classes:


Class Arachnida


Arachnida, a class belonging to subphylum Chelicerata is composed of a diverse group of organisms including spiders, scorpions, mites, and ticks among many others. Currently, over 90,000 species belonging to class Arachnida have been identified with more than 8,000 of these living in North America.

Depending on the species, Arachnids can be found in aquatic and terrestrial environments. For instance, some mites (Hydrachnidia), which are some of the most ubiquitous organisms on earth. Hydrachnidia (water mites) in particular can be found in freshwater environments.

Apart from Hydrachnidia, there are other different types of mites in particular that can live in water (semi-aquatic). A majority of Arachnids, however, are terrestrial organisms where they mostly live as predators of smaller insects.

* The name Arachnida comes from the Greek word aráchnē that means spider. Here, it is also worth noting that spiders represent the majority of arachnids.



Major Characteristics - Class Arachnida


  • Two body parts - consist of an abdomen (posterior part of the body) and a cephalothorax which is the anterior part of the body. The cephalothorax is composed of two the dorsal carapace and the ventral sternum.
  • Six pairs of appendages - These include the chelicerae (fangs), which are the first pair of appendages, pedipalps that either act as sensory organs in some organisms (spiders) or as pincers in others (scorpions) and the rest that are used for walking (legs) - The mouthparts of arachnids are made for ingesting a fluid meal rather than chewing (they do not have a jaw)
  • Four pairs of legs
  • They do not possess an antennae
  • Venom glands of arachnids vary from one species to another
  • Possess a dorsal heart that allows for an open circulatory system
  • Have a nervous system that consists of a ventral nerve cord and fused ganglia
  • Reproduction is largely through indirect fertilization- Here, pedipalps are used to transfer spermatophores for fertilization.


Class Merostomata


Class Merostomata consists of two types of organisms including the horseshoe crabs (four species of horseshoe crabs) as well as eurypterids (sea scorpions). While horseshoe crabs are still in existent, sea scorpions are now extinct.

Compared to other classes that consist of organisms found in different environments, Horseshoe crabs which are the only surviving organisms in this class are largely marine organisms. For this reason, most species, apart from Carcinoscorpius rotundicauda, are marine organisms.



Main Characteristics - Class Merostomata

 

  • Possess a dorsal plate that resembles a horseshoe - Thus the name horseshoe crabs
  • Are predators of smaller organisms (worms, some small fish etc)
  • They can be found on the sea floor where they may be buried in mud
  • Six pairs of appendages. five for moving (four pairs are used for walking while the last pair is used for pushing the animal) and a chelicera used for feeding
  • Two main body parts that include the cephalothorax (the part covered by a hard carapace) and an abdomen. The posterior part of the organisms consists of a tail spine. The tail spine (caudal spine), also referred to as a telson is used for turning the crabs when they are overturned
  • Females lay eggs at the shallow coastal waters that are then externally fertilized by the male
  • Have book gills on their abdominal legs that are used for gaseous exchange
  • Well developed circulatory system
  • A nervous system that is made up of a circum-esophageal brain, a ventral nerve cord as well as ganglia
  • Five pairs of eyes that make up the two compound lateral eyes


Class Pycnogonida


Commonly known as sea spiders, Pycnogonida is made up of marine organisms consisting of about 1,000 species. Although they are referred to as sea spiders, Pycnogonida are not really spiders, but only organisms that resemble spiders.


Main Characteristics - Pycnogonids


  • Long legs and a small abdomen- For this reason, they are also known as Pantopoda which means all legs
  • Long legs possess claws
  • Although a majority of Pycnogonids are small, some of those that live in the deep sea can grow to reach 70 centimeters in diameter
  • A long proboscis through which they suck juices from the prey (small invertebrates with soft bodies such as cnidarians)
  • Also possess a pair of claws on their head as well as a pair of ovigers used for carrying their eggs
  • While most Pycnogonids have four pairs of legs, some may have more than four pairs


Major Characteristics - Subphylum Chelicerata

 

  • Two main body segments
  • Have four pairs of legs
  • Have pedipalps, but lack antennae
  • Have Chelicerae that are used to hold food
  • With an exception of a few, a majority of organisms in this subphylum are terrestrial


Subphylum Myriapoda




Subphylum Myriapoda is composed of about 13,000 species spread across four major classes. As is the case with all arthropods, Myriapods have a segmented body as well as many legs that are segmented.

They also have an exoskeleton made up of chitin. With regards to size, Myriapods range from tiny to about 30 centimeters in length. Some, however, are almost microscopic.

Myriapods can be found in different terrestrial environments such as soil, littler and moist forests where they break down plant material. A majority of Myriapods are herbivores and thus feed on plant material. However, some live as predators of small animals.


Examples of Myriapods include:

  • centipedes
  • symphylans
  • millipedes
  • pauropods


Classes of subphylum Myriapoda:


Class Chilopoda


Class Chilopoda consists of centipedes. These are uniramian arthropods with an elongated, flattened body consisting of between 12 and 100 segments. However, the foremost and last body segments of centipedes may vary in shape when compared to the other segments.

In addition to a pair of jointed antennae, the head (first segment) of centipedes also posses appendages that are modified to form jaw-like, poisonous claws/fangs. While the name centipede means "one hundred legs" members of class Chilopoda have between 30 and 300 legs depending on the species.

Each of the body segments, apart from the first and last body segment, consists of a pair of jointed legs that are not only used for walking, but also for holding the prey when it is captured.  

Members of class Chilopoda are terrestrial and thus live on land. However, they need moist habitats and are therefore commonly found in such microhabitats as under rocks, logs, litter and dark, moist areas among others. In these habitats, a majority of centipedes live as predators of other smaller animals and arthropods. However, Giant centipedes (Scolopendra gigantea) found in parts of South America have been shown to prey on much larger animals such as frogs and mice. 

Among members of class Chilopoda, fertilization is internal. Following fertilization, the female lays eggs in moist habitats, which (nymphs) in turn hatch and go through a series of molts before reaching adulthood.

As adults, centipedes protect themselves using a number of special adaptations that include: inflicting painful bites, throwing a sticky substance using their hind legs or simply curl into a ball. This, however, is dependent on the species.


With over 3,000 species of Chilopoda identified today, some examples of Chilopoda species include:

  • Bothropolys xanti
  • Lithobius microps
  • Brown Centipede
  • Zygethobius ecologus
  • Great Plains Soil Centipede
  • Strigamia bidens
  • Strigamia epileptica


Class Diplopoda


Class Diplopoda is composed of about 10,000 species of millipedes. While the name (millipede) suggests that they have 1,000 legs, the largest species have about 350 legs with a majority having about 30 pairs of legs.

Compared to centipedes, millipedes have diplosomite segments (each of the segments consists of two fused segments) with two pairs of legs on each of the fused segments.

The head does not have legs while the first three segments only have a single pair each. The head segment has one pair of antennae as well as chewing mouthparts used for feeding on plants.


Other Characteristics - Diplopoda


  • Simple eyes among a few species
  • Two pairs of ganglia and heart arteries in each of the segments
  • Measure between 2mm and 280 mm in length
  • Majority of millipedes have an exoskeleton that is made up of calcareous dorsal plates
  • Defense strategy involves curling into a coil as well as the production of a toxic fluid/gas among many of the species
  • Fertilization is internal (males use specialized legs in the seventh segment to transfer sperm cells to the female)
  • Female millipedes release eggs in moist habitats that ultimately hatch to release young ones that go through several molts before they mature
  • Majority of millipedes are herbivores that feed on decaying plant matter and other plant material. However, some are live as predators and scavengers

Species of millipede include:

  • Archispirostreptus gigas
  • Narceus americanus
  • Tachypodoiulus niger
  • Greenhouse millipede
  • Ommatoiulus sabulosus


Class Pauropoda


Compared to members of class Chilopoda and Diplopoda, Pauropods are tiny organisms that range between 0.5-1.5 mm in length. Their bodies are pale (whitish or pale brown) and softer when compared to the aforementioned classes and consist of between 8 and 11 segments.


Other Characteristics - Class Pauropods


  • About 700 species have been described so far
  • Majority of species have separate sexes
  • Abundant in moist habitats (under rocks, soils, decaying logs etc)
  • They feed on tiny root material including root-like structures of some fungus
  • Can move very fast
  • Possess a poorly developed head that has branched antennae and small, weak chewing mouthparts (jaws and tiny curved teeth)


Class Symphyla


Examples:


  • Scolopendrellidae
  • Scutigerellidae
  • Symphyleila cubae Hilton

Members of class Symphyla, known as Symphylans, are terrestrial organisms that resemble centipedes. However, they are smaller and more translucent in appearance. Compared to the other classes, Symphylans are not well understood with only about 160 species identified today.


Characteristics - Symphylans


  • Have tomosvary organs - These allow them to detect changes in humidity and chemical composition
  • Have a pair of antennae on their head and a diffuse nervous system
  • Their bodies consists of a head and segmented trunk
  • Range from 2 to 30mm in length

·      They have a well developed mouth part consisting of mandibles

  • Live in moist microhabitats where they feed on plants and some animal material
  • They are separate sexes
  • Can be found across the world but are abundant in the tropics
  • Slender with a soft body
  • The base of each leg has a stiff spine (aids in movement) and a special sac that regulates water and salts


Subphylum Crustaceae




Consisting of some of the most recognizable organisms (lobsters, crabs etc), Subphylum Crustaceae is one of the most popular groups of invertebrates in the world. Currently, about 70,000 species of Subphylum Crustaceae have been identified spread across six classes.

Depending on the species, Crustaceans can be found in a wide range of habitats from terrestrial to aquatic environments. As such, they also greatly vary in their size, form, habit and life cycle among other characteristics.


Classes and Characteristics of Crustaceans



Class Branchiopoda


Organisms in class Branchiopoda are known as branchiopods and are further divided into three main orders that include: Anostraca, Notostraca, and Diplostraca.

This group is composed of about 800 species that lie in four major (living) groups that include Anostraca, Cladocera, Conchostraca and Notostraca.

A majority of these organisms (sea monkeys, tadpole shrimp and clam shrimp among others) are aquatic and can be found in fresh (majority) and marine environments.

They vary in size (0.25ml to 10cm in length) with diverse body forms that make it difficult to generalize them.


Some of the other characteristics of these organisms include:

  • Thorax and abdomen are fused (in indistinguishable in some case)
  • The number of appendages vary from one species to another
  • Compound and/or simple eyes
  • Can reproduce through parthenogenesis or sexual arrangement in some species
  • Whereas most feed on suspended/settled organic matter, others prey on smaller animals in their environment
  • Well developed circulatory and nervous system, digestive and excretory system


Class Remipedia


The bodies of members of class Remipedia are divided into two regions that include a cephalon and an elongated, segmented trunk.


Some of the characteristics associated with these organisms include:

  • A pair of antennae and laterally directed limbs
  • Worm-like
  • Measure between 0.30 to 1.8 inches in length
  • 10 to 32 body segments
  • Mouth parts include a pair of mandibles, two pairs of maxillae (one pair resembles fangs that bite and inject chemicals that can paralyze the prey)
  • Can be found on island shores and submerged sea caves
  • Feed on some shrimp and fish
  • Hermaphrodites with both male and female reproductive organs


Class Cephalocarida


Members belonging to this group have a head and a segmented thorax and abdomen.

Whereas the thorax (8 segments) has limbs on each segment, limbs are lacking on the abdomen segments. 12 benthic species of this group exist and are characterized by their small size (2-4mm), a large head section, no eyes, two pairs of antennae (in front of the mouth), mandibles and two pairs of maxillae as well as a telson on the abdomen.

They are often found in the intertidal zones where they feed on marine detritus.


Class Macillopoda


Class Maxillopoda is not particularly well understood due to the diverse organisms that include copepods and barnacles. Some of the few characteristics that unite these organisms include their small size and short bodies (apart from barnacles) and use of their feet for filter feeding (apart from barnacles).

On the other hand, they may have a similar number of segments on their divided bodies: a cephalic section with 5 segments, thoracic part with 6 segments and an abdominal section with 4 segments.


Class Ostracoda



Class Ostracode consists of about 30,000 species that look like small bivalve mollusks. They are commonly found in aquatic environments across the world with a majority of them living on the sea floor. However, some species are also found in freshwater habitats and moist terrestrial environments such as forests. A majority of ostracods reproduce sexually while a few are parthenogenic. In their habitats (lagoons, deep ocean and freshwater bodies etc) Ostracods have been shown to be filter feeders. However, some use their claws to prey on smaller animals.


Some of the other characteristics of Ostracods include:

  • 0.1mm to 32 mm in length
  • They look like shrimp They have pores on their shells
  • Compared to other classes, they have the least number of limbs


Class Malacostraca


Class Malacostraca consists of more than 30,000 species distributed in marine, freshwater and terrestrial environments. They have a hard exoskeleton with a body that is divided into three main parts including the thorax, abdomen, and Cephalon (with the head and thorax being fused into a cephalothorax).

Malacostracans are segmented (five segments on each body part/section) with each of the segment having a pair of appendages for a majority of the species.


Some of the other characteristics of Malacostracans include:

  • They have internal body gills
  • Highly developed open circulatory system among larger members
  • A large brain connected to ganglia
  • Stomach with two chambers
  • They are dioecious
  • A majority of these organisms are carnivorous in nature


Subphylum Hexapoda


This is one of the most diverse groups consisting of about 700,000 species. They include many winged animals (as adults) and some wingless groups such as proturans, springtails and bristletails.

A majority of organisms in this group are terrestrial (soil dwelling) while a few and various larval forms can be found in freshwater environments.


Some of the primary characteristics of this sub-phylum include:

  • Three body parts (head, thorax, and abdomen)
  • Three pairs of jointed legs attached to the thorax
  • They have compound eyes


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References


Anthony J. Wright and Roger A. Cooper. (2011). Phylum arthropoda, subphylum trilobitomorpha:trilobites. University of Wollongong. 

(2008) Millipedes (Class Diplopoda). In: Capinera J.L. (eds) Encyclopedia of Entomology. Springer, Dordrecht

Burden B. (2004) Symphylans (Class Symphyla). In: Encyclopedia of Entomology. Springer, 

Dordrecht Richard C. Brusca and Joel

W. Martin. Phylum Arthropoda. Crustacea: Crabs, Shrimps, and Thier Kin. 

Links 

https://entomology.unl.edu/class-arachnida

https://www.infoplease.com/encyclopedia/plants-and-animals/animals/zoology-invertebrates/arthropoda/subphylum-trilobitomorpha


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