n., plural: arthropods
Definition: any of the animals of the phylum Arthropoda
What is an arthropod? An arthropod is an invertebrate living organism characterized by the presence of multiple joints, a chitinous exoskeleton, segmentation, and an open circulatory system.
The chitinous exoskeleton has a protective function for the internal organs of the organisms. Moreover, it provides an attachment point to the muscles of the organisms, specifically for the arthropods with moving appendages.
The arthropod’s exoskeleton is made up of layers consisting of chitin and proteins. Segmentation is arthropods that accounts for the specialization of the structures and organs. Additionally, segments provide a region where the multiple limbs or appendages of arthropods arise. These appendages have several functions including sensation, walking, copulation, feeding in addition to other functions.
The open circulatory system of the arthropods consists mainly of the heart and short arteries where the heart delivers the hemolymph to sinuses surrounding the internal organs through shirt arteries.
The phylum Arthropoda is further subdivided into sub-phylum groups where each group is characterized by specific features of the organisms within the group, such as subphylum Trilobitomorpha. Members of this group are characterized by their extinct trilobites, Crustacea includes lobsters, shrimps, and crabs. Chelicerata group members have multiple limbs. They include scorpions and spiders. Members of the Tracheata sub-phylum include myriapods and insects.
The Arthropoda phylum includes over 85% of animals, such as insects, shrimps, millipedes, spiders, and crabs. The number of known and described arthropods is about 30 million species. Over one million of these species are insects.
Arthropod species live in various habitats as they may live in deep oceans or high altitudes. Therefore, members of the phylum Arthropoda vary greatly in their structure and history.
Arthropod biomass is suggested to be greater than the sum of all other animals. They have special distinctive characteristics that distinguish them from other invertebrates. These features include the jointed limbs and the exoskeletons.
Exoskeletons of arthropods develop from the hard cuticle covering. The cuticle present between the body segments and limbs of Arthropods are attached to the muscles allowing their flexible movement. The cuticle is formed mainly of chitin carbohydrate, which is greatly abundant on earth.
Another specific feature of arthropods is their segmented bodies that appear bilaterally symmetric and the different number of segments in their bodies with their appendages have several functions according to the region where they originate.
The circulatory system of Arthropods consists of a dorsal heart in addition to a vascular space that makes up a hemocoel. The central nervous system consists of a brain or an oesophageal center anteriorly, and a nerve cord with ganglions ventrally. Arthropod muscles are mainly striated fibers without epithelial cells with cilia.
Arthropods were discovered millions of years ago. For example, dominant arthropods, the trilobites, were radiated about 550 million years ago in the Palaeozoic seas of the early ages. Moreover, it is suggested that arthropods had periods of development where they were developed from their early annelid ancestors. About 400 million years ago, the first arachnid appeared as well as the insects, sea spiders, and millipedes in the Devonian period that appeared about 350 million years ago.
Structure of Arthropods
The exoskeletons of Arthropods are essential for their successes and limitations. Exoskeletons have various functions such as protection from the external environment, help in locomotion, support their bodies, reduce the loss of water, and energy storage. The exoskeleton consists of a non-chitinous, impermeable, thin outer layer called the epicuticle and an elastic, permeable, thick inner layer called the endocuticle.
The endocuticle is mainly composed of chitin and protein. The endocuticle outer layer may be hardened by the action of calcium carbonate. This feature was found in various marine crustaceans or in arachnids and insects that sclerotize the cuticle proteins. The mouthparts are hardened so many food sources can be ingested by arthropods due to the great diversity in their structures. Moreover, the exoskeleton also lines the trachea or the respiratory tubes of insects and myriapods; additionally, they extend to the gut. This great combination of soft membranes along with hard materials provides rigidity and strength in addition to their flexibility.
The natural composition of the arthropod gives it special advantages over the endoskeleton of vertebrates such as the greater bending resistance since the solid endoskeletons of vertebrates are almost three times weaker than the same area of an exoskeleton. Consequently, for the endoskeleton to have a strength similar to the exoskeleton, it must be thicker leaving less space for muscles.
The endoskeleton of arthropods is mechanically efficient because of its flexibility; therefore, it forms a wide range of body structures and forms in arthropods resulting in a great variation in the composition of different arthropods. This variation is not present elsewhere in kingdom Animalia. However, exoskeletons may lead to the presence of some limitations that include the limited growing physical size of the organism. Therefore, an increase in the size of the organism will lead to the shedding of the outer exoskeleton covering. In order to monitor the outer world, the exoskeleton must be perforated with sensilla.
Despite the growth limitations, some extinct aquatic arthropods grew to a length of 1.8 meters and few crustaceans such as the giant spider-crabs can weigh up to 6.4 kg but now most arthropods are relatively small in size.
Marine arthropods are able to grow larger than land arthropods. The weight of the largest land spiders and insects is not more than 100 g. The measures of the largest insect called the Goliath beetles are 10 cm wide and 15 cm long. The smallest arthropods, exemplified by mites and parasitic wasps, are less than 0.23 mm. Even though they are small (their weight is less than a nucleus of a cell) they have complicated behaviors and complex structures.
Respiration in Arthropods
The respiration in arthropods differs in various arthropods. Myriapods and insects have trachea, which are small tubes that permit the diffusion of oxygen to all the body parts. The diffusion process in arthropods is governed by the physical constraints where the growth of an organism does not affect the amount of oxygen delivered to its tissues. However, tracheal respiration is very efficient and enables respiration with a small partial pressure difference between the outside atmosphere and the pressure in the trachea.
Respiration through the trachea can deliver oxygen to the muscles of the insect. Marine arthropods use either diffusion through the cuticle or gills (such as in the Crustacea). Arachnids have enclosed gills (book lung).
Even though exoskeletons cause many limitations, arthropods have many other adaptations that make them survive and thrive successfully over millions of years.
Life Cycle and Reproduction
Arthropods reproduce mainly by sexual reproduction. However, some arthropods are hermaphroditic, meaning they have the organs of both sexes, such as in barnacles. Few crustaceans and insects can reproduce by means of parthenogenesis. They revert to reproducing sexually when conditions become more favorable again.
Arthropods that live in aquatic habitats often reproduce sexually typically by means of external fertilization. Some Crustaceans and Opiliones reproduce sexually by using their specialized appendages that serve as penises or gonopods.
All terrestrial arthropods reproduce by internal fertilization. They do so usually by males producing packets of sperms called spermatophores, which the females take into their bodies.
Arthropods usually lay eggs. However, gravid scorpions produce eggs that hatch inside their body.
Arthropods may be fully developed as adults at birth. Others, though, are helpless until the first molt.
Insects that do not have cuticles hatch as caterpillars or grubs in order to build their tissues as adults.
The growth of arthropods with an outer hardcover occurs by a process called molting, which is controlled by various hormones that coordinate with each other to promote the growth of the Arthropod.
The enzymes digest and break down the inner old layers of the cuticle to be separated from the new cuticle. To harden the new flexible cuticle, the arthropod swallows air or water.
Marine crustaceans develop directly without hatching. Large eggs hatch after being supplied with water. In contrast, the Chinese mitten crab migrates to the surface and to the bottom during its life cycle to breed. Thus, it has a highly resistant impermeable cuticle that withstands great changes in osmotic pressure.
Watch the video below to see an example of an arthropod life cycle. This video presents the life cycle of a mosquito (insect).
Types of Arthropods
What are the different types of Arthropods based on habitat? Arthropods can live either on land or in freshwater. In freshwater, most crayfish and brachiopods live in freshwater. Some members of other groups also live in freshwaters, such as amphipods, copepods, crabs, shrimps, isopods, and ostracods.
Freshwater animals adapt to their habitat by providing mechanisms that prevent the entrance of the water into their bodies. Although the fluids inside their bodies are more diluted than in their environment they resist the clearance of ions from their bodies to the surrounding environment.
The cuticle of crustaceans is almost impermeable; thus, water cannot penetrate their body. The gills are the only permeable part to allow gas exchange. Therefore, gills are responsible for the osmoregulation in Crustacean.
Crustaceans liberate amino acids from the muscles into their blood in order to resist the dilution of water inside their bodies. They get rid of excess water in their bodies through the excretory organs.
Semi-terrestrial species, such as the fiddler crab, spend their life between air and water. They have fluids in their bodies that made their bodies to possess a relatively high osmotic pressure. This finding suggests that semi-terrestrial could have evolved and developed from seawater. For example, land crabs (representing a complete family of amphipods) have stiffened gills that act as a lung and permit some gaseous exchange enabling them to live in water or on land. Even though they mostly live on land, they breed in the sea and leave their eggs to develop in a direct way.
Classification of Arthropods
Different classifications of the phylum Arthropoda have been proposed since the introduction of arthropods. Therefore, there is no specific scheme for the classification of arthropods. The difference between different schemes lies in the principal structural features of arthropods, such as the genital opening position and the mandible’s articulation. These features are further used to classify arthropods hierarchically.
This classification lies mainly within two types. One proposes that arthropods originated from common ancestors so they are originally monophyletic. This is the most commonly used classification of arthropods in literature. The other view suggests that arthropods originally arose from different ancestors then they became similar to each other structurally by evolution so this scheme suggests that arthropods are polyphyletic.
Each view has its specific subdivisions for the arthropods. The monophyletic group assumes that all arthropods arose from an arthropod ancestor with distinct segmentations in its body. The monophyletic arthropods are classified into several groups such as crustaceans, insects, arachnids, pentastomids, trilobites, myriapods, etc.
One scheme of classification divides the arthropods into the trilobitomorpha, which includes the extinct trilobites, the Mandibulata (containing the articulated mandibles divided into seven classes, with antenna), and the Chelicerata (without antenna but with a head and with appendages used for walking). One of the major limitations of this scheme is the undefined relationship amongPentastomida, Tardigrada, and Onychophora, and the other arthropods.
Another similar classification is grouping the arthropods into six groups: (1) Chelicerata, (2) Trilobita, (3) Myriapoda, (4) Insecta, (5) Onychophora, and (6) Crustacea.
As for the polyphyletic-based classification of arthropods, there are three groups recognized. These groups are
- Uniramia, which consist of Myriapoda, the Onychophora, and Hexapods.
- The second group is the Crustacea.
- The third group is the Chelicerata.
Phylum Arthropoda is not recognized since each group represents a separate phylum. Some groups are left uncertain, such as the trilobites, which are suggested to represent a separate phylum because they are distinct. The Uniramia (including the Tardigrada) and the Pentastomida are not clearly classified as well.
The most recent arthropods classification classifies arthropods into 5 subphyla which are:
- Aquatic Crustacea (e.g. shrimp, crab species, and lobsters)
- Chelicerata (e.g. marine horseshoe crabs, mites, and scorpions)
- Myriapods (a subphylum containing centipedes and millipedes)
- Hexapods or Insecta, which is relatively close to crustacean or hexapods,
- Trilobites, which includes marine animals
Another way of classifying arthropods is shown in the video below. The arthropods are classified into three subphyla: Trilobita, Chelicerata, and Mandibulata.
Evolution of Arthropods
Trilobites were probably the first arthropods. Their fossil was discovered about 500 million years ago. Trilobites are marine arthropods with several segments and walking appendages After trilobites, more arthropods surfaced and evolved. One of their distinctive features is fused segments. Also, their appendages were lost. Eventually, arthropods evolved to develop a generally three-segmented body.
Arthropods were probably the earliest animals to live on earth. The first arthropods that moved to land were millipedes. They developed a trachea or lungs to breathe, as well as the exoskeleton that protects arthropods from drying, and provides structural support to the animal.
The common segmented structure of arthropods gives rise to several classes of great structural diversity. Even though all arthropods have exoskeletons, they are morphologically different from each other in terms of thorax, head, and abdomen. For instance, few of them lack segmented appendages, which are a common feature in many other groups of arthropods. The difference in the structure between these species depends on the genetic diversity among them.
A group of animals that are considered to be intermediate between the arthropods and the annelids is the Onychophora. Their bodies contain regular segments while they have many features similar to the arthropods such as breathing by the trachea, having claws at the end of their paired limbs, and the nature of their reproductive system. On the other hand, they are similar to annelids in their repeated segments of excretory organs, the structure of the head and eyes, and their unstriated muscles. Various evidence suggests that Onychophora, as well as, arthropods originated from common ancestors, the aquatic lobopods.
Crustaceans and insects are suggested to have common roots. This view is supported by the order of the mitochondrial gene, which is similar in both classes. Such findings should consider both classes to be grouped together. However, the second segment in Crustacea bears an additional pair of antennae, which is considered the main difference between the two groups.
Several crustaceans and insects have chewing mandibles. Fossils reveal that these mandibles in the third segment evolved by truncation from the whole limb. The great variety between members of crustacea is not found in insects. This accounts for a constant structure of three pairs of limbs in the thorax without any limbs in the abdomen. The diversity among crustaceans results from the different genetic patterns where isopods (the appendages responsible for feeding) are formed in the position of walking legs. The first segment in the thorax contains the head.
Arthropods are a main component of the ecosystem since they occupy an important place in food chains. Arthropods may act as decomposers, pollinators, herbivores, parasites, seed dispersers, and predators. They possess different characteristics that give them the ability to play a vital role in the ecological system. In particular, they have a great reproductive capacity, various adaptations for different environmental conditions, small size, and great biodiversity. Arthropods are used as biological indicators to indicate the integrity of the ecosystem. Biological indicators are used mainly in the study of fluctuations and status of the ecosystem by providing information that helps in the management of ecosystem plans.
Arthropods have more temporal and explicit scales in comparison to birds and vertebrates due to their wide geographical distribution, large patch size, quick turnover, large population size, patch dynamics.
Arthropods act as providers for carnivorous, parasitoids, and other consumers. Additionally, they act as eliminators where they act as detritivores and decomposers of dead organisms. They also serve as facilitators for the dispersion of seeds, pollination, and designing microhabitats. Therefore, the presence of arthropods in the ecosystem indicates its integrity.
Soil arthropods act as decomposers of the forest ecosystem while flying arthropods act as pollinators. Consequently, the loss of pollinators would eventually lead to a decrease in seed production, while the loss of soil arthropods may lead to an imbalance in the ecological system.
Watch this video to see what happens should bees become extinct.
Medical and Economic Uses
Arthropods are a huge part of human lives. For instance, several species of crustaceans are consumed by humans as a source of food (e.g. lobsters, crabs, etc.), Others, though, can be harmful to humans if they have recently fed on particular plants that acquire neurotoxins. Moreover, some of them can transmit parasites to humans. Insects and other animals are useful for the production of bees, wax, silk, and dye. They can also serve in the pollination of flowers.
Crustaceans are characterized by the presence of a pair of mandibles, two pairs of antennae, two maxillae pairs, and five or more pairs of legs. In the more advanced species such as lobsters and decapods, the body is divided into a segmented abdomen and a cephalothorax. Their appendages are specialized in order to perform specific functions. Most Crustacea species are marine. Other species live in freshwater and only 3% are completely terrestrial. Crustaceans breathe through the cuticle where gas exchange is performed all over their bodies or by gills. Crustacea species vary in their length, evolution, and habitat. The chelicerate body is usually divided into an opisthosoma and prosoma without the presence of an antenna. In chelicerates, the appendages are used as legs, unlike insects and crustacea which use it as jaws.
Crab species are one of the most abundant crustaceans. They belong to the Decapoda order. Crabs can colonize almost anywhere; they are found in terrestrial and marine habitats. They can live in deep oceans in a depth that reaches 6,000 meters or above sea level on mountains with a height up to 2,000 meters. Moreover, crabs can survive in estuarine habitats where the temperature or salinity fluctuates every day. The crabs have evolved to live in a terrestrial habitat. They occasionally return to the water. Other spices are totally freshwater, and some need only a transient source of water such as small water bodies in holes of trees.
About 6,793 species of crabs were identified in the infraorder Brachyura, crabs communicate by either waving or drumming, they are usually aggressive and fight with each other either for a home or a female crab. They work together to protect their family or to find food. Some crabs dig in mud or sand to hide, mate, or rest. Crabs usually feed on algae; however, they may consume other foods such as worms, detritus, worms, bacteria, or mollusks depending on their species. Some crab species evolve over time. Their evolution is characterized by a reduction in the size of the abdomen or an increase in the robust body.
Crabs mate sexually. They attract each other by acoustic, chemical, vibratory, or visual means. Most aquatic crabs use chemical attractors while other terrestrial crabs use visual methods of attraction. Most crabs have internal fertilization. After fertilization, eggs are elaborated into the female’s abdomen below the tail for protection.
Myriapods bear antennae in their anterior segment, while the second segment has long mandibles in contrast to crustaceans and insects. Myriapods cuticle contains a little wax, therefore, it is not very impermeable in comparison to arachnids. Excretion is not developed in myriapods, such as in insects. Malpighian tubules are responsible for the excretion in myriapods. Myriapods breathe through the trachea, the tracheal respiratory system delivers oxygen to the blood instead of cells. To limit the conservation of water, the spiracles in their trunk cannot be closed. To sum up, myriapods have greatly adapted to the terrestrial habitat, even more than spiders.
There are over a million known species of insects. In fact, they are the greatest known animals’ order. Insects built a relationship of adaptation with flowering plants on land; however, many immature insects live in freshwater but only a few species of insects are present in the sea, unlike crustaceans. Therefore, insects are considered terrestrial organisms due to their adaptation by special characteristics, such as their ability to fly, flexibility in their life cycle, and their ability to conserve water. The small size of insects enables them to avoid competition with large vertebrates. Moreover, the reproductive capacity of insects increases their population size. It also contributes to rapid genetic evolution. Their respiratory tracheal system can supply oxygen to all cells directly due to the small size of the organism where the oxygen diffuses along branched tracheal tubes that end into a cell or between two cells. The tubes are always open due to thickening with the cuticle.
READ: FREE TUTORIALS ON INSECTS AND OTHER ARTHROPODS
Try to answer the quiz below to check what you have learned so far about arthropods.
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- Lane, R. P., & Crosskey, R. W. (2012). Medical insects and arachnids. Springer Science & Business Media.
- Maleque, M. A., Ishii, H. T., & Maeto, K. (2006). The use of arthropods as indicators of ecosystem integrity in forest management. Journal of Forestry, 104(3), 113-117.
- Moore, J. (2001). An introduction to the invertebrates. Cambridge University Press.
- Ng, P. K., Guinot, D., & Davie, P. J. (2008). Systema Brachyurorum: Part I. An annotated checklist of extant brachyuran crabs of the world. The Raffles Bulletin of Zoology, 17(1), 1-286.
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