In arthropods, morphological segmentation is built upon a more fundamental developmental unit, the ‘parasegment’ (Martinez-Arias and Lawrence, 1985 ). Parasegment boundaries are established during embryogenesis by ‘segment-polarity’ genes, such as engrailed and wingless, which are expressed in a series of persistent stripes along the Anterior-Posterior axis. Interestingly, parasegments are offset slightly from morphological segments: parasegment boundaries fall at the anterior edge of each engrailed domain and line up with the middle of each appendage, whereas segment boundaries fall at the posterior edge of each engrailed domain and lie in between the appendages.
Analogous to vertebrate ‘re-segmentation’ (each vertebra being formed from portions of two different somite pairs), this developmental phase shift makes sense if the role of the parasegments is chiefly to organize the nervous system and associated appendicular structures, whereas the role of morphological segmentation is to protect these centers and form exoskeletal articulations between them (Deutsch, 2004 ). Each segment-polarity gene is expressed at a particular position within a segmental unit, and the overall arrangement is remarkably conserved across Panarthropoda (Damen, 2002 ; Janssen and Budd, 2013 ). A central goal of segmentation research is to understand how upstream regulatory processes establish this important pattern within the embryo.
Read more: Arthropod segmentation Erik Clark, Andrew D. Peel, Michael Akam. Development 2019 146: dev170480 doi: 10.1242/dev.170480. Published 25 September 2019.
Arthropods are covered with a tough, resilient integument or exoskeleton of chitin. Arthropods are bilaterally symmetrical and their body possesses an external skeleton.
The rigid cuticle of the exoskeleton inhibits growth, so arthropods replace it periodically by undergoing ecdysis (molting) or shedding the old exoskeleton after growing a new one that is not yet hardened. Molting cycles run nearly continuously until an arthropod reaches full size.
Arthropod bodies are also segmented internally, and the nervous, muscular, circulatory and excretory systems have repeated components.
Arthropods have a wide variety of respiratory systems. The most common respiratory pigment in arthropods is copper-based hemocyanin; this is used by many crustaceans and a few centipedes. A few crustaceans and insects use iron-based hemoglobin, the respiratory pigment used by vertebrates.
The arthropod nervous system from the brain to the rear end consists of a dorsal brain cord and a ventral, ganglionated longitudinal nerve cord (primitively paired) below the gut, from which lateral nerves extend in each segment; the cords form a pair of ganglia from which sensory and motor nerves run to other parts of the segment. The two parallel cords are connected by repeating segments of nerves. These repeating, rung-like segments of nerves are known as commissures, and they branch out from each of an arthropod’s body segments to connect the parallel nerve cords across the ventral surface. The spots where the commissures branch out form big bundles of sensory and motor nerves in each body segment; these bundles are called segmental ganglia (singular ganglion), and they serve as the central warehouse where other nerves branch to the arthropod’s extremities in that segment. Sensory nerves carry sense-based messages like the taste and touch toward the brain; motor nerves carry messages from the brain in order to move an arthropod’s muscles and react to a sensory stimulus. Most arthropods have a secondary ganglion bundle beneath the esophagus, called the subesophageal ganglion.
There are two different types of arthropod excretory systems. In aquatic arthropods, the end-product of biochemical reactions that metabolize nitrogen is ammonia, which is so toxic that it needs to be diluted as much as possible with water. The ammonia is then eliminated via any permeable membrane, mainly through the gills. Various groups of terrestrial arthropods have independently developed a different system: the end-product of nitrogen metabolism is uric acid, which can be excreted as dry material.
Most arthropods have sophisticated visual systems that include one or more usually both compound eyes and pigment-cup ocelli (“little eyes”). In most cases, ocelli are only capable of detecting the direction from which light is coming.
Arthropods belong to phylum Euarthropoda. The phylum is sometimes called Arthropoda, but strictly this term denotes a (putative > see Tactopoda) clade that also encompasses the phylum Onychophora. Euarthropoda is typically subdivided into five subphyla, of which one is extinct:
- Trilobites (extinct)
Arthropods represent the evolutionary pinnacle of the protostomes. It seems likely that arthropods evolved from the same root as the annelids and that the three main lineages of arthropods – the Chelicerata, the Crustacea, and the Insecta – evolved independently from a common ancestor. Little is known of the ancestors of living arthropods. They may have resembled living onychophorans like Peripatus, an animal with a distinctly annelid-like body wall and excretory system but arthropod-like appendages and trachea.
The following factors no doubt contributed to the success of terrestrial arthropods as they adapted to solve the problems of support, stability, desiccation, and respiration associated with living in the air:
- Development of the exoskeleton, which gives support and shape to bodies in the air;
- The waxy cuticle of arachnids and insects prevents water loss and protects against desiccation;
- The adaptability of body structure and organs;
- Small size and motility, including flight in insects, confers an advantage over vertebrates;
- Stability for a light body, due to being ‘swung low’, allows for long legs to increase speed;
- The evolution of a tracheal system of tubes for respiration, which carries air around the body allowing the gaseous exchange to occur;
- Care for the young – making nests and cocoons, providing food stores;
- Giving birth to live young;
- Existence of a huge range of terrestrial niches: the majority of arthropods are only 3-4 mm long and their habitats are correspondingly small;
- Absence of other terrestrial invertebrates competing for space and food.
The life cycles of terrestrial and marine arthropods differ markedly. Adult marine arthropods are usually sedentary or bottom-living depending upon planktonic larvae for dispersal. Vast numbers of offspring are produced as so few survive without being eaten by other members of the plankton. Adult terrestrial arthropods are motile and can range over relatively large areas (often by flying). They only produce relatively few larvae because the adults build shelters to protect them as they grow to adulthood. Such protective behavior has been perfected in insects where adults and larvae live together in structures made of insect products or building material gleaned from the environment. On land, arthropods farm, hunt and trap as a way of providing sustenance for their young and themselves.
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- Deutsch, J. S. (2004). Segments and parasegments in Arthropods: a functional perspective. BioEssays 26, 1117-1125. doi:10.1002/bies.20111
- Damen, W. G. M. (2002). Parasegmental organization of the spider embryo implies that the parasegment is an evolutionary conserved entity in arthropod embryogenesis. Development 129, 1239-1250. doi:10.3410/f.1004866.56706
- Janssen, R. and Budd, G. E. (2013). Deciphering the onychophoran ‘segmentation gene cascade’: Gene expression reveals limited involvement of pair rule gene orthologs in segmentation, but a highly conserved segment polarity gene network. Dev. Biol. 382, 224-234. doi:10.1016/j.ydbio.2013.07.010
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