Arthropods    -    Phylum:   Arthropoda
"So important are insects and other land-dwelling arthropods that if all were to disappear,
humanity probably could not last more than a few months."   -   Edward Osborne Wilson,  (1929-2021).
      The name Arthropoda is derived from the ancient Greek: 'arthron' (=joint) and 'pous' (=foot).   As the name suggests they have jointed limbs, they also have segmented bodies and a cuticle* : the major part of the exoskeleton made of chitin.  The total number of species of Arthropods is subject to speculation and estimates vary wildly between 1.2 and 10 million species.   They are a diverse group of animals: the smallest insect weighs in at less than 25 micrograms; the heaviest, the Atlantic Lobster, may tip the scales at 20 kilos or more.
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      The body of most Arthropods is segmented and this is usually obvious from their appearance although mites do not have obvious segmentation and in many Crustaceans and some Arachnids it would be hard to pick as well.   In general the body can be divided into three (or two) 'tagmata' (=segments), the head, thorax and abdomen and, all three can have their own set of paired specialized appendages.  In those animals that have two tagmata (many Arachnids and some Crustaceans) the head and thorax are usually one.  With most Arthropods, it is the thoracic appendages that are the legs.
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​       During most of their life cycle, arthropods have segmented bodies. Early on in arthropod development, however, some species exist in intermediate stages that are very different from the adult form. These are called larval stages. Development involving larval stages is called indirect development. There are also many species that undergo direct development, in which the young hatch looking essentially like a smaller version of the adult. These species do not go through larval stages or metamorphosis.
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Image 1. Exoskeleton shed by a Cycada.
      The exact relationships between the various subphyla of the Arthropoda, and, between the Arthropoda and other phyla in the kingdom: Animalia has been a hot topic of debate for many years.   Since the 1990s when molecular studies of DNA sequences became more commonplace, it has become more and more accepted that the Arthropods could be considered part of a superphylum, Ecdysozoa or 'moulting animals', containing the Nematodes, Priapulids and Tardigrades as well.  The Annelids, long since considered close relatives to the Arthropods due to them also having segmented bodies, are to be placed in the superphylum: Lophotrochozoa.  (See here)
    Of the 33 phyla that make up the Kingdom: Animalia, the Arthropoda are by far the most numerous.  Some estimates ascribe up to 84 % of all living animals to the phylum.  Despite the fact that most species are small, the total accumulation of their biomass by far outstrips, by a factor of multiples, the total mass that all eight billion plus of us represent.
       Arthropods have an external skeleton or exoskeleton made of chitin interlaced with a variety of proteins.    Chitin is a large, structural polysaccharide made from chains of modified glucose. It is a naturally occurring biopolymer, which is a polymer created by an organism. In terms of abundance, chitin is second only to cellulose. Over 1 billion tons of chitin are synthesized each year by organisms. This extremely versatile molecule can form solid structures on its own as in insect wings, or can combine with other components like calcium carbonate to make even stronger substances like the shell of a clam.    Instead of a hydroxyl group (OH), the glucose molecules in chitin have an amyl group attached that consists of carbon and nitrogen. This produces a dipole in the molecule, which increases the hydrogen bonds that can be formed between these molecules and the molecules around them. When combined in a matrix with various compounds and other chitin molecules, the resulting structure can be very hard because of all the weak interactions between nearby molecules.

      Having a hard exoskeleton has consequences for the growth of the animal as it is quite inflexible and needs to be shed now and then for growth spurts to occur.  This happens by means of a process called moulting or ecdysis.   Prior to the shedding of the old exoskeleton the animal grows a new one underneath and the animal literally breaks out of the old exoskeleton and then experiences a  developmental burst before the new exoskeleton has the opprtunity to harden.  The stages between each moult is called an instar.  The progression of instars can be kept track of by the actual appearance of the animal at each instar stage.
​      Another drawback of the exoskeleton is that it blocks sensory input.  It is therefor necessary that either the sensors, or the neural connectors thereto, protrude through the hard chitin wall.  Many arthropods have done this so elaborately that much of their exoskeleton has become an array of sensory organs.  Most touch sensors are like bristles or setae as often are the sensors for taste and smell.  Pressure sensors commonly taking the form of membranes, serve as hearing organs and the antennae of most hexapods serve multiple sensory purposes including detecting environmental conditions such as temperature and humidity. 
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      Arthropods breathe in different ways depending on their environment:
Aquatic Arthropods:    Aquatic arthropods, such as crustaceans and horseshoe crabs, possess gills for respiration. The gills are always outgrowths of the integument (skin) and are therefore covered by the exoskeleton, which is thin in this area and not a barrier to the exchange of gases.
Terrestrial Arthropods: Terrestrial arthropods have special respiratory structures to exchange gases with the air. Most arthropods breathe through a process called tracheal respiration. This involves the movement of air through a series of tiny tubes (tracheae) that branch off from the main respiratory organ (the spiracles). The spiracles are located on the sides of the arthropod’s body and open to the outside environment. Some terrestrial arthropods possess tracheae and book lungs as respiratory organs.
Diffusion: Arthropods also breathe through a process called diffusion. This process allows oxygen to move from areas of high concentration to areas of low concentration.
        Arthropods are well represented in the fossil record thanks mainly to their exoskeleton.   Often mineralized with inorganic compounds the hard shell does not decay rapidly and so is often caught in sediments and remains there even after the sediment has turned into rock.   It is generally accepted that many Crustaceans originated in the Cambrian period some 540 - 485 million years ago.  All of these were living in a marine environment however.   Land-based arthropods do not appear in the fossil record until the Silurian period around 445 - 420 million years ago.   The oldest Arachnid fossil is estimated at 420 million years old and the oldest insect around 405 million years although there is some evidence to suggest they may have been around for at least some 30 million years prior to that.    Myriapod fossils have been unearthed from sediments of the Devonian Period but it was not until the Carboniferous Period, about 360 - 300 Mya, that a good record of centipedes, millipedes and insects was laid down in the sediments.
      Edward Osborne Wilson  (see the quote at the top of the page)  was an American biologist who was generally regarded as the worlds' foremost expert on ants.  He developed the science of sociobiology, which aims to describe social behaviours in terms of evolutionary developments.   Two of his books,  On Human Nature  and  The Ants  have won Pullitzer prizes.   Wilson discovered and describe in excess of 400 species of ants and he was one of the main proponents behind the establishment of the  Encyclopedia of Life.
References  and  links:
>         Image 1.   Exoskeleton shed by a Cicada.  Photo by the editor.
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​>      Budd, G.,  and  Telford, M.,  (2009).   'The origin and evolution of arthropods."  Nature. 457. 812-7.   10.1038/nature07890. 
>      Frode, Ø.,   (2000).   'How many species of arthropods? Erwin's estimate revised.'   Biological Journal of the Linnean Society, 71 (4): 583-597,   doi:10.1006/bijl.2000.0468
>        Liu, X.,  Zhang, J.  and   Zhu, K.Y.,  (2019).   'Chitin in Arthropods: Biosynthesis, Modification, and Metabolism.'   Adv Exp Med Biol. 2019;1142:169-207.
doi: 10.1007/978-981-13-7318-3_9
>      Ruppert, E.E.,  Fox, R.S.  and  Barnes, R.D.,  (2004).  'Invertebrate Zoology: A functional and evolutionary approach.'  (7th Ed.)  Brooks/Cole-Thomson Learning,  Belmont, CA, U.S.A.  
>      Williams, D.M.,  (2001).   Largest.'  Book of Insect Records, University of Florida    or   https://entnemdept.ufl.edu/walker/ufbir/chapters/chapter_30.shtml

       https://evolution.berkeley.edu/the-arthropod-story/exoskeleton-and-the-jointed-limb/arthropod-adaptability/
      https://biologydictionary.net/chitin/
       https://www.thoughtco.com/facts-about-arthropods-4069412
       https://en.wikipedia.org/wiki/Antenna_(biology)​
       https://en.wikipedia.org/wiki/Arthropod_cuticle
     https://en.wikipedia.org/wiki/Diffusion​
​       https://en.wikipedia.org/wiki/Ecdysis​
      https://en.wikipedia.org/wiki/Exoskeleton\
      https://en.wikipedia.org/wiki/Instar
       ​https://en.wikipedia.org/wiki/Seta
       https://en.wikipedia.org/wiki/Spiracle_(arthropods)​
       ​https://en.wikipedia.org/wiki/Tagma_(biology)
       https://eol.org/
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Header photo: 
Diana Davey.
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