ARTHROPOD CHARACTERISTICS AND CLASSIFICATION MODULE 1

MODULE 1 CHARACTERISTICS AND CLASSIFICATION Unit 1

General Characteristic Features are bilaterally symmetrical, triploblastic and usually metamerically segmented. The exhibit tagmatization. Tagmatization is the specialization of body regions (segments) of a metameric animal for specific functions; for example, the head of an arthropod is specialized for feeding and sensory functions, the is specialized for locomotion while the is specialized for visceral functions. They are usually encased in a tough, jointed exoskeleton called the cuticle. Some or all the body segments are provided with paired jointed appendages, including a pair of jaws. Respiration in aquatic arthropods is usually by means of gills while terrestrial arthropods respire by means of trachea (air ducts), book lungs or through the body surface. They have an open circulatory system and the muscular walls of the heart is perforated by pairs of lateral openings called ostia. The excretory organs in arthropods are the antennal (or maxillary glands), coxal glands and Malpighian tubules. Nitrogenous wastes are ammonia compounds (crustaceans), guanine (arachnids) and urea (). Sensory organs include antennae and sensory hairs, simple and compound eyes, auditory organs (insects) and statocysts (for balance in crustaceans). Sexes are usually separate with paired reproductive organs and ducts. Fertilization is mostly internal. During development, they undergo one to several larval stages (metamorphosis) to the adult form. Asexual reproduction is by parthenogenesis as occurs in some crustaceans and insects.

Unit 2 Arthropod Cuticle and Moulting Arthropods have a strong, flexible outer covering called an exoskeleton or cuticle composed mainly of chitin, which is a nitrogenous polysaccharide secreted by the epidermis. The cuticle is secreted by a single layered epidermis (also called the hypodermis). The cuticle has a thin impermeable outer layer, the epicuticle, and a relatively thick, tough but elastic permeable inner layer called the procuticle. The epicuticle prevents desiccation as it is impermeable to water. The epicuticle is also involved in the hardening of the exoskeleton. The

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ARTHROPOD CHARACTERISTICS AND CLASSIFICATION MODULE 1

procuticle is composed of an exocuticle and endocuticle. The major component of the procuticle is chitin, which is an amino polysaccharide. It is freely permeable to gases and many dissolved substances. Chitin is insoluble in water, alcohol, alkali, dilute acids or the digestive juices of many animals. The cuticle protects the internal organs, provides attachment for muscles and prevents water loss in terrestrial forms. The arthropod has to periodically moult in order to increase in size (that is to grow), this is because the outer layer of the cuticle is non expansible. Moulting in all arthropod is regulated by a hormone called ecdysone; and the process of moulting is referred to as ecdysis. Therefore, real tissue growth takes place between two moults known as intermoult stages or instars. The process of moulting terminates in the actual shedding of the skin; the cast off intact skin is known as the exuvium. While the cuticle is still soft, the arthropod body swells rapidly by taking up air or water as soon as it emerges from the old cuticle. The new cuticle formed is therefore enlarged and becomes sclerotized thereby increasing its protective ability. Many arthropods are vulnerable after ecdysis; they therefore find somewhere to hide until the new exoskeleton hardens.

Unit 3

Classification Phylum Arthropoda can be divided into five groups (subphyla): Subphylum Trilobitomorpha These comprise all fossil forms with body divided by three longitudinal furrows into three lobes. Example are the trilobites. Subphylum Chelicerata Members have the first pair of appendages modified into chelicerae with claws, one pair of pedipalps and four pairs of walking legs. They lack antennae, compound eyes and true jaws; body divided into cephalothorax (or prosoma) and abdomen (or ). In

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ARTHROPOD CHARACTERISTICS AND CLASSIFICATION MODULE 1

, the opisthosoma may be subdivided into the mesosoma and . Examples include sea , horseshoe crab, spider, , pseudoscorpion, mite and tick. Subphylum Crustacea Members have biramous appendages. They have two pairs of antennae, the first pair being the antennule. They are aquatic mandibulate arthropods. The body usually comprise two tagma: a cephalon with six segments and many segmented trunk or a cephalothorax with about 13 to 14 segments and six segmented abdomen. Examples include water flea, barnacle, copepod, ostracod, shrimp, lobster and crabs. Subphylum Myriapoda Members have body divided into two tagmata; the head with four pairs of appendages and long many segmented trunk with paired appendages. Examples are the centipedes and millipedes. Subphylum Hexapoda Members have three pairs of legs with body divided into three tagmata. The body comprises the head with five pairs of appendages, three segmented thorax with three pairs of uniramous legs and abdomen with greatly reduced appendages. There is the presence of compound eyes. Examples include dragonfly, cockroach, locust, grasshopper, termites, body louse, bed bug, butterfly, , mosquito, housefly, tsetse fly and flea.

Conclusion Arthropods are the largest of all animal phyla, especially as it relates to species diversity; and occurring in virtually all environments on Earth, exploiting every imaginable lifestyle. Arthropods show tagmatization and body segments, bear pairs of jointed appendages, bear cuticle and undergo moulting (ecdysis), and development is through metamorphosis. The phylum comprise five subphyla: Trilobitomorpha (extinct), Chelicerata, Crustacea, Myriapoda and Hexapoda.

References/Further Reading 1. Barrington EJW (1979). Invertebrate Structure and Function. 2. Structure and functions of the cuticle by Larry Keeley Sep 2, 2011. YouTube video; https://youtu.be/fZkNlvKdK3g; (accessed on 30 March, 2020). 3. Molting of the insect cuticle by Larry Keeley Sep 2, 2011. YouTube video; https://youtu.be/QfeEZl0VGs0; (accessed on 30 March, 2020).

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