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28.4D: Subphyla of Arthropoda

  • Page ID
    13728
  • The Phylum Arthropoda includes a wide range of species divided into the subphyla: Hexapoda, Crustacea, Myriapoda, and Chelicerata.

    LEARNING OBJECTIVES

    Differentiate among the subphylums hexapoda, myriapoda, crustacea, and chelicerata

    KEY TAKEAWAYS

    Key Points

    • The Hexapoda include insects; the Crustacea include lobster, crabs, and shrimp; the Myriapoda include centipedes and millipedes; and the Chelicerata include spiders, scorpions.
    • The Hexapoda are the largest grouping of Arthropods, containing the more than one million species of insects, having representatives with six legs and one pair of antennae.
    • The Myriapoda are terrestrial, prefering humid environments; they have between 10 and 750 legs.
    • The Crustacea are primarily aquatic arthropods, but also include terrestrial forms, which have a cephalothorax covered by a carapace.
    • The Chelicerata, which includes the spiders, horseshoe crabs, and scorpions, have mouth parts that are fang-like and used for capturing prey.

    Key Terms

    • cephalothorax: the fused head and thorax of spiders and crustaceans
    • forcipule: a modified pincer-like foreleg in centipedes, capable of injecting venom

    Representatives of Phylum Arthropoda

    Subphylum Hexapoda

    The name Hexapoda denotes the presence of six legs (three pairs) in these animals, which differentiates them from the number of pairs present in other arthropods. Hexapods are characterized by the presence of a head, thorax, and abdomen, constituting three tagma. The thorax bears the wings as well as six legs in three pairs. Many of the common insects we encounter on a daily basis, including ants, cockroaches, butterflies, and flies, are examples of Hexapoda.

    Among the hexapods, the insects are the largest class in terms of species diversity as well as biomass in terrestrial habitats ). Typically, the head bears one pair of sensory antennae, mandibles as mouthparts, a pair of compound eyes, and some ocelli (simple eyes), along with numerous sensory hairs. The thorax bears three pairs of legs (one pair per segment) and two pairs of wings, with one pair each on the second and third thoracic segments. The abdomen usually has eleven segments and bears reproductive apertures. Hexapoda includes insects that are winged (like fruit flies) and wingless (like fleas).

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    Insect showing wings and body segments: Protaetia fieberi in flight posture. Hexapods are characterized by having three distinct tagma, or body segments. This beetle is just one of over one million different species of insects that inhabit the Earth.

    Subphylum Myriapoda

    Subphylum Myriapoda includes arthropods with numerous legs. Although the name is hyperbolic in suggesting that myriad legs are present in these invertebrates, the number of legs may vary from 10 to 750. This subphylum includes 13,000 species; the most commonly-found examples are millipedes and centipedes. All myriapods are terrestrial animals, prefering a humid environment.

    Myriapods are typically found in moist soils, decaying biological material, and leaf litter. Centipedes, such as Scutigera coleoptrata,are classified as chilopods. These animals bear one pair of legs per segment, mandibles as mouthparts, and are somewhat dorsoventrally flattened. The legs in the first segment are modified to form forcipules (poison claws) that deliver venom to prey such as spiders and cockroaches, as centipedes are predatory. Millipedes bear two pairs of legs per diplosegment, a feature that results from embryonic fusion of adjacent pairs of body segments, are usually rounder in cross-section, and are herbivores or detritivores. Millipedes have visibly more numbers of legs as compared to centipedes, although they do not bear a thousand legs.

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    House centipede: The house centipede (Scutigera coleoptrata) is one of the 13,000 species of Myriapoda. They bear one pair of legs per segment and can inject venom. The Myriapods contain the millipedes and centipedes.

    Subphylum Crustacea

    Crustaceans are the most dominant aquatic arthropods since the total number of marine crustacean species stands at 67,000. However, there are also freshwater and terrestrial crustacean species. Krill, shrimp, lobsters, crabs, and crayfish are all examples of crustaceans. Terrestrial species like the wood lice (Armadillidium spp.) (also called pill bugs, rolly pollies, potato bugs, or isopods) are also crustaceans, although the number of non-aquatic species in this subphylum is relatively low.

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    Mediterranean green Crab: This crab (Carcinus aestuarii) is one of the 67,000 species of crustaceans inhabiting the world’s oceans. Most crustaceans are decapods, having ten legs.

    Crustaceans possess two pairs of antennae, mandibles as mouthparts, and biramous (“two branched”) appendages: their legs are formed in two parts, as distinct from the uniramous (“one branched”) myriapods and hexapods.

    Unlike that of the Hexapoda, the head and thorax of most crustaceans is fused to form a cephalothorax, which is covered by a plate called the carapace, thus producing a body structure of two tagma. Crustaceans have a chitinous exoskeleton that is shed by molting whenever the animal increases in size. The exoskeletons of many species are also infused with calcium carbonate, which makes them even stronger than in other arthropods. Crustaceans have an open circulatory system where blood is pumped into the hemocoel by the dorsally-located heart. Hemocyanin and hemoglobin are the respiratory pigments present in these animals.

    Subphylum Chelicerata

    This subphylum includes animals such as spiders, scorpions, horseshoe crabs, and sea spiders and is predominantly terrestrial, although some marine species also exist. An estimated 77,000 species, found in almost all habitats, are included in subphylum Chelicerata.

    The body of chelicerates may be divided into two parts: prosoma and opisthosoma, which are basically the equivalents of cephalothorax (usually smaller) and abdomen (usually larger). A “head” tagmum is not usually discernible. The phylum derives its name from the first pair of appendages, the chelicerae, which are specialized claw-like or fang-like mouthparts. These animals do not possess antennae. The second pair of appendages is known as pedipalps. In some species, such as sea spiders, an additional pair of appendages, called ovigers, is present between the chelicerae and pedipalps.

    Chelicerae are used primarily for feeding, but in spiders, these are often modified into fangs that inject venom into their prey before feeding. Members of this subphylum have an open circulatory system with a heart that pumps blood into the hemocoel. Aquatic species have gills, whereas terrestrial species have either trachea or book lungs for gaseous exchange.

    image

    Chelicera of spiders: This photo shows the chelicera of a spider being held open with a stick. Some chelicerae, such as those found in spiders, are hollow and contain (or are connected to) venom glands which are used to inject venom into prey or a (perceived) threat.

    The nervous system in chelicerates consists of a brain and two ventral nerve cords. These animals use external as well as internal fertilization strategies for reproduction, depending upon the species and its habitat. Parental care for the young ranges from absolutely none to relatively-prolonged care.

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