28.6 Phylum Arthropoda (in Superphylm Ecdysozoa)

Learning Outcomes

  • Compare the internal systems and appendage specializations of phylum Arthropoda

The superphylum Ecdysozoa also includes the phylum Arthropoda, one of the most successful clades of animals on the planet. Arthropods are coelomate organisms characterized by a sturdy chitinous exoskeleton and jointed appendages. There are well over a million arthropod species described, and systematists believe that there are millions of species awaiting proper classification. Like other Ecdysozoa, all arthropods periodically go through the physiological process of molting, followed by ecdysis (the actual shedding of the exoskeleton), as they grow. Arthropods are eucoelomateprotostomic organisms, often with incredibly complicated life cycles.

Phylum Arthropoda includes animals that have been successful in colonizing terrestrial, aquatic, and aerial habitats. This phylum is further classified into five subphylaTrilobita (trilobites, all extinct), Chelicerata (horseshoe crabs, spiders, scorpions, ticks, mites, and daddy longlegs or harvestmen), Myriapoda (millipedes, centipedes, and their relatives), Crustacea (crabs, lobsters, crayfish, isopods, barnacles, and some zooplankton), and Hexapoda (insects and their six-legged relatives).

The fossilized trilobite resembles a footprint, with a rounded front end and ridges extending across the body.
Figure 28.36 A trilobite. Trilobites, like the one in this fossil, are an extinct group of arthropods. Their name “trilobite” refers to the three longitudinal lobesmaking up the body: right pleural lobe, axial lobe, and left pleural lobe (credit: Kevin Walsh).


Characteristic features of the arthropods include the presence of jointed appendages, body segmentation, and chitinized exoskeleton. Fusion of adjacent groups of segments gave rise to functional body regions called tagmata (singular = tagma). Tagmata may be in the form of a headthorax, and abdomenor a cephalothorax and abdomenor a head and trunk, depending on the taxon.  Jointed arthropod appendages, often in segmental pairs, have been specialized for various functions: sensing their environment (antennae), capturing and manipulating food (mandibles and maxillae), as well as for walking, jumping, digging, and swimming.

In the arthropod body, a central cavity, called the hemocoel (or blood cavity), is present, and the hemocoel fluids are moved by contraction of regions of the tubular dorsal blood vessel called “hearts.” Groups of arthropods also differ in the organs used for nitrogenous waste excretion, with crustaceans possessing green glands and insects using Malpighian tubules, which work in conjunction with the hindgut to reabsorb water while ridding the body of nitrogenous waste. The nervous system tends to be distributed among the segments, with larger ganglia in segments with sensory structures or appendages. The ganglia are connected by a ventral nerve cord.

Respiratory systems vary depending on the group of arthropod. Insects and myriapods use a series of tubes (tracheae) that branch through the body, ending in minute tracheoles.  The major tracheae open to the surface of the cuticle via apertures called spiracles.  In contrast, aquatic crustaceans utilize gills,  (Figure 28.37).

The characteristic morphology of representative animals from each subphylum is described below.

Part A is a diagram of a spider showing an outline of the body, with the heart and lung inside. The book lung looks like a book with many pages and is located just anterior to a spiracle in the ventral abdomen. The heart is a long tube located in the dorsal portion of the abdomen. Part B is a photo of the underside of a horseshoe crab. The book gills are 5 pairs of plates near the tail.
Figure 28.37 Arthropod respiratory structures. The book lungs of (a) arachnids are made up of alternating air pockets and hemocoel tissue shaped like a stack of books (hence the name, “book lung”). The book gills of (b) horseshoe crabs are similar to book lungs but are external so that gas exchange can occur with the surrounding water. (credit a: modification of work by Ryan Wilson based on original work by John Henry Comstock; credit b: modification of work by Angel Schatz)

Subphylum Chelicerata

This subphylum includes animals such as horseshoe crabssea spiders, spiders, mites, ticks, scorpions, whip scorpions, and harvestmen. Chelicerates are predominantly terrestrial, although some freshwater and marine species also exist. An estimated 77,000 species of chelicerates can be found in almost all terrestrial habitats.

Figure 28.38 Chelicerae. The chelicerae (first set of appendages) are well developed in the scorpion. (credit: Kevin Walsh)
Figure 28.39 Spider. The trapdoor spider, like all spiders, is a member of the subphylum Chelicerata. (credit: Marshal Hedin)

Link to Learning

Visit this site to click through a lesson on arthropods, including interactive habitat maps, and more.

Subphylum Myriapoda

Subphylum Myriapoda comprises arthropods with numerous legs. Although the name is misleading, suggesting that thousands of legs are present in these invertebrates, the number of legs typically varies from 10 to 750. This subphylum includes 16,000 species; the most commonly found examples are millipedes and centipedes. Virtually all myriapods are terrestrial animals and prefer a humid environment.

Figure 28.40 Myriapods. The centipede Scutigera coleoptrata (a) has up to 15 pairs of legs. The North American millipede Narceus americanus (b) bears many legs, although not a thousand, as its name might suggest. (credit a: modification of work by Bruce Marlin; credit b: modification of work by Cory Zanker)

Subphylum Crustacea

Crustaceans are the most dominant aquatic (both freshwater and marine) arthropods, with the total number of marine crustaceans standing at about 70,000 species. Krill, shrimp, lobsters, crabs, and crayfish are examples of crustaceans (Figure 28.41). However, there are also a number of terrestrial crustacean species as well: Terrestrial species like the wood lice (Armadillidium spp), also called pill bugs, roly-polies, potato bugs, or isopods, are also crustaceans. Nonetheless, the number of terrestrial species in this subphylum is relatively low.

Photo a shows a crab on land, and photo b shows a bright red shrimp in the water. Image c is of a pill bug walking on sand.
Figure 28.41 Crustaceans. The (a) crab and (b) shrimp krill are both aquatic crustaceans. The pill bug Armadillidium is a terrestrial crustacean. (credit a: modification of work by William Warby; credit b: modification of work by Jon Sullivan credit c: modification of work by Franco Folini. https://commons.wikimedia.org/w/index.php?curid=789616)
Illustration A shows the biramous, or two-branched leg of a crayfish. Illustration B shows the uniramous, or one-branched leg of an insect. From proximal to distal, the coxa is the portion of the leg that attaches to the body. The trochanter is small and round and attaches to the coxa. The femur is the largest part of the leg, and is long and somewhat oval. The tibia resembles the femur in shape, but is much thinner. The end of the leg are the tarsus sections.
Figure 28.42 Arthropod appendages. Arthropods may have (a) biramous (two-branched) appendages or (b) uniramous (one-branched) appendages. (credit b: modification of work by Nicholas W. Beeson)
An illustration of a midsagittal cross section of a crayfish shows the carapace around the cephalothorax, and the heart in the dorsal thorax area.
Figure 28.43 Crustacean anatomy. The crayfish is an example of a crustacean. It has a carapace around the cephalothorax and the heart in the dorsal thorax area. (credit: Jane Whitney)
Micrograph a shows a shrimp nauplius larva, which has a teardrop-shaped body with tentacles and long, frilly arms at the wide end. Micrograph b shows a barnacle cypris larva, which is similar in shape to a clam. Micrograph c shows green crab zoea larva, which resembles a shrimp, with a large plated section covering its head, and its long tail tapering from from broader to thinner.
Figure 28.44 Crustacean larvae. All crustaceans go through different larval stages. Shown are (a) the nauplius larval stage of a tadpole shrimp, (b) the cypris larval stage of a barnacle, and (c) the zoea larval stage of a green crab. (credit a: modification of work by USGS; credit b: modification of work by Mª. C. Mingorance Rodríguez; credit c: modification of work by B. Kimmel based on original work by Ernst Haeckel)

Subphylum Hexapoda

The insects comprise the largest class of arthropods in terms of species diversity as well as in terms of biomass—at least in terrestrial habitats.

Many of the common insects we encounter on a daily basis—including ants, beetles, cockroaches, butterflies, crickets and flies—are examples of Hexapoda. Among these, adult ants, beetles, flies, and butterflies develop by complete metamorphosis from grub-like or caterpillar-like larvae, whereas adult cockroaches and crickets develop through a gradual or incomplete metamorphosis from wingless immatures.  Variations in wing, leg, and mouthpart morphology all contribute to the enormous variety seen in the insects. Insect variability was also encouraged by their activity as pollinators and their coevolution with flowering plants.  Some insects, especially termites, ants, bees, and wasps, are eusocial, meaning that they live in large groups with individuals assigned to specific roles or castes, like queen, drone, and worker. Social insects use pheromones—external chemical signals—to communicate and maintain group structure as well as a cohesive colony.

Visual Connection

The illustration shows the anatomy of a bee. The digestive system consists of a mouth, pharynx, stomach, intestine, and anus. The respiratory system consists of spiracles, or openings, along the side of the bees body that connect to tubes that run up and join a larger dorsal tube that connects all the spiracles together. The circulatory system consists of a dorsal blood vessel that has multiple hearts along its length. The nervous system consists of cerebral ganglia in the head that connect to a ventral nerve cord.
Figure 28.45 Insect anatomy. In this basic anatomy of a hexapod insect, note that insects have a well-developed digestive system (yellow), a respiratory system (blue), a circulatory system (red), and a nervous system (purple). Note the multiple “hearts” and the segmental ganglia.

Link to Learning

Watch this Crash Course Biology video “Annelids & Arthropods.”


Icon for the Creative Commons Attribution-ShareAlike 4.0 International License

General Biology Copyright © by Mary Ann Clark; Matthew Douglas; and Jung Choi is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

Share This Book