Modern Biology Answer Key Section 38-1 Review Phylum Arthropoda

Learning Objectives

By the end of this department, you lot will be able to practice the following:

• Compare the internal systems and appendage specializations of phylum Arthropoda
• Discuss the environmental importance of arthropods
• Discuss the reasons for arthropod success and abundance

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. In that location are well over a million arthropod species described, and systematists believe that there are millions of species pending proper classification. Like other Ecdysozoa, all arthropods periodically go through the physiological process of molting, followed past ecdysis (the bodily shedding of the exoskeleton), as they grow. Arthropods are eucoelomate, protostomic organisms, oftentimes with incredibly complicated life cycles.

Phylum Arthropoda

The name "arthropoda" means "jointed feet." The proper name aptly describes the invertebrates included in this phylum. Arthropods have probably ever dominated the animate being kingdom in terms of number of species and likely will continue to do so: An estimated 85 per centum of all known species are included in this phylum! In effect, life on World could feasibly be called the Age of Arthropods offset nearly 500 million years ago.

The principal characteristics of all the animals in this phylum are the structural and functional segmentation of the torso and the presence of jointed appendages. Arthropods accept an exoskeleton made principally of chitin—a waterproof, tough polysaccharide composed of N-acetylglucosamine. Phylum Arthropoda is the near speciose clade in the animal world (Table 28.1), and insects class the single largest grade within this phylum. For comparing, refer to the gauge numbers of species in the phyla listed beneath.

Phylum	# species
Ctenophora	100
Porifera	v,000
Cnidaria	11,000
Platyhelminthes	25,000
Rotifera	ii,000
Nemertea	i,200
Annelida	22,000
Mollusca	112,000
Nematoda	28,000+
Tardigrades	>1,000
Arthropoda	1,134,000
Echinodermata	7,000
Chordata	100,000

Tabular array 28.1

Phylum Arthropoda includes animals that have been successful in colonizing terrestrial, aquatic, and aerial habitats. This phylum is further classified into five subphyla: Trilobita (trilobites, all extinct), Chelicerata (horseshoe crabs, spiders, scorpions, ticks, mites, and daddy longlegs or harvestmen), Myriapoda (millipedes, centipedes, and their relatives), Crustacea (venereal, lobsters, crayfish, isopods, barnacles, and some zooplankton), and Hexapoda (insects and their six-legged relatives). Trilobites, an extinct group of arthropods constitute chiefly in the pre-Cambrian Era (virtually 500 one thousand thousand years agone), are probably most closely related to the Chelicerata. These are identified based on their fossils; they were quite various and radiated significantly into thousands of species before their complete extinction at the end of the Permian about 240 meg years ago (Effigy 28.36).

Effigy 28.36 A trilobite. Trilobites, like the i in this fossil, are an extinct group of arthropods. Their proper noun "trilobite" refers to the iii longitudinal lobes making up the body: right pleural lobe, axial lobe, and left pleural lobe (credit: Kevin Walsh).

Morphology

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 caput, thorax, and abdomen, or a cephalothorax and belly, or a caput and trunk, depending on the taxon. Commonly described tagmata may be equanimous of different numbers of segments; for instance, the head of most insects results from the fusion of six ancestral segments, whereas the "head" of another arthropod may exist made of fewer ancestral segments, due to contained evolutionary events. Jointed arthropod appendages, oftentimes in segmental pairs, have been specialized for various functions: sensing their environment (antennae), capturing and manipulating food (mandibles and maxillae), too as for walking, jumping, earthworks, and swimming.

In the arthropod trunk, a key crenel, called the hemocoel (or blood cavity), is nowadays, and the hemocoel fluids are moved by wrinkle of regions of the tubular dorsal blood vessel called "hearts." Groups of arthropods besides differ in the organs used for nitrogenous waste excretion, with crustaceans possessing greenish glands and insects using Malpighian tubules, which work in conjunction with the hindgut to reabsorb water while ridding the torso of nitrogenous waste. The nervous system tends to be distributed amidst the segments, with larger ganglia in segments with sensory structures or appendages. The ganglia are connected past a ventral nerve string.

Respiratory systems vary depending on the group of arthropod. Insects and myriapods apply a serial of tubes (tracheae) that branch through the body, ending in infinitesimal tracheoles. These fine respiratory tubes perform gas exchange direct betwixt the air and cells within the organism. The major tracheae open up to the surface of the cuticle via apertures called spiracles. We should note that these tracheal systems of ventilation have evolved independently in hexapods, myriapods, and arachnids. Although the tracheal system works extremely well in terrestrial environments, it also works well in freshwater aquatic environments: In fact, numerous species of aquatic insects in both immature and adult stages possess tracheal systems. However, although in that location are insects that live on the surface of marine environments, none is strictly marine—meaning that they complete their entire metamorphosis in table salt water.

In contrast, aquatic crustaceans utilise gills, terrestrial chelicerates apply book lungs, and aquatic chelicerates use book gills (Figure 28.37). The book lungs of arachnids (scorpions, spiders, ticks, and mites) contain a vertical stack of hemocoel wall tissue that somewhat resembles the pages of a volume. Betwixt each of the "pages" of tissue is an air infinite. This allows both sides of the tissue to be in contact with the air at all times, greatly increasing the efficiency of gas exchange. The gills of crustaceans are filamentous structures that exchange gases with the surrounding water.

The cuticle is the hard "covering" of an arthropod. It is made up of two layers: the epicuticle, which is a sparse, waxy, water-resistant outer layer containing no chitin, and the layer beneath information technology, the chitinous procuticle, which itself is composed of an exocuticle and a lower endocuticle, all supported ultimately by a basement membrane. The exoskeleton is very protective (information technology is sometimes difficult to squish a big beetle!), only does not sacrifice flexibility or mobility. Both the exocuticle (which is secreted before a molt), and an endocuticle, (which is secreted after a molt), are composed of chitin bound with a protein; chitin is insoluble in water, alkalis, and weak acids. The procuticle is not only flexible and lightweight, simply also provides protection against dehydration and other biological and physical stresses. Some arthropods, such equally the crustaceans, add calcium salts to their exoskeleton, which increases the strength of the cuticle, merely does reduce its flexibility. In some cases, such as lobsters, the amount of calcium salt deposited within the chitin is extreme. In society to abound, the arthropod must "shed" the exoskeleton during the physiological process chosen molting, following by the actual stripping of the outer cuticle, called ecdysis ("to strip off"). At first, this seems to be a dangerous method of growth, because while the new exoskeleton is hardening, the animal is vulnerable to predation; still, molting and ecdysis also permit for growth and change in morphology, too equally for corking diversification in size, but because the numbers of molts can be modified through development.

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

Figure 28.37 Arthropod respiratory structures. The book lungs of (a) arachnids are made up of alternate air pockets and hemocoel tissue shaped like a stack of books (hence the name, "book lung"). The book gills of (b) horseshoe crabs are like to book lungs just are external so that gas exchange tin occur with the surrounding water. (credit a: modification of piece of work by Ryan Wilson based on original work by John Henry Comstock; credit b: modification of work past Angel Schatz)

Subphylum Chelicerata

This subphylum includes animals such as horseshoe crabs, sea 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 tin be institute in almost all terrestrial habitats.

The body of chelicerates is divided into 2 tagmata: prosoma and opisthosoma, which are basically the equivalents of a cephalothorax (usually smaller) and an abdomen (usually larger). A distinct "head" tagma is not usually discernible. The phylum derives its name from the first pair of appendages: the chelicerae (Effigy 28.38), which serve as specialized clawlike or fanglike mouthparts. We should note here that chelicerae are actually modified legs, just they are not the exact serial equivalent of mandibles, which are the modified leglike chewing mouthparts of insects and crustaceans: The chelicerae are borne on the showtime segment making upwardly the prosoma, whereas the mandibles are embryonically on the fourth segment of the mandibulate caput. The chelicerates take secondarily lost their antennae and hence exercise non have them. Some of the functions of the antennae (such as touch on) are now performed by the 2d pair of appendages— the pedipalps, which may besides be used for general sensing the environment equally well as the manipulation of food. In some species, such as sea spiders, an boosted pair of derived leg appendages, called ovigers, is present between the chelicerae and pedipalps. Ovigers are used for preparation and by males to deport eggs. In spiders, the chelicerae are oft modified and cease in fangs that inject venom into their prey before feeding (Effigy 28.39).

Effigy 28.38 Chelicerae. The chelicerae (first set of appendages) are well developed in the scorpion. (credit: Kevin Walsh)

Virtually chelicerates ingest food using a preoral cavity formed by the chelicerae and pedipalps. Some chelicerates may secrete digestive enzymes to pre-assimilate food before ingesting it. Parasitic chelicerates like ticks and mites have evolved blood-sucking apparatuses. Members of this subphylum have an open circulatory organization with a center that pumps claret into the hemocoel. Aquatic species, like horseshoe crabs, have gills, whereas terrestrial species take either tracheae or book lungs for gaseous exchange. Chelicerate hemolymph contains hemocyanin a copper-containing oxygen send protein.

Effigy 28.39 Spider. The trapdoor spider, like all spiders, is a fellow member of the subphylum Chelicerata. (credit: Marshal Hedin)

The nervous system in chelicerates consists of a brain and two ventral nerve cords. Chelicerates are dioecious, meaning that the sexes are split up. These animals use external fertilization as well equally internal fertilization strategies for reproduction, depending upon the species and its habitat. Parental treat the young ranges from absolutely none to relatively prolonged care.

Link to Learning

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 proper noun 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 constitute examples are millipedes and centipedes. Nigh all myriapods are terrestrial animals and prefer a boiling environment. Ancient myriapods (or myriapod-like arthropods) from the Silurian to the Devonian grew up to 10 feet in length (3 meters). Unfortunately, they are all extinct!

Myriapods are typically found in moist soils, decaying biological cloth, and leaf litter. Subphylum Myriapoda is divided into four classes: Chilopoda, Symphyla, Diplopoda, and Pauropoda. Centipedes similar Scutigera coleoptrata (Figure 28.40) are classified as chilopods. These animals deport one pair of legs per segment, mandibles as mouthparts, and are somewhat dorsoventrally flattened. The legs in the beginning segment are modified to form forcipules (poison claws) that deliver poisonous substance to prey like spiders and cockroaches, equally these animals are all predatory. Symphyla are similar to centipedes, but lack the poison claws and are vegetarian. Millipedes bear 2 pairs of legs per diplosegment—a feature that results from the embryonic fusion of next pairs of torso segments. These arthropods are usually rounder in cross-section than centipedes, and are herbivores or detritivores. Millipedes have visibly more numbers of legs as compared to centipedes, although they do non accept a thousand legs (Figure 28.40b). The Pauropods are like to millipedes, but have fewer segments.

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 non a thousand, equally its name might propose. (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 seventy,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 likewise: Terrestrial species like the forest lice (Armadillidium spp), also called pill bugs, roly-polies, potato bugs, or isopods, are too crustaceans. However, the number of terrestrial species in this subphylum is relatively depression.

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 past Jon Sullivan credit c: modification of work by Franco Folini. https://commons.wikimedia.org/w/alphabetize.php?curid=789616)

Crustaceans typically possess two pairs of antennae, mandibles as mouthparts, and biramous ("two branched") appendages, which means that their legs are formed in two parts called endopods and exopods, which announced superficially distinct from the uniramous ("one branched") legs of myriapods and hexapods (Figure 28.42). Since biramous appendages are also seen in the trilobites, biramous appendages represent the ancestral condition in the arthropods. Currently, we draw various arthropods as having uniramous or biramous appendages, merely these are descriptive only, and exercise not necessarily reverberate evolutionary relationships other than that all jointed legs of arthropods share common ancestry.

Figure 28.42 Arthropod appendages. Arthropods may have (a) biramous (2-branched) appendages or (b) uniramous (ane-branched) appendages. (credit b: modification of work by Nicholas W. Beeson)

In most crustaceans, the caput and thorax is fused to form a cephalothorax (Effigy 28.43), which is covered past a plate called the carapace, thus producing a torso program comprising two tagmata: cephalothorax and belly. Crustaceans have a chitinous exoskeleton that is shed past molting and ecdysis whenever the animal requires an increment in size or the next stage of development. The exoskeletons of many aquatic species are also infused with calcium carbonate, which makes them even stronger than those of other arthropods. Crustaceans accept an open circulatory organisation where blood is pumped into the hemocoel past the dorsally located heart. Hemocyanin is the major respiratory pigment present in crustaceans, merely hemoglobin is plant in a few species and both are dissolved in the hemolymph rather than carried in cells.

Figure 28.43 Crustacean beefcake. 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)

Every bit in the chelicerates, well-nigh crustaceans are dioecious. However, some species like barnacles may be hermaphrodites. Serial hermaphroditism, where the gonad tin can switch from producing sperm to ova, is too exhibited in some species. Fertilized eggs may exist held within the female of the species or may be released in the water. Terrestrial crustaceans seek out damp spaces in their habitats to lay eggs.

Larval stages—nauplius or zoea—are seen in the early development of aquatic crustaceans. A cypris larva is also seen in the early development of barnacles (Effigy 28.44).

Figure 28.44 Crustacean larvae. All crustaceans go through different larval stages. Shown are (a) the nauplius larval phase of a tadpole shrimp, (b) the cypris larval phase of a barnacle, and (c) the zoea larval phase of a green crab. (credit a: modification of work by USGS; credit b: modification of piece of work by Mª. C. Mingorance Rodríguez; credit c: modification of work by B. Kimmel based on original work past Ernst Haeckel)

Crustaceans possess a encephalon formed by the fusion of the first three segmental ganglia, as well as two compound optics. A ventral nerve cord connects additional segmental ganglia. Most crustaceans are carnivorous, but herbivorous and detritivorous species, and even endoparasitic species are known. A highly evolved endoparasitic species, such as Sacculina spp, parasitizes its crab host and ultimately destroys it later on it forces the host to incubate the parasite'due south eggs! Crustaceans may too be cannibalistic when extremely loftier populations of these organisms are present.

Subphylum Hexapoda

The insects contain the largest course of arthropods in terms of species diversity as well every bit in terms of biomass—at least in terrestrial habitats.

The name Hexapoda describes the presence of half dozen legs (three pairs) in these animals, which differentiates them from other groups of arthropods that have different numbers of legs. In some cases, nonetheless, the number of legs has been evolutionarily reduced, or the legs have been highly modified to adapt specific conditions, such equally endoparasitism. Hexapod bodies are organized into three tagmata: head, thorax, and abdomen. Individual segments of the head have mouthparts derived from jointed legs, and the thorax has three pairs of jointed appendages, and also wings, in most derived groups. For instance, in the pterygotes (winged insects), in addition to a pair of jointed legs on all three segments comprising the thorax—prothorax, mesothorax, and metathorax—there are veined wings on the mesothorax and metathorax.

Appendages plant on other body segments are too evolutionarily derived from modified legs. Typically, the head bears an upper "lip" or labrum and mandibles (or derivation of mandibles) that serve every bit mouthparts; maxillae, and a lower "lip" chosen a labium: both of which dispense food. The caput besides has ane pair of sensory antennae, equally well every bit sensory organs such as a pair of compound eyes, ocelli (unproblematic eyes), and numerous sensory hairs. The belly usually has 11 segments and bears external reproductive apertures. The subphylum Hexapoda includes some insects that are winged (such as fruit flies) and others that are secondarily wingless (such as fleas). The but lodge of "primitively wingless" insects is the Thysanura, the bristletails. All other orders are winged or are descendants of formally winged insects.

The development of wings is a major, unsolved mystery. Unlike vertebrates, whose "wings" are simply preadaptations of "arms" that served as the structural foundations for the evolution of functional wings (this has occurred independently in pterosaurs, dinosaurs [birds], and bats), the evolution of wings in insects is a what we call a de novo (new) development that has given the pteryogotes domination over the Earth. Winged insects existed over 425 million years ago, and by the Carboniferous, several orders of winged insects (Paleoptera), most of which are now extinct, had evolved. In that location is good physical prove that Paleozoic nymphs with thoracic winglets (peradventure hinged, former gill covers of semi-aquatic species) used these devices on land to drag the thoracic temperature (the thorax is where the legs are located) to levels that would enable them to escape predators faster, find more food resources and mates, and disperse more easily. The thoracic winglets (which can be found on fossilized insects preceding the advent of truly winged insects) could have easily been selected for thermoregulatory purposes prior to reaching a size that would have allowed them the chapters for gliding or actual flapping flight. Even modern insects with broadly attached wings, such every bit butterflies, use the basal one-third of their wings (the expanse next to the thorax) for thermoregulation, and the outer 2-thirds for flight, cover-up, and mate selection.

Many of the common insects nosotros encounter on a daily basis—including ants, beetles, cockroaches, butterflies, crickets and flies—are examples of Hexapoda. Among these, adult ants, beetles, flies, and collywobbles develop by consummate metamorphosis from grub-like or caterpillar-similar larvae, whereas adult cockroaches and crickets develop through a gradual or incomplete metamorphosis from wingless immatures. All growth occurs during the juvenile stages. Adults do not grow further (but may become larger) after their last molt. Variations in fly, 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, pregnant that they live in large groups with individuals assigned to specific roles or castes, like queen, drone, and worker. Social insects employ pheromones—external chemical signals—to communicate and maintain group structure as well as a cohesive colony.

Visual Connection

Visual Connection

Figure 28.45 Insect beefcake. In this basic anatomy of a hexapod insect, note that insects accept a well-adult digestive system (yellowish), a respiratory organisation (blueish), a circulatory system (blood-red), and a nervous system (purple). Notation the multiple "hearts" and the segmental ganglia.

Which of the following statements most insects is simulated?

1. Insects have both dorsal and ventral claret vessels.
2. Insects have spiracles, openings that allow air to enter into the tracheal arrangement.
3. The trachea is part of the digestive system.
4. Most insects have a well-adult digestive system with a oral fissure, crop, and intestine.

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Source: https://openstax.org/books/biology-2e/pages/28-6-superphylum-ecdysozoa-arthropods