14.18: Subphylums of Arthropoda - Biology

14.18: Subphylums of Arthropoda - Biology

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Learning Objectives

  • Identify the different subphylums in phylum Arthropoda

Arthropods represent the most successful phylum of animal on Earth, in terms of the number of species as well as the number of individuals. These animals are characterized by a segmented body as well as the presence of jointed appendages. In the basic body plan, a pair of appendages is present per body segment. Within the phylum, traditional classification is based on mouthparts, number of appendages, and modifications of appendages present. Arthropods bear a chitinous exoskeleton. Gills, trachea, and book lungs facilitate respiration. Sexual dimorphism is seen in this phylum, and embryonic development includes multiple larval stages.

Subphylum Hexapoda

The name Hexapoda denotes the presence of six legs (three pairs) in these animals as differentiated 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.

Amongst the hexapods, the insects (Figure 1) 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).

Practice Question

Which of the following statements about insects is false?

  1. Insects have both dorsal and ventral blood vessels.
  2. Insects have spiracles, openings that allow air to enter.
  3. The trachea is part of the digestive system.
  4. Insects have a developed digestive system with a mouth, crop, and intestine.

[reveal-answer q=”4825″]Show Answer[/reveal-answer]
[hidden-answer a=”4825″]Statement c is false.[/hidden-answer]

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 and prefer a humid environment.

Myriapods are typically found in moist soils, decaying biological material, and leaf litter. Subphylum Myriapoda is divided into four classes: Chilopoda, Symphyla, Diplopoda, and Pauropoda. Centipedes like Scutigera coleoptrata (Figure 2a) 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 poison to prey like spiders and cockroaches, as these animals are all 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 (Figure 2b).

Subphylum Crustacea

Crustaceans are the most dominant aquatic arthropods, since the total number of marine crustacean species stands at 67,000, but there are also freshwater and terrestrial crustacean species. Krill, shrimp, lobsters, crabs, and crayfish are examples of crustaceans (Figure 3). 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.

Crustaceans possess two pairs of antennae, mandibles as mouthparts, and biramous (“two branched”) appendages, which means that their legs are formed in two parts, as distinct from the uniramous (“one branched”) myriapods and hexapods (Figure 4).

Unlike that of the Hexapoda, the head and thorax of most crustaceans is fused to form a cephalothorax (Figure 5), 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.

Most crustaceans are dioecious, which means that the sexes are separate. Some species like barnacles may be hermaphrodites. Serial hermaphroditism, where the gonad can switch from producing sperm to ova, may also be seen in some species. Fertilized eggs may be 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 and zoea—are seen in the early development of crustaceans. A cypris larva is also seen in the early development of barnacles (Figure 6). Crustaceans possess a tripartite brain and two compound eyes. Most crustaceans are carnivorous, but herbivorous and detritivorous species are also known. Crustaceans may also be cannibalistic when extremely high populations of these organisms are present.

Subphylum Chelicerata

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

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 (Figure 7), 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, like sea spiders, an additional pair of appendages, called ovigers, is present between the chelicerae and pedipalps.

Chelicerae are mostly used for feeding, but in spiders, these are often modified into fangs that inject venom into their prey before feeding (Figure 8). 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.

Most chelicerates ingest food using a preoral cavity formed by the chelicerae and pedipalps. Some chelicerates may secrete digestive enzymes to pre-digest food before ingesting it. Parasitic chelicerates like ticks and mites have evolved blood-sucking apparatuses.

The nervous system in chelicerates consists of a brain and two ventral nerve cords. These animals use external fertilization 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.

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

14.18: Subphylums of Arthropoda - Biology

Traditionally, it was argued that arthropods share a common ancestor with annelid worms. Annelids and Arthropods have a coelom and a segmented body. However, molecular phylogentics analyses comparing DNA and RNA indicate that arthropods are more closely related to the roundworms or nematodes (Prothero, 2007, p. 193). Arthropods were the first animals to have jointed appendages. Joints permit powerful movement, aid in locomotion and can be modified to serve many functions. The name arthropod means jointed foot. Trilobites were the first animals with eyes that were capable of forming images (Johnson & Raven, 2001). Arthropods were the first animals to invade land during the Silurian period (millipedes, followed by scorpions, spiders, and eventually insects) (Prothero, 1998, p. 248). Arthropods were the first organisms to evolve wings. Insects took to the skyies 100 million years before the flying reptiles of the Mesozoic.

Jointed Appendages & The Exoskeleton

Arthropods success and diversity is due to mainly two factors. First, arthropod jointed appendages are susceptible to adaptive specialization. Legs, mouthparts antennae, claws pincers, swimming paddles, and gills represent some of the specializations. Second, their exoskeleton provides protection and mechanical advantage. The exoskeleton serves as an external armor for protection against the environment and predators. Leverage is increased in arthropods as the muscles pull from the inside of the exoskeleton instead of muscles pulling from the outside of an internal skeleton as in vertebrates. The exoskeleton is not a living tissue and does not grow. Therefore, the exoskeleton must be shed or molted in order for growth to occur. After molting an arthropod can make radical body changes in its body plan before the new skeleton dries and hardens, allowing for the evolution of life cycles that include metamorphic processes.

Having your skeleton on the outside can also make you vulnerable at times as well as place limits on size. After molting the soft exposed body is vulnerable to predators. Arthropods must hide from predators during molting. In molting its exoskeleton the arthropod has lost most of its structural support. If the body were too large it would lose its integrity after the first molt "dissolving" into a blob. The size to which an animal with an external skeleton can evolve also depends on the buoyancy provided by its surroundings. The largest arthropods to evolve did so in the marine environment. On land oxygen levels may also limit arthropod size as respiration through book lungs and tracheae depend partly on diffusion.

The exoskeleton is made of chitin (polysaccharide and protein). In some groups, such as trilobites and ostracodes, the cutile is also mineralized with calcite. Arthropods with chitin reinforced with calcite leave a good fossil record. Unfortunately, most arthropods have exoskeletons made only of chitin, which explains why overall the arthropod fossil record is poor. Luckily, fossil lagerstatten that preserve nonmineralized tissues, such as Baltic amber, the Burgess Shale, and the Florissant fossil beds provide important windows into arthropod evolution.

Arthropods can be divided into four subphylums. The subphylum Tracheata or Uniramia (millipedes, centipedes, and insects) inlude arthropods with jaws. Arthropods with fangs or pincers belong to the subphylums Trilobitomorpha (trilobites), Crustacea (lobsters, crabs, shrimps, crayfish, barnacles, ostracodes, and pill bugs) and Chelicerata (scorpions, mites, spiders, horseshoe crabs, and eurypterids).


Trilobites range from the Cambrian to the Permian. Next to ostracodes they are the most common arthropod fossil. Trilobite means three-lobed and refers to the three longitudinal divisions of their calcified exoskeleton. An axial lobe running down the center is attached to pleural lobes on either side. The head or chephalon is separated from the tail or pygidium by thorasic segments. As juveniles, trilobites were pelagic, swimming in the plankton. As adults, most trilobites were benthic detritus feeders, although there are some exceptions such as the agnostids, which are thought to have lived a pelagic life due to their world-wide distribution. Trilobites were one of the first organisms to have eyes and may have been the first with eyes that focused images. Many trilobite species lost their eyes through selection such as the agnositds and Cryptolithus.

Trilobites have compound eyes made of calcite crystals. Two important types of compound eye can be found in trilobites. Holochroal eyes consisted of hundreds (up to 15,000) of closely packed elongated prisms of calcite arranged in a hexagonal pattern (Fortey, 2000, p. 99). The lenses of these eyes must be viewed with the aid of magnification. Holochroal eyes produced composite images of the world, but with little resolution. Phacops trilobites possessed schizochroal eyes. Schizochroal eyes are made of larger spherical-shaped lenses numbering in the hundreds (up to 700) and separated by exoskeleton. Each lens is made of two calcite crystals arranged as a doublet lens. Ironically, the doublet lens was designed by Dutch scientist Christian Huygens (1629-1695) and the French philosopher Rene Descartes (1586-1650) to correct for spherical aberration. Through selection nature had anticipated the same design 400 million years earlier. The phacopid lens produced larger images bringing them into sharp focus (Fortey, p. 106).

Trilobites enjoyed a great adaptive radiation during the Cambrian reaching their peak in abundance and diversity. In fact, Trilobites are the biostratigraphic standard for the Cambrian. Trilobites declined during the Ordovician and were only a minor part of the seafloor fauna in the Silurian and Devonian. Trilobites did enjoy some increase in diversity during the Devonian however, they never fully recovered from the late Devonian extinction event and vanished during the Permian.

Phacops rana is the state fossil for Pennsylvania. Isotelus is the state fossil for Ohio. Calymene celebra is the state fossil for Wisconsin.


Chelicerates include spiders, scorpions, mites, ticks, horseshoe crabs, and eurypterids. The chelicerate body is divided into a cephalothorax and the abdomen. They do not have antennae. The mouth parts are composed of chelicerae(small claws or fangs) and pedipalps. Pedipalps are modified into pincers in scorpions, eurypterids, and some spiders. The thorax has 4 pairs of legs. The subclass Arachnida includes spiders, scorpions, ticks, and mites. Trigonotarbids are spider-like arachnids that lack poison glands and silk-producing organs, but they do have book lungs. Trigonotarbids make their first appearance in the Silurian. The first true spider, pseudoscorpions, scorpions, and mites make their first appearance during the Devonian.

Eurypterids are an extinct group of arthropods in the subclass Eurypterida. The eurypterid is a chelicerate arthropod and looks like a cross between a scorpion and lobster. Eurypterids range from the Ordovician to the Permian. Eurypterids were major marine predators during the Silurian. The Eurypterid Eurypterus remipes is the state fossil for New York. Horseshoe crabs belong to the subclass Xiphosura. Primitive horseshoe crabs make their first appearance in the Silurian. In the early Paleozoic horseshoe crabs evolved into a variety of body forms. Horseshoe crabs similar to the present day Limulus do not appear until the Pennsylvanian.

Crustaceans range from the Cambrian to recent times. Today, crustaceans are the most successful arthropods in marine environments. Crustaceans also inhabit freshwater environments and a few are terrestrial. Familiar crustaceans include shrimp, lobsters, crabs, crayfish, barnacles, water fleas (Daphnia), and pill bugs. The body and legs of crustaceans are enclosed in a chitinous shell, which in some species is reinforced with calcium containing minerals. Crustaceans have two pairs of sensory antennae, three pairs of limbs used to handle and push food into the mouth, and biramous walking legs that may have gills. The general body plan of crustaceans consists of a head, thorax, and abdomen. The head and throax are often fused into a cephalothorax and the abdomen often ends in a tail-like structure. All crustaceans develop from a distinctive larva type known as nauplius. Many taxa below the class level leave no fossil record however, two classes, Malacostraca and Maxillopoda have a substantial fossil record.

The class Malacostraca includes crabs, shrimps, lobsters, krill, pillbugs, and amphipods. Some of the earliest crustaceans from this class included shrimp-like organisms called phyllocarids, which first appear in the Cambrian. The familiar order of Decapods (crabs, lobsters, crayfish and shrimps) did not appear until the Devonian period. The Decapods underwent a great adaptive radiation during the Mesozoic. Crabs make their first undisputed appearance in the Jurassic. Predation during the Mesozoic from crabs and lobsters may have forced mollusks to burrow and many brachiopods to go extinct (Protheros, 1998, p. 265).

The class Maxillopoda includes ostracodes, copepods, barnacles, and many shrimp-like forms. Barnacles and ostracodes have calcified skeletons that fossilize well. Barnacles (subclass Cirripedia) are strictly marine organisms and range from Ordovician to recent times. The barnacle nauplius larva changes into a shrimp-like, bivalved organism. The shrimp-like organism adheres its head to a surface, casting off its bivalved shell. After a profound metamorphosis the adult form becomes cemented to the substrate and the mantle, which encloses its body, secretes a calcareous shell. The calcareous shell is made of side plates and lid plates. The lid plates are needed for precise identification, but are rarely fossilized. Most barnacles attach to hard surfaces and are filter feeders. Some species burrow into the shells of mollusks and corals. Still others are parasitic and burrow into other organisms. Ostracodes (subclass Ostracoda) are the most common fossil arthropods and range from early Cambrian to recent times. Ostracodes are usually microscopic crustaceans with a pair of kidney-bean-shaped calcareous shells hinged over their back. Ostracodes are rapidly evolving, abundant microfossils, making them very useful for biostratigraphy. Ostracodes are sensitive to depth and water conditions, which makes them useful tools for studies in paleoecology.

Tracheata or Uniramia

Tracheates include millipedes, centipedes, and insects. Millipedes and centipedes belong to the class Myriapoda. Millipedes have two pairs of legs per segment. Millipedes have a long tubular body covered with a calcified cuticle coated with wax and equipped with up to 200 pairs of legs. Millipedes live among rotting vegetation and are mostly scavengers or detritus feeders. Possible millipede trace fossils are found in the Ordovician, which would make them the first animals to invade land. Millipede body fossils first appear in the Silurian. Centipedes have a more flattened body with fewer segments and only one pair of legs per body segment. Centipedes have at most a few dozen pairs of legs. Centipedes are carnivores equipped with poison claws and fangs. Centipedes first appear in the Silurian.

Insects are the most successful animal in terms of numbers and diversity. Insects belong to the epiclass Hexapoda and the class Insecta. Hexapoda consists of entognathous hexipods and the true insects. Entognathus hexipods have their mouthparts recessed within their head, springtails are a familiar example.

The insect body plan consists of a head with antennae, thorax with six legs, and an abdomen. The insect cuticle does not fossilize well. At the species level the insect fossil record is poor however, at higher taxonomic levels the resolution improves. At the family level 63% of living families are represented in the fossil record. At the order level 100% are represented. Primitive wingless insects first appear in the Devonian. Insects with wings (Pterygota) make their first appearance in the Pennsylvanian. There is evidence to suggest that insects with fixed wings (Paleoptera), like mayflies, came before those with folded wings (Neoptera), like cockroaches. Herbivorous insects make their first appearance in the Carboniferous. Insects up through the Carboniferous undergo incomplete metamorphosis (superorder Exopterygota or Hemipterodea). Insects with complete metamorphosis (superorder Endopterygota or Holometabola) make their first appearance in the Permian. Social insects make their first appearance in the Cretaceous. Fleas, which are parasites to mammals, do not appear in the fossil record until the Cretaceous.

Insect Trace Fossils

Insect ichnofossils (trace fossils) can be helpful in determining what types of insects were present at a particular time and provide information about the nature and persistence of past plant-insect associations.

Evidence for herbivory in insects appears in the Carboniferous. Like vertebrates, the first insects were carnivores and detritivores. Herbivory requires hosting cellulose-digesting bacteria through a symbiotic relationship within the gut. The oldest examples of marginal and surface feeding are on Carboniferous seed fern leaves of Neuropteris and Glosspteris (Grimaldi & Engel, 2005, p. 52). It is estimated that only 4% of the leaves in Carboniferous deposits exhibit damage from feeding. Herbivores do not make a significant impact on plant life until the Permian (Kenrick & Davis, 2004, pp. 166-167).

Galls are excessive growths on stems, leaves, cones, and flowers caused by insect feeding or egg laying. The earliest fossil galls are found on the petioles of Psaronius tree ferns of the Late Carboniferous. Insect gall fossil diversity and abundance takes off with the advent of flowering plant evolution in the Cretaceous (Grimaldi & Engel, 2005, p. 53).

Insects produce tunnels in wood known as borings or galleries. Some insects eat the cambial layer while others eat fungus that grows within the galleries, still others eat the wood itself. The oldest borings and galleries in wood, attributed to mites, are known from the Carboniferous. The first definitive beetle borings are from the Triassic. There are some borings in permineralized Triassic-aged wood from Arizona that are attributed to termites or bees however, they may be beetle borings (Grimaldi & Engel, 2005, p. 54 & 55).

Leaf mines are meandering tunnels produced by the feeding larvae of some beetle, fly, and sawfly species. The first definitive leaf mines first appear in the leaves of Triassic conifers and pteridosperms. Interestingly, the abundance and diversity of fossil leaf mines coincides with the radiation of flowering plants (Angiosperms) during the Cretaceous. Leaf mines have been used to establish the peristence of insect and plant associations. For example, the larvae of certain moth families have been eating the leaves of Quercus (oak) and Populus (poplars) for 20 million years and hispine beetles have been eating the leaves of Heliconia for 70 million years (Grimaldi & Engel, 2005, p. 52).

Caddisfly larvae live in lakes, ponds, and rivers. Many build distinctive protective cases from bits of sand, shells and vegetation. Fossil caddisfly cases can often be identified to the family or even genus level. The oldest larval caddisfly cases (Trichoptera) are found in the Jurassic (Grimaldi & Engel, 2005, p. 51).

Celliforma is a fossil bee nest (in the form of subterranean excavations) that is first found in Late Cretaceous deposits. Celliforma is found from the Cretaceous to the Pliocene (Grimaldi & Engel, 2005, p. 51). Termite borings appear in the Cretaceous and represent the oldest undisputed fossil nest for social insects (Grimaldi & Engel, 2005, p. 54). Coprinisphaera is the fossil burrow of a scarabaerine dung beetle, which makes its first apperance during the Paleocene. Coprinisphaera lived from the Paleocene to the Pleistocene and had a wide geographic range being found in South America, Antarctica, Africa and Asia. Coprinisphaera coincide with the evolution of the first ecosystems to have abundant mammalian herbivores. Evidence for the first scarab tunnels are found in the coprolites of herbivorous dinosaurs from the Late Cretaceous of Montana (Grimaldi & Engel, 2005, p. 50).

Science Olympiad Fossil Event

The 2016 Science Olympiad Fossil List includes trilobites, eurypterids, insects, and crustaceans. The following trilobite genera are listed: Phacops, Isotelus, Cryptolithus, Calymene, and Elrathia. Crustaceans mentioned on the list include: shrimp, lobster, crabs, and barnacles.

Elrathia kingii
House Range and Drum Mountains Western Utah, USA
Wheeler Shale Formation
Cambrian: 505 Million Years

Trilobite Molt
Cryptolithus tessellatus

Kope Formation Ordovician
Ft. Mitchell, KY

Phacops megalomanicus
Atlas Mountains, Morocco
9.5 cm long x 5.5 cm wide

Flexicalymene meeki
Arnheim Formation Ordovician
Mt. Orab, OH

Eurypterus remipes
Fiddler Green Formation
Phelps Member
Upper Silurian
Herkimer Co., New York
Specimen is 9 cm long

Baltic Amber with Inclusion
Wasp (Order Hymenoptera)
Cenozoic Paleocene Eocene
Yantarny, Kaliningrad, Russia
4.5 cm long x 3 cm wide x 1.5 cm thick

Cenozoic Paleocene Eocene
Primorskoje, Kaliningrad, Russia

Cenozoic Paleocene Eocene
Primorskoje, Kaliningrad, Russia

Carbonized Insects
Green River Formation
Cenozoic Paleocene Eocene
Kemmerer, Wyoming
Slab 6 cm x 4 cm

Carbonized Insect
Yixian Formation
Lioaning Province, China

Haquel, Lebanon

Conocoryphe sulzeri
Middle Cambrian
Jince, Czech Republic
Larger Tilobite 3 cm

Balanus concavus
Tamiami Formation
Port Charlotte, Florida
10 cm tall x 10 cm wide


Fortey, R. (2000). Trilobite: Eyewitness to Evolution. New York: Vintage Books.

Grimaldi, D. & Engel, M.S., (2005). Evolution of the Insects. New York: Cambridge University Press.

Johnson, G.B. & Raven, P.H. (2001). Biology: Principles & Explorations. New York: Holt, Rinehart & Winston.

Nudds, J.R. & Selden P.A. (2008). Fossil Ecosystems of North America: A Guide to the Sites and Their Extraordinary Biotas. Chicago: University of Chicago Press.

Prothero, D.R. (1998). Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill.

Prothero, D.R. (2007). Evolution: What Fossils Say and Why It Matters. New York: Columbia University Press.

Selden P. & Nudds, J. (2004). Evolution of Fossil Ecosystems. Chicago: The University of Chicago Press.


Segmentation and cuticle Edit

The Chelicerata are arthropods as they have: segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins heads that are composed of several segments that fuse during the development of the embryo a much reduced coelom a hemocoel through which the blood circulates, driven by a tube-like heart. [10] Chelicerates' bodies consist of two tagmata, sets of segments that serve similar functions: the foremost one, called the prosoma or cephalothorax, and the rear tagma is called the opisthosoma or abdomen. [13] However, in the Acari (mites and ticks) there is no visible division between these sections. [14]

The prosoma is formed in the embryo by fusion of the ocular somite (referred as "acron" in previous literatures), which carries the eyes and labrum, [12] with six post-ocular segments (somite 1 to 6), [11] which all have paired appendages. It was previously thought that chelicerates had lost the antennae-bearing somite 1, [15] but later investigations reveal that it retain and correspond to a pair of chelicerae or chelifores, [16] small appendages that often form pincers. somite 2 has a pair of pedipalps that in most sub-groups perform sensory functions, while the remaining four cephalothorax segments (somite 4 to 6) have pairs of legs. [11] In primitive forms the ocular somite has a pair of compound eyes on the sides and four pigment-cup ocelli ("little eyes") in the middle. [13] The mouth is between somite 1 and 2 (chelicerae and pedipalps).

The opisthosoma consists of thirteen or fewer segments, may or may not end with a telson. [11] In some taxa such as scorpion and eurypterid the opisthosoma divided into two groups, mesosoma and metasoma. [11] The abdominal appendages of modern chelicerates are missing or heavily modified [13] – for example in spiders the remaining appendages form spinnerets that extrude silk, [17] while those of horseshoe crabs (Xiphosura) form gills. [18] [11]

Like all arthropods, chelicerates' bodies and appendages are covered with a tough cuticle made mainly of chitin and chemically hardened proteins. Since this cannot stretch, the animals must molt to grow. In other words, they grow new but still soft cuticles, then cast off the old one and wait for the new one to harden. Until the new cuticle hardens the animals are defenseless and almost immobilized. [19]

Chelicerae and pedipalps Edit

Chelicerae and pedipalps are the two pairs of appendages closest to the mouth they vary widely in form and function and the consistent difference between them is their position in the embryo and corresponding neurons: chelicerae are deutocerebral and arise from somite 1, ahead of the mouth, while pedipalps are tritocerebral and arise from somite 2, behind the mouth. [13] [11] [12]

The chelicerae ("claw horns") that give the sub-phylum its name normally consist of three sections, and the claw is formed by the third section and a rigid extension of the second. [13] [20] However, spiders' have only two sections, and the second forms a fang that folds away behind the first when not in use. [17] The relative sizes of chelicerae vary widely: those of some fossil eurypterids and modern harvestmen form large claws that extended ahead of the body, [20] while scorpions' are tiny pincers that are used in feeding and project only slightly in front of the head. [21]

In basal chelicerates, the pedipalps are unspecialized and subequal to the posterior pairs of walking legs. [11] However, in sea spider and arachnids, the pedipalps are more or less specialized for sensory [13] or prey-catching function [11] – for example scorpions have pincers [21] and male spiders have bulbous tips that act as syringes to inject sperm into the females' reproductive openings when mating. [17]

Body cavities and circulatory systems Edit

As in all arthropods, the chelicerate body has a very small coelom restricted to small areas round the reproductive and excretory systems. The main body cavity is a hemocoel that runs most of the length of the body and through which blood flows, driven by a tubular heart that collects blood from the rear and pumps it forward. Although arteries direct the blood to specific parts of the body, they have open ends rather than joining directly to veins, and chelicerates therefore have open circulatory systems as is typical for arthropods. [23]

Respiratory systems Edit

These depend on individual sub-groups' environments. Modern terrestrial chelicerates generally have both book lungs, which deliver oxygen and remove waste gases via the blood, and tracheae, which do the same without using the blood as a transport system. [24] The living horseshoe crabs are aquatic and have book gills that lie in a horizontal plane. For a long time it was assumed that the extinct eurypterids had gills, but the fossil evidence was ambiguous. However, a fossil of the 45 millimetres (1.8 in) long eurypterid Onychopterella, from the Late Ordovician period, has what appear to be four pairs of vertically oriented book gills whose internal structure is very similar to that of scorpions' book lungs. [25]

Feeding and digestion Edit

The guts of most modern chelicerates are too narrow to take solid food. [24] All scorpions and almost all spiders are predators that "pre-process" food in preoral cavities formed by the chelicerae and the bases of the pedipalps. [17] [21] However, one predominantly herbivore spider species is known, [26] and many supplement their diets with nectar and pollen. [27] Many of the Acari (ticks and mites) are blood-sucking parasites, but there are many predatory, herbivore and scavenger sub-groups. All the Acari have a retractable feeding assembly that consists of the chelicerae, pedipalps and parts of the exoskeleton, and which forms a preoral cavity for pre-processing food. [14]

Harvestmen are among the minority of living chelicerates that can take solid food, and the group includes predators, herbivores and scavengers. [28] Horseshoe crabs are also capable of processing solid food, and use a distinctive feeding system. Claws at the tips of their legs grab small invertebrates and pass them to a food groove that runs from between the rearmost legs to the mouth, which is on the underside of the head and faces slightly backwards. The bases of the legs form toothed gnathobases that both grind the food and push it towards the mouth. [18] This is how the earliest arthropods are thought to have fed. [29]

Excretion Edit

Horseshoe crabs convert nitrogenous wastes to ammonia and dump it via their gills, and excrete other wastes as feces via the anus. They also have nephridia ("little kidneys"), which extract other wastes for excretion as urine. [18] Ammonia is so toxic that it must be diluted rapidly with large quantities of water. [30] Most terrestrial chelicerates cannot afford to use so much water and therefore convert nitrogenous wastes to other chemicals, which they excrete as dry matter. Extraction is by various combinations of nephridia and Malpighian tubules. The tubules filter wastes out of the blood and dump them into the hindgut as solids, a system that has evolved independently in insects and several groups of arachnids. [24]

Nervous system Edit

Cephalothorax ganglia fused into brain Abdominal ganglia fused into brain
Horseshoe crabs All First two segments only
Scorpions All None
Mesothelae First two pairs only None
Other arachnids All All

Chelicerate nervous systems are based on the standard arthropod model of a pair of nerve cords, each with a ganglion per segment, and a brain formed by fusion of the ganglia just behind the mouth with those ahead of it. [31] If one assume that chelicerates lose the first segment, which bears antennae in other arthropods, chelicerate brains include only one pair of pre-oral ganglia instead of two. [13] However, there is evidence that the first segment is indeed available and bears the cheliceres. [32] [16]

There is a notable but variable trend towards fusion of other ganglia into the brain. The brains of horseshoe crabs include all the ganglia of the prosoma plus those of the first two opisthosomal segments, while the other opisthosomal segments retain separate pairs of ganglia. [18] In most living arachnids, except scorpions if they are true arachnids, all the ganglia, including those that would normally be in the opisthosoma, are fused into a single mass in the prosoma and there are no ganglia in the opisthosoma. [24] However, in the Mesothelae, which are regarded as the most primitive living spiders, the ganglia of the opisthosoma and the rear part of the prosoma remain unfused, [33] and in scorpions the ganglia of the cephalothorax are fused but the abdomen retains separate pairs of ganglia. [24]

Senses Edit

As with other arthropods, chelicerates' cuticles would block out information about the outside world, except that they are penetrated by many sensors or connections from sensors to the nervous system. In fact, spiders and other arthropods have modified their cuticles into elaborate arrays of sensors. Various touch and vibration sensors, mostly bristles called setae, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of taste and smell, often by means of setae. [34]

Living chelicerates have both compound eyes (only in horseshoe crabs, as the compound eye in the other clades has been reduced to a cluster of no more than five pairs of ocelli), mounted on the sides of the head, plus pigment-cup ocelli ("little eyes"), mounted in the middle. These median ocelli-type eyes in chelicerates are assumed to be homologous with the crustacean nauplius eyes and the insect ocelli. [35] The eyes of horseshoe crabs can detect movement but not form images. [18] At the other extreme, jumping spiders have a very wide field of vision, [17] and their main eyes are ten times as acute as those of dragonflies, [36] able to see in both colors and UV-light. [37]

Reproduction Edit

Horseshoe crabs, which are aquatic, use external fertilization, in other words the sperm and ova meet outside the parents' bodies. Their trilobite-like larvae look rather like miniature adults as they have full sets of appendages and eyes, but initially they have only two pairs of book-gills and gain three more pairs as they molt. [18]

Being air-breathing animals, the living arachnids (excluding horseshoe crabs) use internal fertilization, which is direct in some species, in other words the males' genitalia make contact with the females'. However, in most species fertilization is indirect. Male spiders use their pedipalps as syringes to "inject" sperm into the females' reproductive openings, [17] but most arachnids produce spermatophores (packages of sperm) which the females take into their bodies. [24] Courtship rituals are common, especially in the most powerful predators, where males risk being eaten before mating. Most arachnids lay eggs, but all scorpions and a few mites keep the eggs inside their bodies until they hatch and offspring rather like miniature adults emerge. [24]

Levels of parental care for the young range from zero to prolonged. Scorpions carry their young on their backs until the first molt, and in a few semi-social species the young remain with their mother. [38] Some spiders care for their young, for example a wolf spider's brood cling to rough bristles on the mother's back, [17] and females of some species respond to the "begging" behavior of their young by giving them their prey, provided it is no longer struggling, or even regurgitate food. [39]

Fossil record Edit

There are large gaps in the chelicerates' fossil record because, like all arthropods, their exoskeletons are organic and hence their fossils are rare except in a few lagerstätten where conditions were exceptionally suited to preserving fairly soft tissues. The Burgess shale animals like Sidneyia from about 505 million years ago have been classified as chelicerates, the latter because its appendages resemble those of the Xiphosura (horseshoe crabs). However, cladistic analyses that consider wider ranges of characteristics place neither as chelicerates. There is debate about whether Fuxianhuia from earlier in the Cambrian period, about 525 million years ago , was a chelicerate. Another Cambrian fossil, Kodymirus, was originally classified as an aglaspid but may have been a eurypterid and therefore a chelicerate. If any of these was closely related to chelicerates, there is a gap of at least 43 million years in the record between true chelicerates and their nearest not-quite chelicerate relatives. [40]

Sanctacaris, member of the family Sanctacarididae from the Burgess Shale of Canada, represents the oldest occurrence of a confirmed chelicerate, Middle Cambrian in age. [3] Although its chelicerate nature has been doubted for its pattern of tagmosis (how the segments are grouped, especially in the head), [40] a restudy in 2014 confirmed its phylogenetic position as the oldest chelicerate. [3]

The eurypterids have left few good fossils and one of the earliest confirmed eurypterid, Pentecopterus decorahensis, appears in the Middle Ordovician period 467.3 million years ago million years ago, making it the oldest eurypterid. [41] Until recently the earliest known xiphosuran fossil dated from the Late Llandovery stage of the Silurian 436 to 428 million years ago , [42] but in 2008 an older specimen described as Lunataspis aurora was reported from about 445 million years ago in the Late Ordovician. [43]

The oldest known arachnid is the trigonotarbid Palaeotarbus jerami, from about 420 million years ago in the Silurian period, and had a triangular cephalothorax and segmented abdomen, as well as eight legs and a pair of pedipalps. [44]

Attercopus fimbriunguis, from 386 million years ago in the Devonian period, bears the earliest known silk-producing spigots, and was therefore hailed as a spider, [45] but it lacked spinnerets and hence was not a true spider. [46] Rather, it was likely sister group to the spiders, a clade which has been named Serikodiastida. [47] Close relatives of the group survived through to the Cretaceous Period. [48] Several Carboniferous spiders were members of the Mesothelae, a primitive group now represented only by the Liphistiidae, [45] and fossils suggest taxa closely related to the spiders, but which were not true members of the group were also present during this Period. [49]

The Late Silurian Proscorpius has been classified as a scorpion, but differed significantly from modern scorpions: it appears wholly aquatic since it had gills rather than book lungs or tracheae its mouth was completely under its head and almost between the first pair of legs, as in the extinct eurypterids and living horseshoe crabs. [50] Fossils of terrestrial scorpions with book lungs have been found in Early Devonian rocks from about 402 million years ago . [51] The oldest species of scorpion found as of 2021 is Dolichophonus loudonensis, which lived during the Silurian, in present-day Scotland. [52]

Relationships with other arthropods Edit

The "traditional" view of the arthropod "family tree" shows chelicerates as less closely related to the other major living groups (crustaceans hexapods, which includes insects and myriapods, which includes centipedes and millipedes) than these other groups are to each other. Recent research since 2001, using both molecular phylogenetics (the application of cladistic analysis to biochemistry, especially to organisms' DNA and RNA) and detailed examination of how various arthropods' nervous systems develop in the embryos, suggests that chelicerates are most closely related to myriapods, while hexapods and crustaceans are each other's closest relatives. However, these results are derived from analyzing only living arthropods, and including extinct ones such as trilobites causes a swing back to the "traditional" view, placing trilobites as the sister-group of the Tracheata (hexapods plus myriapods) and chelicerates as least closely related to the other groups. [56]

Major sub-groups Edit

It is generally agreed that the Chelicerata contain the classes Arachnida (spiders, scorpions, mites, etc.), Xiphosura (horseshoe crabs) and Eurypterida (sea scorpions, extinct). [58] The extinct Chasmataspidida may be a sub-group within Eurypterida. [58] [59] The Pycnogonida (sea spiders) were traditionally classified as chelicerates, but some features suggest they may be representatives of the earliest arthropods from which the well-known groups such as chelicerates evolved. [60]

However, the structure of "family tree" relationships within the Chelicerata has been controversial ever since the late 19th century. An attempt in 2002 to combine analysis of RNA features of modern chelicerates and anatomical features of modern and fossil ones produced credible results for many lower-level groups, but its results for the high-level relationships between major sub-groups of chelicerates were unstable, in other words minor changes in the inputs caused significant changes in the outputs of the computer program used (POY). [61] An analysis in 2007 using only anatomical features produced the cladogram on the right, but also noted that many uncertainties remain. [62] In recent analyses the clade Tetrapulmonata is reliably recovered, but other ordinal relationships remain in flux. [48] [63] [49] [64] [65] [66] [2]

The position of scorpions is particularly controversial. Some early fossils such as the Late Silurian Proscorpius have been classified by paleontologists as scorpions, but described as wholly aquatic as they had gills rather than book lungs or tracheae. Their mouths are also completely under their heads and almost between the first pair of legs, as in the extinct eurypterids and living horseshoe crabs. [50] This presents a difficult choice: classify Proscorpius and other aquatic fossils as something other than scorpions, despite the similarities accept that "scorpions" are not monophyletic but consist of separate aquatic and terrestrial groups [50] or treat scorpions as more closely related to eurypterids and possibly horseshoe crabs than to spiders and other arachnids, [25] so that either scorpions are not arachnids or "arachnids" are not monophyletic. [50] Cladistic analyses have recovered Proscorpius within the scorpions, [47] based on reinterpretation of the species' breathing apparatus. [67] This is reflected also in the reinterpretation of Palaeoscorpius as a terrestrial animal. [68]

A 2013 phylogenetic analysis [69] (the results presented in a cladogram below) on the relationships within the Xiphosura and the relations to other closely related groups (including the eurypterids, which were represented in the analysis by genera Eurypterus, Parastylonurus, Rhenopterus and Stoermeropterus) concluded that the Xiphosura, as presently understood, was paraphyletic (a group sharing a last common ancestor but not including all descendants of this ancestor) and thus not a valid phylogenetic group. Eurypterids were recovered as closely related to arachnids instead of xiphosurans, forming the group Sclerophorata within the clade Dekatriata (composed of sclerophorates and chasmataspidids). This work suggested it is possible that Dekatriata is synonymous with Sclerophorata as the reproductive system, the primary defining feature of sclerophorates, has not been thoroughly studied in chasmataspidids. Dekatriata is in turn part of the Prosomapoda, a group including the Xiphosurida (the only monophyletic xiphosuran group) and other stem-genera. A recent phylogenetic analysis of the chelicerates places the Xiphosura within the Arachnida as the sister group of Ricinulei., [2] but others still retrieve a monophyletic arachnida. [70]

Although well behind the insects, chelicerates are one of the most diverse groups of animals, with over 77,000 living species that have been described in scientific publications. [71] Some estimates suggest that there may be 130,000 undescribed species of spider and nearly 500,000 undescribed species of mites and ticks. [72] While the earliest chelicerates and the living Pycnogonida (if they are chelicerates [60] ) and Xiphosura are marine animals that breathe dissolved oxygen, the vast majority of living species are air-breathers, [71] although a few spider species build "diving bell" webs that enable them to live under water. [73] Like their ancestors, most living chelicerates are carnivores, mainly on small invertebrates. However, many species feed as parasites, herbivores, scavengers and detritivores. [14] [28] [71]

Diversity of living chelicerates
Group Described species [71] [74] Diet
Pycnogonida (sea-spiders) 500 Carnivorous [71]
Araneae (spiders) 34,000 Carnivorous [71] 1 herbivore [26]
Acari (mites and ticks) 32,000 Carnivorous, parasitic, herbivore, detritivore [14] [71]
Opiliones (harvestmen) 6,500 Carnivorous, herbivore, detritivore [28]
Pseudoscorpiones (false scorpions) 3,200 Carnivorous [75]
Scorpiones (scorpions) 1,400 Carnivorous [21]
Solifugae (sunspiders) 900 Carnivorous, omnivorous [76]
Schizomida (small whipscorpions) 180
Amblypygi (whipspiders) 100
Uropygi (Thelyphonida – whipscorpions) 90 Carnivorous [77]
Palpigradi (micro whipscorpions) 60
Xiphosura (horseshoe crabs) 4 Carnivorous [71]
Ricinulei 60

In the past, Native Americans ate the flesh of horseshoe crabs, and used the tail spines as spear tips and the shells to bail water out of their canoes. More recent attempts to use horseshoe crabs as food for livestock were abandoned when it was found that this gave the meat a bad taste. Horseshoe crab blood contains a clotting agent, limulus amebocyte lysate, which is used to test antibiotics and kidney machines to ensure that they are free of dangerous bacteria, and to detect spinal meningitis and some cancers. [78]

Cooked tarantula spiders are considered a delicacy in Cambodia, [79] and by the Piaroa Indians of southern Venezuela. [80] Spider venoms may be a less polluting alternative to conventional pesticides as they are deadly to insects but the great majority are harmless to vertebrates. [81] Possible medical uses for spider venoms are being investigated, for the treatment of cardiac arrhythmia, [82] Alzheimer's disease, [83] strokes, [84] and erectile dysfunction. [85]

Because spider silk is both light and very strong, but large-scale harvesting from spiders is impractical, work is being done to produce it in other organisms by means of genetic engineering. [86] Spider silk proteins have been successfully produced in transgenic goats' milk, [87] tobacco leaves, [88] silkworms, [89] [90] [91] and bacteria, [86] [92] [93] and recombinant spider silk is now available as a commercial product from some biotechnology companies. [91]

In the 20th century, there were about 100 reliably reported deaths from spider bites, [94] compared with 1,500 from jellyfish stings. [95] Scorpion stings are thought to be a significant danger in less-developed countries for example, they cause about 1,000 deaths per year in Mexico, but only one every few years in the USA. Most of these incidents are caused by accidental human "invasions" of scorpions' nests. [96] On the other hand, medical uses of scorpion venom are being investigated for treatment of brain cancers and bone diseases. [97] [98]

Ticks are parasitic, and some transmit micro-organisms and parasites that can cause diseases in humans, while the saliva of a few species can directly cause tick paralysis if they are not removed within a day or two. [99]

A few of the closely related mites also infest humans, some causing intense itching by their bites, and others by burrowing into the skin. Species that normally infest other animals such as rodents may infest humans if their normal hosts are eliminated. [100] Three species of mite are a threat to honey bees and one of these, Varroa destructor, has become the largest single problem faced by beekeepers worldwide. [101] Mites cause several forms of allergic diseases, including hay fever, asthma and eczema, and they aggravate atopic dermatitis. [102] Mites are also significant crop pests, although predatory mites may be useful in controlling some of these. [71] [103]

Metameric Segmentation and Tagmata

Metameric segmentation is where the body is divided into a series of repeated segments, as in a millipede for instance.

Madagascar banded millipede (Aphistogoniolus polleni)

Each segment then performs all the functions of the body trunk sections, has legs, nerves, breathing apparatus, a unit of digestive tract and all the same organs and tissues. Each segment is in fact a copy of the one before it and the one behind. This is obvious in some arthropods, like millipedes, but not so obvious in others.

What has happened in the others is called tagmatization.

This is where groups of segments become specialised to perform specific functions for the whole body. These groups of segments are called Tagmata (Singular = Tagma). Careful dissection and analysis can reveal the underlying form of the original metameric segment in most cases.

Watch the video: 3840Chapter 19: Arthropoda- trilobites, chelicerates and myriapods (August 2022).