CHAPTER 19 Trilobites, Chelicerates, and Myriapods 19-1
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Anthropodization Characteristics Soft cuticle of the arthropod ancestors was stiffened by deposition of protein and chitin Joints had to provide flexibility Sequence of molts was necessary to allow for growth Molting required hormonal control Hydrostatic skeleton function was lost Coelom regressed and was replaced by open sinuses Motile cilia were lost 19-3
Characteristics Phylum Arthropoda Contains over 3/4 of all known species Approximately 1,100,000 species of arthropods have been recorded Rich fossil history dating to late Precambrian Eucoelomate protostomes with welldeveloped organ systems Segmented Molecular analyses indicate annelids and arthropods evolved from different ancestors 19-4
Characteristics Sizes range from the Japanese crab (four meters in leg span) to the 0.1 mm long follicle mite Abundance and wide ecological distribution makes them the most diverse animal group Some are agents of disease and compete with humans for food Others are beneficial All modes of feeding occur among arthropods but most are herbivorous 19-5
Relationships among arthropod subgroups Divided into subphyla based on relationships between subgroups Groupings among subphyla based on molecular data Centipedes, millipedes, pauropods, and symphylans are placed in subphylum Myriapoda Insects are placed in subphylum Hexapoda Spiders, ticks, horseshoe crabs and their relatives form subphylum Chelicerata Lobsters, crabs, barnacles, and others form subphylum Crustacea 19-6 Characteristics
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Also included in Crustacea are tongue worms Extinct trilobites are placed in subphylum Trilobita Relationships are controversial Mandibulate hypothesis Myriapods, hexapods, and crustaceans more closely related due to a shared mouthpart, the mandible Molecular evidence of a close relationship between hexapods and crustaceans unites subphylum Crustacea with subphylum Hexapoda in clade Pancrustacea 19-9 Characteristics
Great Diversity and Abundance of Arthropods Versatile Exoskeleton Cuticle is highly protective and jointed, providing mobility Consists of inner thick procuticle and outer thin epicuticle Procuticle has an exocuticle secreted before a molt and an endocuticle secreted after molting Both layers of procuticle contain chitin bound with protein Procuticle is lightweight, flexible, and protects against dehydration Chitin content varies from 40% of the procuticle in insects to as much as 80% in crustaceans 19-10
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Great Diversity and Abundance of Arthropods Calcium salts responsible for hardness of procuticle of lobsters and crabs Cuticle is laminated and further hardened by tanning Cuticle is thin between segments, allowing for movement at the joints Ecdysis, or molting Process of shedding outer covering and growing a new, larger one Arthropods typically molt four to seven times Weight is a limit to ultimate body size 19-12
Great Diversity and Abundance of Arthropods Segmentation and Appendages for Efficient Locomotion Primitive pattern Linear series of similar somites with jointed appendages Many somites may be fused or combined into specialized groups called tagmata Appendages often highly specialized for division of labor Limb segments are hollow levers with internal striated muscles Appendages may function in sensing, food handling, walking, or swimming 19-13
Great Diversity and Abundance of Arthropods Air Piped Directly to Cells Terrestrial arthropods Use an efficient tracheal system for oxygen transport Aquatic arthropods Respire via various forms of gills Highly Developed Sensory Organs Eyes vary from simple light sensitive ocelli to a compound mosaic eye Other sensory structures for touch, smell, hearing, balancing, and chemical reception 19-14
Great Diversity and Abundance of Arthropods Complex Behavior Patterns Arthropods surpass most other invertebrates in complex and organized activities Most behavior is innate but some is learned Use of Diverse Resources through Metamorphosis Many arthropods undergo metamorphic changes leading to different larval and adult stages Larvae and adults feed on different organisms and occupy different habitats Avoid competition 19-15
Subphylum Trilobita History of an Ancient Group Trilobites arose before the Cambrian, flourished, and then became extinct 200 million years ago Trilobed body shape due to a pair of longitudinal grooves Bottom dwellers and probably were scavengers Ranging from 2 to 67 centimeters long Could roll up like pill bugs Exoskeleton contained chitin strengthened by calcium carbonate 19-16
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Subphylum Trilobita Body was divided into a cephalon, trunk, and pygidium Cephalon was a fusion of segments Trunk varied in number of somites Pygidium was fused into a plate Cephalon bore antennae, compound eyes, a mouth, and jointed appendages Each body somite except the last had a pair of biramous appendages One of the branches of biramous appendage was fringed May have been a gill 19-18
Subphylum Chelicerata Characteristics Chelicerates have six pairs of cephalothoracic appendages including chelicerae, pedipalps and four pair of legs Lack mandibles and antennae Most suck liquid food from prey 19-19
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Subphylum Chelicerata Class Merostomata: Subclass Eurypterida Eurypterids (giant water scorpions) were the largest of all fossil arthropods 3 m in length Fossils date from Ordovician to Permian periods Resemble both marine horseshoe crabs and terrestrial scorpions Head: 6 fused segments, 6 pairs of appendages, simple and compound eyes, chelicerae, pedipalps, and 4 pairs of walking legs Abdomen: 12 segments and spike-like telson 19-21
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Class Merostomata: Subclass Xiphosurida, Horseshoe Crabs Modern horseshoe crab nearly unchanged from ancestors in the Triassic period 5 species in 3 genera survive Most live in shallow water Structures Unsegmented carapace covers body in front of a broad abdomen and a telson Cephalothorax has 5 pairs of walking legs and a pair of chelicerae Abdomen bears six pairs of broad, thin, appendages fused in the median line Book gills exposed on some abdominal appendages 19-23 Subphylum Chelicerata Carapace has 2 compound and 2 simple eyes
Subphylum Chelicerata Walk with walking legs and swim with abdominal plates Feed at night on worms and small molluscs During mating season, come to shore at high tide to mate Females burrow into sand to lay eggs Males follow to add sperm before eggs are covered Young larvae hatch and return to sea at next high tide Larvae are segmented and resemble trilobites 19-24
Subphylum Chelicerata Class Pycnogonida: Sea Spiders All have small, thin bodies Approximately 1,000 species Some species duplicate somites, and may have five or six pairs of legs Males may have a pair of legs (ovigers) to carry developing eggs Many have chelicerae and palps Mouth, at the tip of a proboscis, sucks juices from cnidarians and soft-bodied animals 19-25
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Subphylum Chelicerata Most have four simple eyes Simple dorsal heart No excretory and respiratory systems Digestive system sends branches to the legs Most gonads are in the legs Occupy all oceans but most common in polar waters Some suggest that pycnogonids belonged to an early-diverging arthropod lineage 19-27
Subphylum Chelicerata Class Arachnida Great diversity Most are free living and more common in warm, dry regions Structures Divided into two tagmata: a cephalothorax and an abdomen Cephalothorax bears a pair of chelicerae, a pair of pedipalps, and 4 pairs of walking legs No antenna and mandibles 19-28
Subphylum Chelicerata Most are predaceous and have claws, fangs, poison glands, or stingers Sucking mouthparts ingest fluids and soft tissues from bodies of their prey Spiders have spinning glands A few spiders may have a segmented abdomen, a primitive character Pedipalps of males are modified, sometimes elaborately, for sperm transfer 19-29
Subphylum Chelicerata Over 80,000 species have been described Scorpions appeared on land in the Silurian, mites and spiders by the end of the Paleozoic Era Most harmless to humans and provide essential control of injurious insects Some spiders are venomous and can cause pain or death in humans Ticks may carry human diseases Mites can be crop pests 19-30
Subphylum Chelicerata Order Araneae: Spiders Approximately 40,000 species Body consists of an unsegmented cephalothorax and abdomen joined by a slender pedicel Anterior appendages are a pair of chelicerae with terminal fangs Pair of pedipalps have sensory functions and are used by males to transfer sperm Basal parts of pedipalps used to handle food Four pairs of walking legs terminate in claws All are predaceous, mostly on insects 19-31
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Subphylum Chelicerata Injected venom liquefies and digests the tissues which is sucked into spider s stomach Breathe by book lungs and/or tracheae Book lungs unique to spiders Parallel air pockets extend into blood-filled chamber Air enters chamber through a slit in body wall Tracheae system is less extensive than in insects Transports air directly to tissues Tracheal systems of arthropods represent a case of evolutionary convergence 19-33
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Subphylum Chelicerata In spiders and insects, Malpighian tubules serve as excretory structures Potassium, other solutes, and waste are secreted into tubules Rectal glands reabsorb the potassium and water, leaving wastes and uric acid for excretion Conserves water and allows the organisms to live in dry environments Many spiders have coxal glands, modified nephridia, at the base of legs 19-35
Subphylum Chelicerata Sensory Systems Most spiders have eight simple eyes, each with a lens, optic rods, and a retina Detect movement and may form images Sensory setae detect air currents, web vibrations, and other stimuli Spider s vision usually poor Awareness of environment depends largely on cuticular mechanoreceptors such as sensory setae 19-36
Subphylum Chelicerata Web-Spinning Habits Spinning silk critical ability for spiders and some other arachnids Two or three pairs of spinnerets contain microscopic tubes that run to silk glands Liquid scleroprotein secretion hardens as it is extruded from spinnerets Silk threads are very strong and will stretch considerably before breaking Silk is used for orb webs, lining burrows, forming egg sacs, and wrapping prey 19-37
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Subphylum Chelicerata Reproduction Before mating, male stores sperm in pedipalps Mating involves inserting pedipalps into the female genital openings A courtship ritual is often required before the female will allow mating Eggs may develop in a cocoon in the web or may be carried by female Young hatch in about two weeks and may molt before leaving the egg cocoon 19-39
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Subphylum Chelicerata Are spiders really dangerous? Most people fear spiders without good reason Spiders are allies of humans in our battle with insects American tarantulas rarely bite, and bite is not dangerous Species of black widow spiders are dangerous Venom is neurotoxic Brown recluse spider Hemolytic venom that destroys tissue around the bite Some Australian and South American spiders are the most dangerous and aggressive 19-41
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Order Scorpionida: Scorpions More common in tropical and subtropical zones but occur in temperate areas Approximately 1,400 species worldwide Nocturnal and feed largely on insects and spiders Sand-dwellers locate prey by detecting surface waves with leg sensillae Appendages attached to cephalothorax Pair of medial eyes and 2 5 lateral eyes Preabdomen has 7 segments Postabdomen has long, slender tail of five 19-44 Subphylum Chelicerata segments that ends in a stinging apparatus
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Subphylum Chelicerata Under the abdomen are comblike pectines Explore the ground and aid in sex recognition Stinger on last segment has venom that varies from mildly painful to dangerous Ovoviviparous or viviparous and produce from 6 to 90 young Perform complex mating dances In some species the male stings the female on pedipalp or on edge of cephalothorax 19-46
Subphylum Chelicerata Order Opiliones: Harvestmen Harvestmen or daddy longlegs Common, particularly in tropical regions Approximately 5,000 species worldwide Unlike spiders, abdomen and cephalothorax join broadly without a narrow pedicel Can lose most of their eight long legs without ill effect Chelicerae are pincerlike Mostly scavengers 19-47
Order Acari: Ticks and Mites Medically and economically the most important arachnids About 30,000 species have been described Both aquatic and terrestrial Inhabit deserts, polar areas, and hot springs Most mites are less than 1 millimeter long Ticks may range up to 2 cm Complete fusion of cephalothorax and abdomen No sign of external segmentation 19-48 Subphylum Chelicerata
Subphylum Chelicerata Mouthparts on tip of the anterior capitulum Chelicerae on each side help pierce, tear, or grip food Other mouthparts include pedipalps with a fused base, hypostome, rostrum, and tectum Adult mites and ticks possess 4 pairs of legs Transfer sperm directly or by spermatophores Egg hatches, releasing a six-legged larva Eight-legged nymphal stages follow 19-49
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Subphylum Chelicerata House dust mites Free-living and often cause allergies Spider mites One of many important agricultural pest mites that suck out plant nutrients Chiggers Larval Trombicula mites Feed on dermal tissues and cause skin irritation Hair follicle mite Demodex Harmless but other species cause mange in domestic animals 19-51
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Subphylum Chelicerata Human itch mite Causes intense itching Tick species of Ixodes Carry Lyme disease Tick species of Dermacentor Transmit Rocky Mountain spotted fever Cattle tick Transmits Texas cattle fever 19-54
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Subphylum Myriapoda Characteristics Myriopods include Chilopoda (centipedes) Diplopoda (millipedes) Pauropoda (pauropods) Symphyla (symphylans) Use trachea to transport Excretion usually by Malpighian tubules 19-57
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Class Chilopoda Natural History Centipedes Found under logs, bark and stones Carnivorous, eating earthworms, cockroaches, and other insects House centipede has 15 pairs of long legs Common in bathrooms and damp cellars Most harmless to humans Few large, tropical centipedes are dangerous Approximately 3,000 species worldwide 19-59 Subphylum Myriapoda
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Subphylum Myriapoda Characteristics Terrestrial and have flattened bodies with up to 177 segments Each segment except the one behind the head and last two Bears a pair of jointed legs, the last pair of which serves a sensory function Appendages of first body segment form poison claws Head has one pair of antennae, a pair of mandibles, and one or two pairs of maxillae Eyes on either side of the head consist of groups of ocelli 19-61
Subphylum Myriapoda Salivary glands empty into anterior end of straight digestive tract Two pairs of Malpighian tubules empty into the hind intestine Elongated heart has pair of arteries in each somite Ostia provide return flow of hemolymph Pair of spiracles in each somite allows air to diffuse through branched air tubes of the tracheae Arthropod nervous system includes a portion that serves as a visceral nervous system 19-62
Subphylum Myriapoda Reproduction Sexes separate with unpaired gonads and paired ducts Some lay eggs and others are viviparous Young resemble adults and do not undergo metamorphosis 19-63
Class Diplopoda Natural History Millipedes Less active than centipedes Walk with graceful rather than wriggling motion Most eat decayed plants but a few eat living plant tissue Most are slow moving and roll into a coil for defense Some secrete toxic or repellant fluids from special repugnatorial glands on side of body More than 10,000 species of worldwide 19-64 Subphylum Myriapoda
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Characteristics Cylindrical bodies with 25 to more than 100 segments Short thorax consists of 4 segments, each bearing one pair of legs Head has 2 clusters of simple eyes and a pair each of antennae, mandibles, and maxillae Each abdominal somite has 2 pairs of spiracles opening into air chambers and tracheal air tubes Two genital apertures located toward anterior end 19-66 Subphylum Myriapoda
Subphylum Myriapoda Reproduction Appendages of seventh segment specialized as copulatory organs After copulation, female lays eggs in a nest and guards them Larvae have only one pair of legs per segment 19-67
Life History Subphylum Myriapoda Class Pauropoda Live in moist soil, leaf litter, decaying vegetation, or under bark and debris Least well known of myriapods Characteristics Soft-bodied, small (2 mm or less) Approximately 500 species Head lacks true eyes, has branched antennae, and a pair of sense organs 19-68
Subphylum Myriapoda 12 trunks segments bear 9 pairs of legs but none on the last 2 segments A tergal plate covers each of the two segments Lack tracheae, spiracles, and circulatory system Probably most closely related to diplopods 19-69
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Subphylum Myriapoda Class Symphyla Life History and Reproduction Live in humus, leaf mold, and debris Male Scutigerella places a spermatophore at end of a stalk Female stores the sperm in special pouches Removes and smears eggs with sperm before attaching them to moss or lichen Young hatch with only 6 or 7 pairs of legs 19-71
Characteristics Small (2 10 mm) with centipede-like bodies Soft-bodied with 14 segments 12 segments bear legs and one bears a pair of spinnerets Antennae are long and unbranched About 160 species are known Eyeless with sensory pits at base of antennae Tracheal system connects to a pair of spiracles on the head and tracheal tubes to the anterior only 19-72 Subphylum Myriapoda
Phylogeny and Adaptive Diversification Phylogeny Relationships between subphyla are debated Taxon of Pancrustacea, which includes hexapods and crustaceans, is well-supported Phylogenies using molecular data rarely support grouping Myriapoda with Pancrustacea There is support for placement of Myriapoda as the sister taxon for Cheliceratae 19-73
Phylogeny and Adaptive Diversification Biologists assume that the ancestral arthropod had a segmented body with one pair of legs per segment Evolution caused adjacent segments to fuse and to make body regions Hox gene studies indicate that the first five segments fused to form the head tagma in all four extant subphyla In spiders, Hox gene studies indicate that the entire prosoma corresponds to the head of other arthropods 19-74
Phylogeny and Adaptive Diversification Sea spiders remain within subphylum Chelicerata because Hox gene studies have found that their head appendage arose from the region of the head that corresponds to the second segment Genetic studies have been helpful in understanding the evolution of uniramous and biramous appendages Molecular evidence repeatedly places hexapods with crustaceans even though hexapods have uniramous appendages and crustaceans have biramous appendages. 19-75
Phylogeny and Adaptive Diversification Leads to the question: Did uniramous appendage development evolve more than once? Numbers of appendages per segment is another variable character among arthropods that lends itself to more testing 19-76
Phylogeny and Adaptive Diversification Adaptive Diversification In contrast to annelids, arthropods have pronounced tagmatization by fusion of somites Those with primitive characters have appendages on each somite Derived forms are specialized Modification of exoskeleton and appendages allowed variation in feeding and movement Adaptations made possible by cuticular exoskeleton and small size fostered high diversity 19-77
Phylogeny and Adaptive Diversification Classification Subphylum Trilobita Subphylum Chelicerata Class Merostomata Class Pycnogonida Class Arachnida Subphylum Myriapoda Class Diplopoda Class Chilopoda Class Pauropoda Class Symphyla Subphylum Crustacea 19-78 Subphylum Hexapoda