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Arthropods

Arthropods. Chapter 18. I. Phylum Arthropoda. ¾ of all known animal species; abundance and wide ecological distribution makes them most diverse group Dates from late Precambrian

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Arthropods

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  1. Arthropods Chapter 18

  2. I. Phylum Arthropoda • ¾ of all known animal species; abundance and wide ecological distribution makes them most diverse group • Dates from late Precambrian • Distinct metameres: linear somites with jointed appendages; somites may be fused or combined into specialized groups called tagmata; appendages specialized for division of labor • Size ranges from 0.1 mm to 4 m • All modes of feeding are used but most herbivorous

  3. II. Body Plan A. Exoskeleton • Cuticle protective but jointed for mobility; inner thick procuticle and outer thin epicuticle • Consists of chitin bound with protein making a lightweight, flexible barrier that protects against dehydration • Laminated and tanned making it harder • Thins at joints to allow movement • Infolds to line foregut, hindgut, and trachea • Ecdydis, molting, sheds outer covering and growing new, larger one; exocuticle secreted by procuticle prior to molt and endocuticle secreted afterwards. • Arthropods molt 4-7 times, with weight of exoskeleton the ultimate limit of size of animal

  4. B. Segmentation & Appendages • Each somite bears pair of segmented legs • Segments and appendages modified for various adaptive functions such as food handling, sensing, walking, or swimming • Limb segments are hollow levers with internal striated muscles

  5. C. Respiration • Tracheal system efficiently delivers oxygen to cells in terrestrial arthropods • Gills are used by aquatic arthropods

  6. D. Sense Organs • Eyes vary from simple ocelli to a compound mosaic eye • Other senses are used for touch, smell, hearing, balancing, and chemical reception

  7. E. Behavior • Surpass most other invertebrates in complex and organized activities • Most behavior innate but some is learned

  8. F. Metamorphoses • Most undergo metamorphoses that result in different larval and adult forms • Larval and adult forms occupy different habitats and eat different foods to avoid competing with each other

  9. III. Subphylum Trilobita • Arose before Cambrian, flourished, died out 200 mya • Trilobed body shape; head, thorax, pygidium • Head fused segments; bore compound eyes, antennae, mouth, and jointed appendages • Gills • Bottom dwellers; probably scavengers • 2- 67 cm long; could roll up like pill bugs

  10. Subphylum Chelicerata A. General Features • Six pairs of appendages: chelicerae (most anterior on head), pedipalps (2nd pair), and 4 pairs of legs • No mandibles or antenna • Most suck liquid from prey

  11. B. Class Merostomata • Horseshoe crabs; unchanged since Triassic • 5 species • Unsegmented carapace covers body; 2 compound eyes and 2 simple eyes • Cephalothorax bears 1 pair chelicerae and 5 pairs of walking legs • Abdomen bears book gills on appendages in median line • Larvae resemble trilobites

  12. C. Class Pycnogonida • Sea spiders • Many have chelicerae and palps; 5-6 pairs of legs • Proboscis sucks fluids from cnidarians and other soft-bodied marine organisms • Live in all oceans but more common in polar

  13. Class Arachnida1. General Features • Spiders, scorpions, daddy longlegs, ticks, and mites; over 70,000 species • Live mainly in warm dry regions • Body consists of cephalothorax (chelicerae, pedipalps, 4 pairs of walking legs) and abdomen • Predaceous with claws, fangs, poison glands, or stingers • Sucking mouth parts ingest fluids and soft tissues • Most harmless and control undesirable insects

  14. Order Araneae: Spidersa. Feeding • Pair chelicerae with terminal fangs • Pedipalps help handle food • All spiders predaceous, feeding mainly on insects which are killed with poison and fangs • Injected venom liquefies and digests the tissues; sucked into stomach

  15. b. Respiration • Book lungs or tracheae used • Book lungs unique to spiders; parallel air pockets extend into blood-filled chamber; air enters chamber through lit in body wall • Tracheae are tubes that carry air from the inside directly into the tissues

  16. c. Excretion • Malpighian tubules are used for excretion • Potassium, other solutes, and wastes are secreted into tubules • Rectal glands reabsorb the potassium and water, leaving wastes and uric acid for excretion; this conserves water allowing spiders to live in dry environments

  17. d. Sensory Systems • Most have 8 simple eyes, each with a lens, optic rods, and a retina; used to detect movement and may form images • Sensory setae detect air currents, web vibrations, and other stimuli

  18. e. Web-spinning • Spinning silk critical ability • 2-3 pairs of spinnerets contain tubes that go to silk glands • A liquid protein hardens as it is extruded; the silk is very strong and will stretch considerably before breaking • Silk is used for webs, lining burrows, forming egg sacs, and wrapping prey

  19. f. Reproduction • Male stores sperm in pedipalps before mating • A courtship ritual is required by female • Male inserts pedipalps into female genital openings • Eggs develop in a cocoon in the web or may be carried by the female • Young hatch in 2 weeks

  20. g. Harmfulness • Spiders consume undesirable insects • American tarantulas rarely bite and bite is not dangerous • Some black widow spiders are venomous to humans, being neurotoxic • Brown recluse spider has hemolytic venom that destroys tissue around bite • Some Australian and South American spiders are the most dangerous and aggressive

  21. Order Scorpionida: Scorpionsa. Features • Most common in tropical and subtropical areas • Nocturnal and feed largely on insects or spiders • Sand scorpions detect prey by feeling surface waves with leg sensillae • Cephalothorax has appendages, medial eyes, and 2-5 lateral eyes • Preabdomen has 7 segments • Postabdomen has tail that ends in a stinger • Stinger has venom that varies from mildly painful to dangerous

  22. b. Reproduction • Comblike pectines under abdomen are used in sex recognition • During mating dance, male guides female over spermatophore • Ovoviviparous or viviporous producing 6-90 young

  23. 4. Order Opiliones:Harvestmen • Daddy long legs • Common in tropical regions • Cephalothorax and abdomen join broadly without any narrowing as in spiders • Chelicerae are pincer-like and used for scavenging

  24. Order Acari: Ticks and Mitesa. Features • 30,000 species • All environments • Fusion of cephalothorax and abdomen with no external segmentation • Mouth on end of tube • Chelicerae pierce, tear, or grip food • pedipalps, and 4 pairs of legs • Hatchlings are 6-legged larvae, followed by 8-legged nymphal stage

  25. b. Relationship with Humans • House dust mites are free-living and often cause allergies • Spider mites suck out plant nutrients and is common agricultural pest • Chiggers are larval mites that feed on skin and cause rashes • Hair follicle mite is harmless but others can cause mange in domestic animals • Human itch mite causes intense itching • Ticks may transmit Lyme disease, Rocky Mountain spotted fever, or Texas cattle fever

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