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CHAPTER 26 - ARTHROPODS

26-1: SPIDERS AND THEIR RELATIVES (Class ARACHNIDA)

  1. Characteristics of Arachnids (Spiders, Scorpions, Mites, and Ticks)
  • 2 Key Appendages (legs)
    1. chelicerae: Legs located at the front of the arachnid. They are used for feeding. In spiders, they are used to deliver poison.
    2. Pedipalps: These appendages are located right behind the chelicerae, and are used to capture and manipulate prey and for courtship displays & mating (feeding mechanism).
  • 2 main body regions
  1. cephalothorax: The front portion of the arachnid (the first "bubble" you see, the head region). The cephalothorax is attached to 4 pairs of legs
  2. abdomen: The stomach or gut area; the last "bubble you see, furthest away from the head.
  • 4 pairs of legs
  • Respiratory organs = "Book Lungs"
  • Excretory Organ = Malpighian Tubules

     B.   Spiders

  • 35,000 species
  • Habitat: Majority land dwellers; some freshwater
  • 1st Arthropods to colonize land
  • predators (they will eat any other animal of manageable size)

     C.   Adaptations that contribute to spiders' success on land:

  • Efficient excretory & water balancing organs (they can rid their bodies of waste while conserving water). Malpighian Tubules are the organs responsible for filtering blood in insects
  • Having the respiratory membrane inside the body allows the surface where gas exchange occurs to remain moist without losing large amounts of water to evaporation during gas exchange. Book Lungs are highly folded sacs that are located inside the body of the spider. Air reaches the book lungs via an opening from the outside of the body, or a spiracle. Book lungs moisten the oxygen in the air so that it can be dissolved and pass into the blood.
  • Poison production in chelicerae allows spiders to obtain prey on land (defense mechanism, feeding mechanism)
  • Silk production. Spiders have spinnerets, which are small, nozzle-like structures at the end of the spider's abdomen. They direct the flow of silk from silk producing glands in the abdomen. Some common functions of silk include:
    1. Safety lines when dangling from branches or dropping to the ground
    2. Line nests or burrows; Habitat
    3. Fashion cocoons for their young
    4. Wrap or ensnare prey (when an insect strikes the web and struggles, the vibrations in the web alert the spider. The spider runs to the insect, bites it, and injects it with poison & digestive enzymes from chelicerae.
  • Although spiders produce poison, only about 20 species are harmful to humans

     D.   Courtship Rituals of spiders

  • Males are smaller than females (he could be mistaken for a meal if he doesn't properly identify himself!)
  • Spiders are usually solitary animals (as opposed to social)
  • Female will sometimes kill the male after mating, wrap him up & save him for her offspring's first dinner (Black widow)

     E.   Scorpions

  • Share many adaptation spiders have that allow them to live on land, such as book lungs & efficient excretory organs
  • Spider/ Chelicerae Scorpion/ Ripping Claws
  • Spider/ Pedipalps Scorpion/ Enlarged Pinchers
  • Eat insects
  • Stinger is at tip of abdomen (p. 504)
  • Feeding mechanism: Grabs prey with pinchers, brings abdomen forward to jab stinger into prey, and tear it to pieces
  • Habitat: Warmer climates (Hide during day, hunt at night)
  • Sting is painful but not lethal to humans
  • Elaborate Mating rituals

     F.   Mites & Ticks

  • Habitat: land/ water (free living) OR on a host (parasites)
  • Feeding Mechanism: Can be scavengers or predators of other tiny creatures
  • Ticks' chelicerae are what pierce your skin!
  • Most are harmless; some can cause disease

     G.   Mites

  • More diverse than ticks
  • Parasitize all groups of animals and many plants
  • Direct/ Indirect effects on humans:
    1. Punch holes in skin, cause severe itching
    2. Carry viruses that plague food crops
    3. Cause feather loss in birds
    4. Cause decreased wool production in sheep
    5. Cause mange in dogs

     H.   Ticks

  • Blood sucking parasites
  • Sharp chelicerae specialized for slicing skin
  • Transmit disease; Lyme disease, Rocky Mountain Fever

     I.   Horseshoe crabs

  • Changed very little in 200 million years
  • Habitat: Shallow parts of the ocean
  • Feeding Mechanism: Plow through sandy bottoms of the shallow ocean, and use clawlike legs to grasp small animals on which they feed.

 

 

26-2: INSECTS, MILLIPEDES, & CENTIPEDES (Class INSECTA)

  1. Characteristics of Insects (Grasshoppers, beetles, termites, fleas, ants)
  • 3 Body regions: Head, Thorax, Abdomen
  • 3 leg pairs
  • Diverse, specialized appendages attached to head
  • Have antennae, appendages that are specialized for sensing the environment (unlike Arachnids)
  • Mouthparts vary between insects, enabling them to feed on a greater variety of food
  • Mouthparts resemble pedipalps of spiders
  • Mouthparts have evolved into highly specialized shapes for scraping, grinding, piercing, and sucking
  • Have mandibles (jaws) instead of chelicerae
  • Legs of insects have evolved into a variety of shapes
    1. Beetles' legs - walking
    2. Grasshopper's legs - jumping
    3. Lice legs - clinging (to host's hair)
  • Adaptations to live on land:
    1. Insects have evolved mechanisms for gas exchange. They have trachae, which are tubules that deliver oxygen directly to the muscle cells, where it is needed.
    2. Insects conserve water with their exoskeleton
    3. Insects have Malpighian tubules that allow for excretion of nitrogenous wastes with little water loss

     B.   Development In Insects:

  1. Direct Development - (spiders, scorpions, mites, ticks) A miniature copy of the adult form hatches from the egg.
  2. Incomplete Metamorphosis - (Grasshoppers, dragonflies, mayflies, cockroaches). Metamorphosis into adult form involves a series of gradual changes. The insect that emerges from the egg is called a nymph. A nymph is a smaller version of the adult, similar structure without wings or a mature reproductive system.
  3. Complete Metamorphosis - (flies, beetles, ants, bees, wasps, butterflies, and moths) - 90% of all insects. The immature form of the insect DOES NOT resemble the adult at all.
    1. Egg of insect hatches
    2. Larva (immature form) emerges (ex: caterpillar)
    3. Larva can't fly, so it feeds & grows until it is ready to transform into an adult
    4. When the larval stage is complete, insect enters a pupa stage; it forms a cocoon. During this stage, tissues & organs are replaced with new ones.
    5. When the pupal stage ends, a fully mature adult insect emerges (ex: butterfly)
    • Ecological significance of various life stages

  C. Ants, bees, wasps, and termites live socially

  • Highly organized societies of related individuals
  • Division of labor among society members
  • Entomology - study of insect
  1. Insects can be harmful:
  • Eat plants
  • Transmit disease to plants (boll weevils cotton) and animals (mosquitoes transmit malaria)
  • Pesticides are used to eliminate pests, but most insects are resistant to them. Pesticides are also carcinogenic/toxic to animals & humans

     E.   Insects can be beneficial:

  • Many fruits depend upon insects for pollination
  • Silk (caterpillars of silk moths)
  • Honey, beeswax (bees)
  • Food source in some parts of the world (Australian aborigines consider larvae of certain beetles a delicacy)
  • Stabilize ecosystems (population control of destructive insects) - used as a safe alternative to pesticides

     F.   Millipedes

  • "One thousand feet"
  • Herbivores (Feeding mechanism: eat decaying plant material)
  • Habitat: forest floors
  • Two pairs of legs/ segment

     G.   Centipedes

  • "One hundred feet"
  • Predators (they eat worms & arthropods…some eat frogs & lizards!)
  • Habitat: varies (desert, land)
  • One pair of legs/ segment
  • First pair of limbs has evolved into large clawlike structures used as a feeding mechanism - to grasp prey & inject venom

 

25-3: CRUSTACEANS (Class CRUSTEACEA)

  1. Characteristics of Crustaceans - Successful Aquatic Arthropods
  • Most have 5 pairs of legs (decapods)
  • 2 Body regions
  • Have antennae
  • Range in size from microscopic forms to huge lobsters
  • Habitat: Aquatic
  • Have appendages for many types of locomotion
    1. Swimming, crawling, burrowing, jumping

2. Specialized appendages for:

    1. sensing environment
    2. brooding eggs and embryos
    3. feeding
    4. gas exchange
    5. attracting mates
    6. defense
  • Have mandibles (like insects)
  • Cannot live on land because they lack key adaptations that are necessary for life on land:
    1. Crustaceans' exoskeletons are heavier and less water-tight than insects; Crustaceans face water loss problems
    2. Crustaceans' excretory organs (Green gland) are less efficient than insects (Malpighian tubules), resulting in less water retention during excretion
    3. Crustaceans' respiratory organs are gills, which collapse in air, unlike the insects' trachae and the arachnids' book lungs.

     B.   Decapods (Lobsters, shrimp, crabs, crayfish)

  • "10 feet"
  • 5 pairs of thoracic legs
  • 1st pair of legs is a large set of claws for food gathering & defense
  • large, muscular abdomen (lobster, crayfish, shrimp)
  • Small, folded abdomen (crab)

     C.   Isopods (Pill bugs & relatives)

  • "Equal feet"
  • 7 pairs of similar walking legs
  • Habitat: moist environment/ land
  • Respire through gills

     D.   Amphipods (Beach hoppers)

  • "Two different kinds of feet"
  • Habitat: Sandy coastlines (under rocks/ stones)

     E.   Barnacles (Acorn, Volcano)

  • Live inside the walls of their calcium shells, which are attached to solid objects like rocks, pilings, ships' hulls, whales

     F.   Daphnia (Water Flea)

  • Eat phytoplankton
  • Preyed upon by young fishes
  • Play a vital role in marine food web

     G.   Copepod

  • Part of the zooplankton, the heterotrophic organisms that feed on phytoplankton
  • Few are parasitic & live on the sea bottom
  • Preyed upon my small predators
  • Important link in the marine ecosystem

 

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