Arthropods

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Arthropods

• Chapter 18

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Phylum Arthropoda

• Two out of every three known species of animals
  are arthropods.
• Members of the phylum Arthropoda are found in
  nearly all habitats of the biosphere.

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Phylum Arthropoda

• Arthropods are
• Multicellular
• Bilaterally symmetrical
• Triploblastic
• Have a true coelom (protostomes)
• Segmented

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General Characteristics of Arthropods

• The diversity and success of arthropods are
  largely related to their segmentation, hard
  exoskeleton (made of chitin), and jointed
  appendages.

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General Characteristics of Arthropods

• Segments have combined into functional groups
  called tagmata.
• Tagmata have specialized purposes.

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General Characteristics of Arthropods

• As arthropods evolved, the segments fused, and
  the appendages became more specialized.
• The appendages of some living arthropods are
  modified for many different functions.

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General Characteristics of Arthropods

• Arthropods have an open circulatory system in
  which fluid called hemolymph is circulated into
  the spaces surrounding the tissues and organs.
• A variety of organs specialized for gas exchange
  have evolved in arthropods.

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A Versatile Exoskeleton

• The exoskeleton of arthropods is very protective,
  but still flexible.
• The exoskeleton is made of chitin.
• Prevents desiccation.
• Provides places for muscle attachment.
• Does not allow for growth, the outer covering
  must be molted ecdysis.

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More Efficient Locomotion

• Usually, each segment bears a pair of jointed
  appendages.
• The appendages have sensory hairs and may be
  modified for sensory functions, food handling, or
  walking swimming.

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Air Piped Directly to Cells

• Most terrestrial arthropods have an efficient
  tracheal system of air tubes, which delivers
  oxygen directly to the tissues and cells.
• Limits body size.
• Aquatic arthropods breathe using internal or
  external gills.

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Highly Developed Sensory Organs

• Arthropods have a variety of sensory organs.

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Complex Behavior Patterns

• Arthropods show complex behavior patterns.
• Mostly innate behaviors.
• Some learned.

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Metamorphosis

• Intraspecific competition (between members of one
  species) is reduced because of metamorphosis.
• Larval forms may be quite different from adults.

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Subphylum Trilobita

• Early arthropods, such as trilobites showed
  little variation from segment to segment.

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Subphylum Trilobita

• Trilobites arose during the Cambrian maybe
  earlier and lasted for 300 million years.

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Subphylum Trilobita

• Trilobites had a trilobed shape.
• Three tagmata
• Head (cephalon) with a mouth, compound eyes,
  antennae, and 4 pairs of leglike appendages.
• Trunk with a variable number of segments each
  with a pair of biramous appendages.
• Pygidium segments fused into a plate.

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Subphylum Trilobita

• Most could roll up like pill bugs.
• Probably benthic scavengers.
• Many (especially later species) had large,
  complex, many-faceted eyes.

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Subphylum Chelicerata

• Chelicerate arthropods include eurypterids,
  horseshoe crabs, spiders, ticks, mites,
  scorpions, sea spiders.

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Subphylum Chelicerata

• They have 6 pairs of cephalothoracic appendages
• Chelicerae (mouthparts)
• Pedipalps
• 4 pairs of walking legs

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Class Merostomata

• Class Merostomata includes the eurypterids and
  horseshoe crabs.
• Eurypterids were giant water scorpions up to 3 m
  in length.
• Cambrian through Permian.
• Predators, some with large crushing claws.

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Class Merostomata

• Three genera of horseshoe crabs live today.
• Limulus, found in North America, has existed on
  earth almost unchanged since the Triassic period.

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Class Merostomata

• Horseshoe crabs have an unsegmented carapace
  (hard dorsal shield), a broad abdomen, and a long
  telson (tail piece).
• Cephalothorax
• Chelicerae
• Pedipalps
• 4 pairs walking legs
• Abdomen
• 6 pairs of thin appendages
• Book gills found on 5.

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Class Merostomata

• Horseshoe crabs have simple and compound eyes.
• Feed at night on worms and small molluscs.
• Come to shore in large numbers to mate at high
  tide.
• Trilobite larvae resemble trilobites.

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Class Pycnogonida

• Sea spiders, class Pycnogonida, have small, thin
  bodies and usually 4 pairs of walking legs.
• Found in all oceans, most common in polar seas.
• Some have chelicerae and pedipalps.

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Class Arachnida

• Class Arachnida includes spiders, scorpions,
  mites, and ticks.

50 µm
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Class Arachnida

• Two tagmata
• Cephalothorax
• Chelicerae
• Pedipalps
• 4 pairs walking legs
• Abdomen

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Class Arachnida

• Most spiders order Araneae have 8 simple eyes
  that can detect light and motion.
• Some hunting jumping spiders may form images.

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Class Arachnida

• Many spin a web used for prey capture.
• Some chase catch prey.

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Class Arachnida

• Scorpions order Scorpiones feed on insects
  spiders which they seize with their pedipalps.
• The last segment contains a bulbous base and a
  curved barb that injects venom.
• Scorpions are viviparous females brood young
  within their reproductive tract.

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Class Arachnida

• Harvestmen order Opiliones differ from
  spiders in that the abdomen and cephalothorax are
  broadly joined rather than constricted.
• Only two eyes
• Abdomen shows segmentation
• Long legs end in tiny claws.

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Class Arachnida

• Mites and ticks order Acari have a fused
  cephalothorax abdomen.
• Mites are tiny 1mm or less.
• Some feed on plant juices and can be major pests.
• Several species of ticks carry diseases such as
  Lyme disease.

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Phylogeny

• Annelids and arthropods share a number of shared
  derived characters.
• Recent molecular evidence has shown them to
  belong to separate superphyla.
• Segmentation arose independently.
• Arthropods did not descend from annelids.