20 Arthropods - Chelicerata

Myriapoda and Chelicerata

  • Myriapoda

    • Includes various groups such as:
    • Diplopoda (millipedes)
    • Chilopoda (centipedes)
    • Pauropoda
    • Symphyla

  • Chelicerata

    • Includes:
    • Arachnida (spiders, mites, ticks, scorpions, etc.)
    • Eurypterida (sea scorpions)
    • Xiphosura (horseshoe crabs)
    • Pycnogonida (sea spiders)

Taxonomic Overview of Arachnida

  1. Subphyla of Arachnida
    • Palpigradi: Micro whipscorpions
    • Araneae: Spiders
    • Amblypygi: Tailless whipscorpions
    • Thelyphonida: Whiptailed scorpions (vinegaroons, uropygids)
    • Schizomida: Short-tailed whipscorpions
    • Ricinulei: Hooded tickspiders
    • Acari: Mites and ticks
    • Opiliones: Harvestmen, daddy-longlegs
    • Scorpionida: Scorpions and their extinct relatives
    • Pseudoscorpiones: Pseudoscorpions, bookscorpions
    • Solifugae: Wind scorpions, camel spiders

Physical Characteristics of Arachnids

  • Body Structure
    • Composed of two body segments:
    • Prosoma: Fused head and thorax
    • Opisthosoma: Abdomen
    • Legs: Eight walking legs
    • Antennae: Absent
    • Pedipalps: Used as sensory organs or for feeding
    • Chelicerae: Used to grasp or chew food

Respiratory System in Arachnids

  • Respiratory Structures
    • Book lungs: Spiracles open to respiratory areas in the abdomen.
    • Tracheal system: Spiracles open to tubes that extend throughout the body.

Specific Arachnid Orders and their Characteristics

1.1 Araneae (Spiders)

  • Approximately 50,000 species.
  • Most are terrestrial; some inhabit freshwater environments.
  • Key anatomical features include:
    • Patella, Coxa, Urticating hairs, Spinnerets, Tibia, Eyes, Femur, Metatarsus, Tarsus, Leg IV, Carapace, Leg III, Fovea, Leg II, Prosoma, Trochanter, Leg I, Chelicerae, and Tarsal claws.
  • Example: Jumping spider (Youtube link)

1.2 Amblypygi (Whip Spiders)

  • Approximately 150 species. Predatory and terrestrial.
  • Use whip-like front legs to locate prey, which is seized with pedipalps.
  • Example of predation (Youtube link).

1.3 Thelyphonida (Vinegaroons, Whip Scorpions)

  • Approximately 100 species; similar ecology as Amblypygi, terrestrial and predatory.
  • Reproduction: Males produce spermatophore; females lay eggs and carry them until the first molt.
  • Example: Vinegaroon (Youtube link).

1.4 Acari (Mites and Ticks)

  • Approximately 40,000 species. Includes:
    • Mites: Herbivores, parasites, predatory; size < 1 mm, found in terrestrial, marine, and freshwater habitats.
    • Ticks: Parasites primarily found in terrestrial environments.
  • Disease transmission via ticks:
    • Anaplasmosis: Transmitted mainly by blacklegged tick (Ixodes scapularis).
    • Babesiosis: Caused by Babesia microti, transmitted by blacklegged ticks.
    • Lyme disease: Caused by Borrelia burgdorferi, also transmitted by Ixodes scapularis.
    • Various other diseases identified and transmitted by specific tick species (details of transmission mechanisms included).

1.5 Opiliones (Daddy Long-legs, Harvestmen)

  • Approximately 7,000 species. They are terrestrial and omnivorous.
  • Example of Opiliones group (Youtube link).

1.6 Scorpionida (Scorpions)

  • Approximately 2,000 species. Terrestrial predators.
  • Anatomical details:
    • Four pairs of walking legs, chelicerae (mouthparts), pedipalps (pincers), opisthosoma (abdomen), telson (venom bulb), and various segments.
  • Example: Nature footage of giant scorpion in Thailand (Youtube link).

1.7 Pseudoscorpiones

  • Approximately 3,000 species, most measure less than 1 cm.
  • They are terrestrial and predatory on other arthropods.

1.8 Solifugae (Camel Spiders)

  • Approximately 1,000 species, they are terrestrial and predatory.

Eurypterida

  • Eurypterida: Extinct group of arachnids.
  • Approximately 250 species, known to inhabit marine to freshwater environments.

Xiphosura (Horseshoe Crabs)

  • Four extant species, including one species in the US and three in Asia, closely resembling ancient fossils.
  • Characteristics:
    • Marine, feeding on benthic invertebrates,
    • Come into shallow waters to mate; their eggs serve as important food sources for other species.
  • Economic Importance: Blood is used in LAL (Limulus amebocyte lysate) test for detecting bacterial endotoxins; methods of harvesting mentioned.

Pycnogonida (Sea Spiders)

  • Approximately 1,300 species, primarily marine.
  • Most feed on sessile invertebrates, exhibiting a unique morphology including:
    • Terminal claws, tarsus, tibia, femur, patella, features such as a proboscis and oviger.
  • Reproductive Strategy: Males carry eggs.