Lecture 11 - Arthropoda

• Members of the phylum Arthropoda are found in nearly all habitats of the biosphere.
• The diversity and success of arthropods are largely related to their segmentation, hard exoskeleton (made of chitin), and jointed appendages. 
• Segments have combined into functional groups called tagmata. 
• Arthropods have an open circulatory system in which fluid called hemolymph is circulated into the spaces surrounding the tissues and organs. 
• The exoskeleton of arthropods is very protective, but still flexible. 
• The exoskeleton is made of chitin. 
• Does not allow for growth, the outer covering must be molted – ecdysis. 
• The appendages have sensory hairs and may be modified for sensory functions, food handling, or walking & swimming. 
• 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 
• Intraspecific competition (between members of one species) is reduced because of metamorphosis. 
• Clade Panarthropoda, Phylum Arthropoda

  • Divided into subphyla based on relationships between subgroups based on molecular data.
• Phylum Arthropoda

  1. Subphylum Myriapoda
  2. Subphylum Hexapoda
  3. Subphylum Chelicerata
  4. Subphylum Crustacea
• 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.
• Formerly, insects and myriapods were placed together in uniramia.
• Subphylum Trilobita

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

  Three tagmata:

  1. Head
  2. Trunk
  3. Pygidium
• 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.

  • One of the branches of biramous appendage was fringed and may have been a gill.
• Pygidium – segments fused into a plate.
• Subphylum Chelicerata

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

  They have 6 pairs of cephalothoracic appendages:

  • Chelicerae (mouthparts)
  • Pedipalps
  • 4 pairs of walking legs
  • Lack mandibles and antennae.
• What are the three classes under Subphylum Chelicerata?

  Merostomata , Pycnogonida, Arachnida
• Class Merostomata - includes the eurypterids and horseshoe crabs.
• Class Merostomata

  Eurypterids were giant water scorpions up to 3m in length.
• Class Merostomata

  Limulus, found in North America, has existed on

  earth almost unchanged since the Triassic period.
• Class Merostomata

  Horseshoe crabs have an unsegmented carapace (hard dorsal shield), a broad abdomen, and a long telson (tail piece).
• Class Pycnogonida - Sea spiders, have small, thin bodies and usually 4 pairs of walking legs.
• Class Arachnida - includes spiders, scorpions, mites, and ticks.
• Class Arachnida

  Two tagmata:

  • Cephalothorax - Chelicerae, Pedipalps ,4 pairs walking legs
  • Abdomen
• What are the orders under Class Arachnida?

  Araneae (spiders), Scorpiones (scorpions), Opiliones (harvestmen), Acari (ticks and mites)
• Class Arachnida - Order Araneae

  • Most spiders – have 8 simple eyes that can detect light and motion.
  • All are predaceous, mostly on insects.
  • Many spin a web use for prey capture. Injected venom liquefies and digests the tissues which is sucked into spider’s stomach.
• Class Arachnida - Order Araneae

  • Two or three pairs of spinnerets contain microscopic tubes that run to silk glands.
• Class Arachnida - Order Araneae

  • In spiders and insects, Malpighian tubules serve as excretory structures.
• Class Arachnida - Order Araneae

  • Many spiders have coxal glands, modified nephridia, at the base of legs.
• Class Arachnida - Order Araneae

  • Reproduction - before mating, male stores sperm in pedipalps.
• Class Arachnida - Order Scorpiones

  Scorpions – 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 or ovoviviparous – females brood young within their reproductive tract.