Lecture 14: Arthropods

Arthropods are the most numerous of all animal phyla.
Defined by distinct body regions and segmented bodies.
Possess jointed appendages, which provide versatility for various tasks.
Occupy all levels of the food chain with diverse ecological roles:
  - Herbivorous
  - Parasites
  - Carnivores
Found in various habitats including soil and water.
Size varies from extremely small to relatively large.

Classified as deizoans and also as protostomes.
Triploblastic organisms with coeloms, which can be reduced.
Exhibit metamerization:
  - Repeating embryonic segments called metamers.
  - Through taigmatization, these segments fuse to form distinct body parts.
  - Example of tagma formations include head, thorax, and abdomen.
Covered by a chitinous exoskeleton for:
  - Protection against dehydration
  - Structural support
  - Segmentation that aids in function.
Exhibit paired and jointed appendages for locomotive and sensory functions.
Ektizoans that undergo molting throughout their life cycles.

Nervous system:
- Ventralized similar to annelids.
- Centralized ganglion functioning as a primitive brain.
Complete digestive system that aids both muscular movement and nutrient processing.
Circulatory system:
- Open circulatory system that opens into a hemocele.
- Hemolymph bathes and nourishes tissues.
Metamorphosis:
- Substantial body form changes between larval and adult stages.
- Reduces competition for resources between life stages.

Monophyletic group subdivided into four extant subphyla and one extinct group:
  1. f
  2. Crustacea
  3. Hexapoda
  4. Myriapoda
  5. Extinct group: Trilobitomorpha (similar in structure to horseshoe crabs)
Transition explained:
- Chelicerata used to be classified separately but now groups with arachnids into broader taxa.

Chelicerata: (spiders, mites, tick, horseshoe crabs, sea spiders)
- Includes horseshoe crabs (Xiphosura), arachnids (spiders, scorpions).
- Pycnogonida (sea spiders).
Crustacea:
- Characterized by water-dwelling species (crabs, lobsters, shrimp, copepods).
- Possess distinctive mandibles and typically biramous appendages.
Hexapoda:
- Known for distinct body divisions (head, thorax, abdomen).
- Includes insects with specialized appendages (five pairs of head appendages, three pairs of thoracic appendages).
Myriapoda:
- Consists of centipedes (Chilopoda) and millipedes (Diplopoda).
- Defined by number of legs per segment.

Composed of two main layers:
- Epicuticle (non-living)
- Pericuticle (initially soft, becomes hardened).
Hypodermis (living part) secretes the materials for the exoskeleton.
The exoskeleton:
- Provides structural integrity and muscle attachment.
- Prevents dehydration and facilitates various movements related to locomotion, feeding, and sensory perception.

Chelacerata: A primary taxon of interest.
- Commonly associated with arachnids (e.g., spiders, scorpions).
Characteristics:
  - Specialization of appendages around mouth regions.
  - Present cephalothorax, which combines head and thorax regions.
  - Comprised of 10 appendages, divided into 5 sets.

Cephalothorax:
- Structure that merges the head and thorax.
- Horseshoe crabs are a notable example.
Abdomen:
- Distinct region located posterior to the cephalothorax.
Specialized Appendages:
- Structures around the mouth heavily modified for feeding (pincers).
- Appendages differ from typical leg-like structures.

Distinction from Crustacea:
- Horseshoe crabs lack the necessary mouth structure to classify as crustaceans.
- The appendage number and specialization differ, especially concerning feeding mechanisms.
Trilobitomorpha:
- Extinct group resembling modern horseshoe crabs.
- Contains remnants of ancient arthropods (trilobites).

Included Groups:
- Scorpions, spiders, and mites (Acarina).
Body Segments:
- Prosoma: includes head and thorax.
- Opisthosoma: abdominal region.
Appendages:
- Four to six appendages typically, with variations (such as modified legs and pincer-like extensions).
Venomous Adaptations:
- Unique adaptations in spiders such as venom glands for subduing prey.
- Not all spiders are venomous to humans; most lack mouth size capable of human bites.

Mites (Acarina):
  - Bodies fused into a structure called carapace (fusion of head, thorax, and abdomen).
  - Most are microscopic (1 mm or less)
  - Types of mites include ectoparasites that dwell on animal surfaces (e.g., skin mites).
Disease Transmission:
- Some mites can transmit diseases (e.g., Rocky Mountain Spotted Fever).

Feeding Behavior:
  - Consume dead skin and some species are strictly vegetarian.
  - Differentiation between ectoparasitic behavior leading to allergic reactions and benign skin-dwelling mites.
  - Some species are linked to diseases including lyme disease.

Sea Spiders (Pycnogonida):
- Misleadingly named; distinct from true spiders.
- Characterized by a modified body plan and unique appendages.
Tardigrades (Water Bears):
- Found in marine and freshwater environments, including lichens and mosses.
- Notable for their cryptobiosis ability, allowing survival in extreme conditions (including space).

Setae:
- Small extensions from the body surface for sensory input (movement detection, temperature changes).
Eye Structure:
- Development of compound eyes; evolution does not mirror cephalopod eye development.
- Complex sensory abilities allow interaction with environments.

Common Ancestry:
- All arthropods share a common ancestor characterized by metamerization (segmental arrangement of body).
Key Shared Characteristics:
- Sheddable outer cuticle (exoskeleton); typically made from chitin.
- Development of compound eyes and specialized mouth appendages (chelicerae).
Significant Subgroups:
  - Includes Myriapoda (centipedes and millipedes).
    - Centipedes have one set of appendages per body segment, whereas millipedes have two.
    - Millipedes are non-venomous, while centipedes can bite.
  - Crustaceans (crabs, shrimp) feature prominently as well.