Introduction to Phylum Arthropoda

Phylum Arthropoda: Detailed Overview and Classification

• Phylum Arthropoda is the largest and most diverse group of animals in the kingdom.

• They are classified within the group Ecdysozoa, which includes protostomal animals that undergo ecdesis—the process of molting or shedding a cuticle or exoskeleton to accommodate growth.

• Biological Diversity: Estimates of total arthropod species range from $2,000,000$ to $10,000,000$. Many species, particularly in unexplored regions like the Amazon, remain unidentified.

• Distribution: Arthropods inhabit virtually every environment on Earth, including marine, freshwater, and terrestrial habitats.

General Body Structure and Physiology

• Segmentation: Arthropods have segmented bodies. In some groups, segments are fused into specialized regions:

  • Head, Thorax, and Abdomen (common in insects).

  • Cephalothorax (a fused head and thorax) and Abdomen (common in spiders, crayfish, and lobsters).

• Appendages: Extensions of the main body modified for specific tasks, such as:

  • Locomotion (walking, swimming).

  • Feeding (capturing prey, chewing).

  • Reproduction.

  • Defense.

  • Sensory reception.

• Exoskeleton: Made of chitin, a polysaccharide chemically similar to starch or glycogen. In some species, such as crustaceans, the chitin matrix is hardened with calcium carbonate ($CaCO_3$).

  • Functions: Provides physical protection, acts as a moisture barrier, and serves as an attachment site for muscles.

• Circulatory System: Arthropods possess an open circulatory system.

• Coelom: They are true coelomates, meaning their organs are suspended within a gut cavity defined by the mesoderm.

Subphylum Chelicerata

• This group is named for their specialized mouthparts called chelicerae, which can be modified as fangs or pincers.

• Body Plan: Characterized by a cephalothorax and an abdomen. They lack antennae.

• Horseshoe Crabs (Genus Limulus):

  • Known as "ancient chelicerates," they have existed for approximately $500,000,000$ years.

  • Anatomy includes five pairs of walking legs. The first pair of walking legs are called pedipalps.

  • Chilaria: A pair of degenerated, small legs located posterior to the walking legs.

  • Respiration: Occurs through book gills—accordion-like structures located under protective plates on the underside.

  • Medical Significance: Horseshoe crab blood contains amoebocytes (similar to human macrophages). These cells produce a substance called coagulogen that clots in the presence of bacterial toxins.

  • Scientists harvest Limulus Amoebocyte Lysate (LAL) to test medical equipment like hypodermic needles and catheters for contamination.

  • Harvesting involves bleeding up to $1/3$ of the crab's blood; the mortality rate of this process is estimated between $7.5\%$ and $10\%$.

Arachnida

• Includes approximately $85,000$ species, primarily terrestrial.

• Scorpions: Considered the most ancient arachnids and the first terrestrial arthropods.

  • Note on the genus Centuroides: Found in Arizona and New Mexico, it possesses neurotoxic venom that can cause convulsions and cardiac or respiratory arrest.

• Spiders: Approximately $30,000$ predatory species.

  • Silk Production: Silk is composed of polypeptide chains containing amino acids such as glycine ($Gly$), alanine ($Ala$), and serine ($Ser$).

  • The substance is stored as a liquid and hardens upon contact with oxygen through molecular cross-linkages.

  • Spider silk is weight-for-weight as strong as nylon.

• Other Arachnids: Includes mites, ticks, and harvestmen (daddy longlegs).

Subphylum Crustacea

• Primarily marine and freshwater organisms.

• Characteristics: Possess two pairs of antennae and "biramous" (two-branched) appendages.

• Examples: Crabs, lobsters, shrimp, and isopods (the only terrestrial crustaceans).

• Crayfish Anatomy:

  • External: Rostrum (pointed "forehead" area), antennae, shorter antennules, and five pairs of walking legs.

  • Chelipeds: The first pair of walking legs, modified into large claws for defense and prey capture.

  • Swimmerets: Small appendages on the abdomen. In males, the first pair are modified into rigid "copulatory swimmerets."

  • Tails: Composed of a central telson and lateral uropods.

  • Internal Structures:

  • Heart: Located in the pericardial sac just under the dorsal shell.

  • Digestive System: Includes a short esophagus, a stomach, and a mushy digestive gland that secretes enzymes.

  • Excretory System: Antennal glands (also called green glands) located near the antennae.

  • Nervous System: Features a tiny brain (a convergence of nerves between the eyes) and a ventral nerve cord with hubs of neurons.

Myriapoda: Centipedes and Millipedes

• Centipedes (Class Chilopoda):

  • Approximately $3,000$ species.

  • Predators with poison fangs.

  • Dorsoventrally flattened body with one pair of legs per body segment.

• Millipedes (Class Diplopoda):

  • Detritivores (eat dead leaf matter).

  • Rounded body with two pairs of legs per body segment.

Hexapoda: Insects and Springtails

• The largest class of organisms on the planet.

• Body Plan: Divided into three regions: Head, Thorax, and Abdomen. They possess six legs ($3$ pairs).

• Respiration: A system of tubes called respiratory trachea that branch into tracheoles. Air enters through external openings on the abdomen called spiracles.

• Grasshopper Anatomy (Romalea):

  • Thorax Segments: Divided into the prothorax (covered by a saddle-like pronotum), mesothorax, and metathorax.

  • Sensory: Compound eyes and three simple eyes (ocelli/osteoma).

  • Tympanum: A circular membrane on the thorax that functions as an eardrum by vibrating in response to sound.

  • Mouthparts: Includes the labrum (upper plate), mandibles (which move side-to-side), maxilla, and maxillary palps.

  • Internal Digestion: Includes the pharynx, esophagus, crop (storage), stomach, and gastric caeca (finger-like glands that secrete enzymes).

  • Excretion: Malpighian tubules located between the stomach and intestine.

  • Reproduction: Females possess an ovipositor at the end of the abdomen for laying eggs. Internally, female ovaries are tube-like and distinct, while male testes appear mushy.

Questions & Discussion

Interviewer/Student: Have you ever seen a horseshoe crab alive? Speaker: Yes, they are very cool. They can get really big. They look scary with the spikes, but they are totally harmless. If you go to Tybee Island off the coast of Savannah, Georgia, you can swim with them. They get up to a very large size.

Interviewer/Student: Is there anything about insects, like a caterpillar turning into a butterfly? Speaker: All arthropods go through different phases in their cycle. Between these phases, they shed their exoskeleton specifically to make room for the next level of their development.

Interviewer/Student: (Discussion on spiders) Speaker: Black widows are the most common poisonous spiders in our specific area. Tarantulas are also shown in the reference materials.