Phylum Arthropoda

Introduction

The phylum Arthropoda is the largest and most diverse animal phylum, encompassing insects, crustaceans, arachnids, and more. Arthropods are characterized by their segmented bodies, jointed appendages, and a hard exoskeleton. This group is essential for understanding animal diversity and evolutionary success.

Phylum Characteristics

Key Features of Arthropods

• Bilateral Symmetry: Body plan with symmetrical left and right sides. Additional info: Most animals exhibit this, except some in other phyla.

• Triploblastic: Possess three embryonic germ layers: ectoderm, mesoderm, and endoderm.

• Coelomate: Have a true coelom (body cavity) lined with mesoderm.

• Segmented Body: Body divided into repeated segments, sometimes fused into functional units (tagmata).

• Chitinous Exoskeleton: Hard, non-living outer shell made of chitin, secreted by the epidermis. Provides protection and support but must be shed for growth (molting).

• Jointed Appendages: Legs and other appendages are jointed, allowing complex movement.

• Sexes Usually Separate: Most arthropods have distinct male and female individuals.

Basis of Arthropod Success

Adaptations Leading to Diversity

• Metameric Segmentation: Repeated body segments allow specialization and flexibility.

• Jointed Appendages: Enable efficient locomotion, feeding, and sensory functions.

• Exoskeleton: Provides protection from predators and environmental hazards, supports muscle attachment, and prevents desiccation.

• Non-living Structure: The exoskeleton does not grow with the animal and must be periodically shed (molting).

Arthropod Diversity

Major Subphyla and Examples

• Subphylum Trilobita: Extinct group, highly diverse during the Cambrian to Permian periods.

• Subphylum Chelicerata: Includes spiders, scorpions, ticks, horseshoe crabs. Characterized by chelicerae (mouthparts) and two main body segments.

• Subphylum Crustacea: Includes crabs, lobsters, shrimp, barnacles. Typically aquatic, with two or three body segments and two pairs of antennae.

• Subphylum Myriapoda: Includes centipedes and millipedes. Many body segments with one or two pairs of legs per segment.

• Subphylum Hexapoda: Insects, the most diverse group, with three body segments (head, thorax, abdomen), three pairs of legs, and usually wings.

Subphylum Chelicerata

Characteristics and Examples

• Body Plan: Two main segments (cephalothorax and abdomen).

• Appendages: Six pairs: chelicerae (feeding), pedipalps (sensory/manipulation), and four pairs of walking legs.

• Respiration: Book lungs or book gills in many species.

• Examples: Spiders, scorpions, ticks, horseshoe crabs.

Subphylum Crustacea

Characteristics and Examples

• Body Segments: Two or three main parts (cephalothorax and abdomen).

• Antennae: Two pairs usually present.

• Mouthparts: Mandibles for chewing.

• Legs: Three or more pairs, often specialized.

• Habitat: Mostly aquatic (marine and freshwater), some terrestrial forms.

• Examples: Crabs, lobsters, shrimp, barnacles, parasitic species like Ourozeuktes bopyroides.

Subphylum Myriapoda

Characteristics and Examples

• Distinct Head: With antennae and chewing mouthparts.

• Multiple Segments: Each bearing legs.

• Millipedes: Two pairs of legs per segment, herbivorous.

• Centipedes: One pair of legs per segment, carnivorous.

• Fossil Record: Includes giant Carboniferous millipedes up to 2.5 m long.

Subphylum Hexapoda (Insects)

Characteristics and Diversity

• Body Segments: Divided into head, thorax, and abdomen.

• Mouthparts: Modified for chewing, sucking, or piercing.

• Legs: Three pairs attached to the thorax.

• Wings: Usually two pairs, attached to the thorax.

• Exoskeleton: Epicuticle (outer layer) is hydrophobic and protects against chemicals and microorganisms; cuticle (inner layer) is sclerotized for rigidity.

• Habitat: Primarily terrestrial, with some aquatic forms.

• Diversity: Over 925,000 described species; estimated 5-20 million species.

Exoskeleton Structure and Function

Layers and Roles

• Epicuticle: Outer layer, composed of waxes and proteins, repels water and chemicals.

• Cuticle: Inner layer, sclerotized for strength, flexible in membranous areas.

• Functions:

  • Protective armor against physical damage and pathogens

  • Prevents dehydration

  • Provides attachment points for muscles

  • Allows for camouflage, warning coloration, and structural coloration

  • Supports sensory structures (chemo- and mechanoreceptors)

• Disadvantages: Limits growth, requires molting, increases vulnerability during molting.

Internal Anatomy

Circulatory and Respiratory Systems

• Circulatory System: Open system with hemolymph (blood) bathing internal organs.

• Respiratory System: Tracheal system delivers oxygen directly to tissues; limits maximum body size.

Insect Life Cycles and Metamorphosis

Developmental Strategies

• Amniote Egg: Adaptation for terrestrial reproduction, resistant to desiccation.

• Metamorphosis:

  • Complete Metamorphosis: Egg → larva → pupa → adult (e.g., flies, butterflies, beetles, ants, bees)

  • Incomplete Metamorphosis: Egg → nymph → adult

  • Reduces competition between life stages by occupying different habitats and food sources.

Insect Body Plan and Tagmatisation

Functional Segmentation

• Head: Feeding and sensory functions

• Thorax: Locomotion (legs and wings)

• Abdomen: Digestion and reproduction

• Tagmatisation: Fusion of segments into functional units

Advantages of Hexapody (Six Legs)

Locomotion and Stability

• Longer Stride: Faster movement

• Increased Stability: Thorax acts as a stable base

• Mechanical Advantage: Powerful legs for jumping, running, or digging

Summary Table: Major Arthropod Subphyla

Additional info:

• Arthropods are the most successful animal group in terms of species diversity and ecological range.

• High oxygen levels during the Carboniferous period allowed for the evolution of giant arthropods.

• Insect pollinators are crucial for terrestrial ecosystems and agriculture.

• Molting (ecdysis) is a vulnerable period for arthropods due to the temporary loss of their protective exoskeleton.