Phylum Arthropoda: Subphylum Hexapoda

Overview

• Hexapoda is a diverse subphylum characterized by six legs and uniramus appendages.
• Distinct body plan: head, thorax, and abdomen.
• Absence of appendages on abdomen due to lack of the expression of the dll gene.

Classes of Hexapods

1. Class Entognatha
   • Includes springtails.
   • Mouthparts are contained within the head capsule.
2. Class Insecta
   • Includes ants, flies, moths, beetles, etc.
   • Ectognathus mouthparts located outside the head capsule.
   • Two groups: Pterygotes (winged) and Aterygotes (wingless).
   • Most diverse and abundant group of arthropods.
   • More species of insects than all other animals combined.

Adaptations and Characteristics

• Insects' success attributed to flying ability and rapid adaptation.
• Insects occupy nearly every habitat except oceans.
• Cuticle: lightweight, strong, waterproof.
• Protects from dehydration, allowing terrestrial dominance.

Body Structure

• Head: Mouthparts, mandibles, maxilla, labial palps, and antennas.
• Thorax: Three parts - prothorax, mesothorax, metathorax.
  • Wings attached to meso/metathorax.
• Abdomen: 9-11 segments, sometimes with cersei or ovipositor.

Locomotion

• Methods: walk, run, jump, swim, burrow, fly.
• Tripod walking method for stability.
• Wings: two pairs (forewings and hindwings), variations exist.
  • Wings composed of cuticle, with veins for strength.

Flight

• Direct and indirect muscles facilitate flight.
• Direct muscles position wings, indirect muscles flap wings.
• Neuronal control: synchronous (one nerve impulse = one stroke) or asynchronous (one impulse = multiple strokes).

Feeding and Digestion

• Diverse ectognathous mouthparts suited to different diets.
• Specialized for specific food sources, limiting competition.
• Complete gut: foregut, midgut (absorption), hindgut (ion/water reabsorption).

Circulation and Respiration

• Open circulatory system, hemolymph distributes nutrients.
• Tracheal system for respiration: spiracles, trachea, tracheoles.
  • More efficient gas exchange than circulatory system.

Excretion and Osmoregulation

• Malpighian tubules for waste removal and osmoregulation.
• Primary waste: uric acid.
• Process of ion and water reabsorption.

Reproduction

• Mostly dioecious, internal fertilization.
• Spermatophores for sperm transfer.
• Various reproductive strategies: viviparous or oviparous.

Growth and Development

• Types: ametabolism (no metamorphosis) vs. metamorphosis (hemi/holo metabolism).
• Metamorphosis stages: egg, larva, pupa, adult.
  • Hormonal control by juvenile hormone.

Defense Mechanisms

• Diapause: dormancy during unfavorable conditions.
• Physical defenses: exoskeleton, chemical defenses, mimicry.
• Behavior: flight, running, hiding.

Ecological and Economic Importance

• Pollinators, pest control, decomposers.
• Products: honey, wax, silk.
• Pests: crop destruction, disease vectors, property damage.

Review Assignment

• Read textbook Chapter 21 (Intro, 21.1-21.4).
• Watch suggested videos for additional understanding of concepts.

Next lecture will cover Deuterostomes: Echinoderms.