Insect respiratory system

General Features

• Insects lack lungs; respiration occurs via tracheal system.
• Tracheae: Air-filled tubes delivering oxygen directly to tissues.
• Tracheoles: Fine terminal branches; site of gas exchange.
• Taenidia: Spiral thickenings in tracheae prevent collapse; absent in tracheoles.
• Spiracles: Paired lateral openings on thorax and abdomen.
• Maximum spiracles: 10 pairs (2 thoracic + 8 abdominal).
• Spiracles have valves to regulate airflow and reduce water loss.
• Haemolymph does not carry oxygen; tracheal system delivers O₂ directly.
• Respiratory pigments (like hemoglobin) are absent.

Types of Respiratory System (Based on Functional Spiracles)

Special Adaptations

• Tracheal gills: Thin extensions for aquatic respiration (e.g., dragonfly nymphs).
• Breathing tubes (siphons): e.g., Mosquito larvae, rat-tailed maggots.
• Plastron respiration: Air film trapped by body hairs for submerged insects (e.g., diving beetles).
• Cutaneous respiration: Through body wall (apneustic larvae).
• Filter chamber (Homopterans): Modifies alimentary canal for fluid feeding; removes excess water.

Facts

• Tracheae originate ectodermally.
• Tracheoles are non-taenidial and end blindly.
• Spiracle regulation prevents water loss in terrestrial insects.
• Gas exchange occurs mainly by diffusion.
• Aquatic insects may use modified tracheal system or siphons.

Insect circulatory system

• Type: Open circulatory system (unlike closed in vertebrates).
• Circulatory fluid: Haemolymph (colorless or pale green).
• Haemolymph = Plasma + Haemocytes.
• No haemoglobin, no respiratory pigments.
• Body cavity filled with haemolymph = Haemocoel.
• Haemolymph not used for oxygen transport – only nutrient and hormone distribution.
• Main pumping organ: Dorsal vessel.
• Dorsal vessel divided into:
  1. Posterior heart (pulsatile, with ostia)
  2. Anterior aorta (non-pulsatile).
• Heart location: Dorsal midline of abdomen.
• Number of chambers: Usually 13 (varies with species).
• Ostia: Paired lateral openings for haemolymph inflow.
• Accessory pulsatile organs: Help blood circulation in appendages (antennae, wings, legs).
• Circulation: Haemolymph pumped forward → aorta → head → sinuses → back to heart via pericardial sinus.
• Diaphragms:
  1. Dorsal diaphragm: Around heart.
  2. Ventral diaphragm: Below gut; assist in circulation.
• Haemocytes functions: Phagocytosis, encapsulation, wound healing, clotting, and nutrient transport.
• Main sugar in haemolymph: Trehalose (a disaccharide).
• Haemolymph pH: Slightly alkaline.
• Heart contraction rate: Temperature dependent.
• Heart enclosed in: Pericardial sinus.
• Circulatory system origin: Mesodermal.