Quick Summary
This chapter uses the cockroach (Periplaneta americana) as the model organism to study organ systems in animals. The cockroach is a well-studied arthropod. Its digestive, circulatory, respiratory, excretory, nervous, and reproductive systems illustrate key concepts in invertebrate biology. All systems are directly tested in NEET.
Cockroach — External Features
Scientific name: Periplaneta americana
| Phylum: Arthropoda | Class: Insecta |
|---|
- Body: 3 regions — head, thorax, abdomen
- Head: Compound eyes, antennae, mouthparts (biting and chewing type)
- Thorax: 3 segments (pro-, meso-, meta-); 3 pairs of legs; 2 pairs of wings (forewings = tegmina, hindwings = membranous)
- Abdomen: 10 segments; anal cerci in both sexes; anal styles in male; brood pouch (oothecal) in female
Digestive System
Path: Mouth → Pharynx → Oesophagus → Crop (food storage) → Gizzard (grinding) → Midgut (digestion + absorption) → Hindgut → Rectum → Anus
- Salivary glands: Open into buccal cavity; secrete saliva (amylase)
- Gizzard (proventriculus): Hard chitinous teeth grind food
- Hepatic caecae (gastric caecae): 6–8 finger-like projections at midgut junction; secrete digestive enzymes
- Malpighian tubules: At midgut-hindgut junction — excretion
Circulatory System
- Type: Open (haemolymph not confined to vessels)
- Heart: Dorsal, muscular tube with 13 chambers; ostia (openings on each side)
- Haemolymph: Colourless; NO haemoglobin; NO respiratory function
- Circulation: Heart → aorta → head → haemocoel → back to heart
Respiratory System
- No lungs — gas exchange via tracheal system
- Spiracles: 10 pairs of openings (2 thoracic, 8 abdominal) on lateral sides
- Tracheae: Air tubes branching into tracheoles → reach all cells directly
- Advantage: O₂ delivered directly to tissues without blood
Excretory System
- Malpighian tubules: ~100 yellow tubules at midgut-hindgut junction
- Absorb uric acid, salts, water from haemolymph
- Pass to hindgut → rectum reabsorbs water → dry uric acid excreted
- Type of excretion: Uricotelic (uric acid)
Nervous System
- Brain: Bilobed supraœsophageal ganglion in head
- Circumœsophageal connectives: Connect brain to suboesophageal ganglion
- Ventral nerve cord: Double cord running ventral to gut; 3 thoracic + 6 abdominal ganglia
- Sense organs: Compound eyes (mosaic vision), antennae (touch and smell), tympanal organ (hearing)
Reproductive System
Male:
- Testes (1 pair in 4th–6th abdominal segments)
- Vas deferens → ejaculatory duct → phallomere (external genitalia)
- Mushroom gland: Accessory reproductive gland
Female:
- Ovaries (2, in 2nd–6th segments)
- Oviducts → genital chamber
- Oothecal (brood pouch): Stores egg capsule (ootheca) — one ootheca per 14–15 eggs
- Spermathecae: Store sperms received during mating
Development: Paurometabolous (incomplete metamorphosis) — egg → nymph → adult (no pupal stage)
Important Questions
One Mark
- Name the excretory organ of cockroach.
- What are spiracles in cockroaches?
- Name the type of circulatory system in cockroach.
- What is an ootheca?
Short Answer (3 marks)
- Describe the digestive system of cockroach from gizzard to rectum.
- Explain the tracheal system of respiration in cockroach.
- What is the function of Malpighian tubules? What type of nitrogen excretion does cockroach show?
Long Answer (5 marks)
- Describe the nervous system of the cockroach with a labelled diagram.
- Give a detailed account of the reproductive system of the cockroach (both male and female).
Common Mistakes
- Cockroach blood (haemolymph): colourless, no haemoglobin, NO respiratory function — O₂ is transported by trachea, not blood.
- Number of spiracles: 10 pairs (not 8 or 2) — 2 thoracic + 8 abdominal.
- Malpighian tubules: at the junction of midgut and hindgut, NOT in the midgut itself.
- Metamorphosis in cockroach: paurometabolous (incomplete) — NO pupal stage (unlike butterfly which is holometabolous).
Board Exam Tips
- Cockroach nervous system diagram with labelling is a standard 5-mark question.
- "Describe the digestive system of cockroach" is another 5-mark staple.
- Comparison table: circulatory system of cockroach vs earthworm vs frog is a 3–5 mark question.
- NEET: Cockroach organ systems contribute 2–3 questions per paper — malpighian tubules, tracheal system, and haemolymph are most tested.
Frequently Asked Questions
Open circulatory system. Blood (haemolymph) is not confined to blood vessels — it flows freely in body cavities (haemocoel). The heart is a dorsal muscular tube with 13 chambers. No respiratory function — oxygen is carried by tracheal system.