🔬 Cockroach: Internal Systems

This lesson covers the internal organ systems of the cockroach (Periplaneta americana), including digestion, respiration, circulation, excretion, nervous system, and reproduction. Understanding these systems is essential because they represent the typical arthropod/insect body plan that appears frequently in competitive exams.

Digestive System

The cockroach has a complete alimentary canal — food enters through the mouth and waste exits through the anus. The digestive tract is divided into three regions based on their embryonic origin:

Foregut (Stomodaeum) — Ectodermal Origin

The foregut is responsible for ingestion, storage, and mechanical breakdown of food. Being ectodermal in origin, it is lined with cuticle.

Part	Function
Mouth	Opening through which food enters
Buccal cavity	Receives food along with saliva from salivary glands
Pharynx	Muscular chamber; helps in swallowing food by pushing it backward
Oesophagus	Short, narrow tube; conducts food from pharynx to crop
Crop	Large, thin-walled sac; serves as the primary food storage organ — food can be held here before being passed to the gizzard
Gizzard (Proventriculus)	Thick-walled grinding chamber with 6 cuticular teeth (chitinous plates) and bristles; mechanically grinds food into fine particles; has a stomodeal valve at its posterior end that controls food passage into the midgut and prevents backflow

Midgut (Mesenteron) — Endodermal Origin

The midgut is the primary site of chemical digestion and nutrient absorption. Unlike the foregut and hindgut, it is not lined with cuticle, allowing direct contact between digestive enzymes and food.

Part	Function
Mesenteron / Ventriculus	The main digestive and absorptive region of the alimentary canal
Hepatic caecae	6–8 finger-like outgrowths (blind pouches) at the junction of foregut and midgut; they secrete digestive enzymes and absorb digested food, greatly increasing the surface area for digestion
Peritrophic membrane	A thin, semi-permeable membrane that surrounds the food mass inside the midgut; it protects the delicate midgut lining from abrasion by food particles while still allowing enzymes and digested nutrients to pass through

Hindgut (Proctodaeum) — Ectodermal Origin

The hindgut is responsible for water reabsorption and waste elimination. Like the foregut, it is lined with cuticle.

Part	Function
Ileum	Short tube; absorbs remaining nutrients
Colon	Absorbs water from the undigested food mass, compacting it into feces
Rectum	Has 6 rectal papillae (specialized pads); these are highly efficient at reabsorbing water and salts from feces — crucial for water conservation in a terrestrial insect
Anus	Terminal opening for egestion (elimination) of feces

Salivary Glands

• 1 pair of salivary glands located in the thorax
• Secrete saliva containing multiple digestive enzymes:
  • Amylase (digests starch into sugars)
  • Cellulase (digests cellulose — unusual for an animal; allows the cockroach to digest plant material)
  • Chitinase (digests chitin — enables the cockroach to eat other insects' exoskeletons)
• A reservoir stores saliva; the salivary duct opens into the buccal cavity

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Respiratory System (Tracheal System)

The cockroach does not have lungs or gills. Instead, it breathes through a tracheal system — a network of air-filled tubes that deliver oxygen directly to every cell in the body. This is the hallmark respiratory system of terrestrial insects.

• Air enters through spiracles (stigmata) → flows through tracheal tubes → branches into finer tracheoles → oxygen diffuses directly into cells

Spiracles

• 10 pairs total:
  • 2 pairs thoracic (on mesothorax and metathorax)
  • 8 pairs abdominal (on segments 1–8)
• Each spiracle has a valve (closing apparatus) that can open and close to regulate air flow and minimize water loss — a critical adaptation for terrestrial life

Tracheal System

• Tracheae: branching tubes lined with cuticle that has spiral thickening called taenidia — these spiral bands keep the tubes from collapsing, much like the rings in a vacuum cleaner hose
• Tracheoles: the finest branches (~1 μm diameter); thin-walled and fluid-filled at their tips; gas exchange (O₂ in, CO₂ out) occurs here by simple diffusion
• Because oxygen is delivered directly to cells, no respiratory pigment (like hemoglobin) is needed — this is why insect blood (haemolymph) does not carry oxygen

Breathing Mechanism

• Tergosternal muscles and abdominal muscles alternately contract and relax, compressing and expanding the abdomen like a bellows:
  • Inspiration: abdominal expansion → air drawn in through spiracles
  • Expiration: abdominal compression → air pushed out

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Circulatory System (Open Type)

Feature	Details
Type	Open circulatory system — blood (haemolymph) flows freely in the body cavity rather than being confined to closed vessels
Blood	Called haemolymph; colourless because it contains no respiratory pigment (hemoglobin/hemocyanin); does NOT transport O₂
Blood cells	Haemocytes — these cells perform phagocytosis (engulf pathogens) and play a role in immune defense and wound healing
Heart	Elongated, tubular organ; located mid-dorsally (along the back); consists of 13 funnel-shaped chambers
Ostia	Small lateral openings in each heart chamber; haemolymph enters the heart through ostia when the heart expands
Heartbeat	~49 beats/min

Sinuses (3 Blood Sinuses)

The haemocoel is divided into three compartments called sinuses by two thin membranes (diaphragms):

Sinus	Location
Pericardial sinus	Around the heart (dorsal — uppermost compartment)
Perivisceral sinus	Around the internal organs (middle — largest compartment)
Perineural / Sternal sinus	Around the ventral nerve cord (ventral — lowermost compartment)

Diaphragms (2)

Diaphragm	Location	Separates
Dorsal diaphragm	Below the heart	Pericardial sinus (above) from perivisceral sinus (below)
Ventral diaphragm	Above the ventral nerve cord	Perivisceral sinus (above) from perineural sinus (below)

Other Structures

• Alary muscles: fan-shaped muscles attached to the dorsal diaphragm; their contraction dilates the heart, creating negative pressure that draws haemolymph in through the ostia — essentially they act as the heart's "filling mechanism"
• Anterior aorta: a single blood vessel extending forward from the heart; opens into the head sinus where haemolymph is distributed to the head region. From there, it flows backward through the sinuses and re-enters the heart through the ostia.
• Nephrocytes: star-shaped cells scattered in the pericardial sinus; they filter haemolymph and absorb waste products, performing an excretory function

Path of blood flow in cockroach

The flow of haemolymph follows this circuit: **Heart pumps forward** → haemolymph exits through the **anterior aorta** into the **head sinus** → flows backward through the **perivisceral sinus** (bathing all organs) → flows into the **perineural sinus** → re-enters the **pericardial sinus** through gaps in the diaphragms → enters the heart through **ostia** (aided by alary muscle contraction). This completes one cycle.

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Excretory System

The cockroach uses multiple structures for excretion, with Malpighian tubules being the primary excretory organs:

Structure	Details
Malpighian tubules	Primary excretory organs; 100–150 in number; yellow, thread-like tubules; attached at the junction of midgut and hindgut. They absorb nitrogenous waste from haemolymph and pass it into the hindgut.
Excretory product	Potassium urate is first formed in Malpighian tubules, then converted to uric acid. The cockroach is therefore uricotelic (excretes uric acid). Uric acid is nearly insoluble and can be excreted with very little water — an important water conservation strategy.
Fat bodies	Store uric acid crystals (accessory excretory role); this stored uric acid is called urate storage excretion
Nephrocytes	Star-shaped cells in the pericardial sinus that filter waste from haemolymph
Body wall / Cuticle	Some excretion occurs through the integument (body surface)
Uricose glands	Found in males only; accessory reproductive glands that also excrete uric acid (used to coat the spermatophore)

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Nervous System

Central Nervous System (CNS)

The cockroach has a relatively simple but well-organized nervous system consisting of a "brain" (actually a pair of ganglia) and a ventral nerve cord:

Structure	Location	Function
Supra-oesophageal ganglion ("brain")	Dorsal to (above) the oesophagus in the head	Receives and processes sensory input from compound eyes, ocelli, and antennae; the main integrating center
Sub-oesophageal ganglion	Ventral to (below) the oesophagus in the head	Controls the mouthparts and salivary glands
Circum-oesophageal connectives	Ring of nerve tissue around the oesophagus	Connect the supra-oesophageal and sub-oesophageal ganglia, forming a nerve ring around the oesophagus
Ventral nerve cord	Runs mid-ventrally through thorax and abdomen	Contains 9 ganglia (3 thoracic + 6 abdominal); each ganglion controls the muscles and organs of its segment — this is why a headless cockroach can still walk!

Peripheral Nervous System (PNS)

• Consists of nerves arising from each ganglion and extending to all body parts, carrying sensory information in and motor commands out

Autonomic / Visceral / Stomatogastric Nervous System

• Arises from 5 ganglia (frontal ganglion, hypocerebral ganglion, ventricular/ingluvial ganglion, etc.)
• Controls involuntary movements of the foregut (pharynx, crop, gizzard) — ensuring the digestive system works automatically

Why can a headless cockroach survive?

A cockroach can survive for about a week without its head because: (1) each ganglion of the ventral nerve cord can independently control local body functions — the cockroach can still stand, walk, and react to touch; (2) it breathes through spiracles (not through the mouth/nose), so decapitation does not block breathing; (3) it has an open circulatory system with no blood pressure, so it does not bleed out. It eventually dies of dehydration because it cannot drink water.

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Reproductive System

Male Reproductive System

Structure	Details
Testes	1 pair; located in the 4th–6th abdominal segments; each testis has 3–4 lobes where sperm are produced
Vas deferens	Tubes from each testis; the two vasa deferentia join to form a single ejaculatory duct
Ejaculatory duct	Opens at the male gonopore (the male reproductive opening)
Seminal vesicles	Mushroom-shaped glands (also called utricular glands); store mature sperm until mating
Phallic / Conglobate gland	Secretes the material used to form the spermatophore (sperm packet)
Phallomeres	3 asymmetric chitinous structures (left, right, ventral) that form the external genitalia; include a titillator (hook-like structure for holding the female during mating) and a pseudopenis
Spermatophore	A 3-layered sperm packet that is transferred to the female during copulation — the layers protect the sperm and provide nutrients

Female Reproductive System

Structure	Details
Ovaries	1 pair; located in the 2nd–6th abdominal segments; each ovary contains 8 ovarioles (egg tubes) where eggs develop
Eggs	16 eggs are produced at a time (8 from each ovary)
Oviducts	Carry eggs from ovaries; the two oviducts join to form a common oviduct which leads to the vagina
Spermatheca	A pair of unequal-sized sacs; stores sperm received from the male during mating; sperm can remain viable for months
Colleterial glands	Secrete the hard, protective material that forms the ootheca (egg case)
Gonapophyses	3 pairs of plate-like structures; form the ootheca chamber and help in egg-laying (oviposition)

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Copulation and Development

Stage	Details
Mating season	March to September
Courtship	Male produces pheromones (chemical signals) to attract the female
Copulation	Male transfers a spermatophore to the female's spermatheca; mating occurs end-to-end (posterior ends touching)
Fertilization	Internal; occurs inside the ootheca as eggs are being laid
Ootheca	The protective egg case; approximately 3/8 inch (~8 mm) long; dark brown; contains 14–16 eggs arranged in 2 rows. The female carries it briefly, then deposits it in a safe, warm location.
Development type	Paurometabolous (incomplete metamorphosis: egg → nymph → adult; no pupal stage). This contrasts with holometabolous insects like butterflies that have a complete metamorphosis with a pupal stage.
Nymph	Resembles the adult but is smaller and wingless; the first nymph instar has small wing pads that grow with each moult
Moulting	13 moults (instars) are required before the nymph becomes a fully developed, winged adult
Time to adult	~1 year from egg to adult

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Cockroach Systems Comparison Table

System	Key Feature
Digestive	Foregut–Midgut–Hindgut; gizzard with 6 teeth; 6–8 hepatic caecae
Respiratory	Tracheal system; 10 pairs spiracles; no blood pigment needed
Circulatory	Open type; 13-chambered heart; haemolymph; 3 sinuses, 2 diaphragms
Excretory	Malpighian tubules (100–150); uricotelic
Nervous	Supra-oesophageal ganglion (brain); ventral nerve cord with 9 ganglia
Reproductive	Spermatophore transfer; ootheca; paurometabolous development; 13 moults

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Beginner's Box — Practice Questions

Set 1: External Morphology

1. The chromosome number of cockroach is: Answer: 2n = 34

2. The type of head orientation in cockroach is: Answer: Hypognathous

3. How many ommatidia are present in each compound eye? Answer: ~2000

4. The forewings of cockroach (thick and leathery) are called: Answer: Tegmina

5. Anal styles are present in: Answer: Male cockroach only

Set 2: Internal Systems

1. The number of Malpighian tubules in cockroach is: Answer: 100–150

2. The heart of cockroach has how many chambers? Answer: 13 (funnel-shaped)

3. The number of spiracles in cockroach is: Answer: 10 pairs (2 thoracic + 8 abdominal)

4. The excretory product of cockroach is: Answer: Uric acid (uricotelic)

5. The gizzard has how many cuticular teeth? Answer: 6

Set 3: Reproduction and Development

1. Each ovary of cockroach has how many ovarioles? Answer: 8

2. The ootheca of cockroach contains how many eggs? Answer: 14–16

3. The development of cockroach is: Answer: Paurometabolous (incomplete metamorphosis)

4. How many moults does a cockroach nymph undergo? Answer: 13

5. Mosaic/apposition vision in cockroach is seen in: Answer: Bright light (daytime)

Set 4: Detailed Anatomy

1. Blood sinuses in cockroach are: Answer: 3 (pericardial, perivisceral, perineural)

2. The "brain" of cockroach is: Answer: Supra-oesophageal ganglion

3. The number of ganglia in the ventral nerve cord is: Answer: 9 (3 thoracic + 6 abdominal)

4. The heartbeat rate of cockroach is approximately: Answer: 49 beats/min

5. Hepatic caecae in cockroach number: Answer: 6–8

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Summary Cheat Sheet

Concept / Topic	Key Details / Explanation
Digestive System — Foregut (Stomodaeum)	Mouth → buccal cavity → pharynx → oesophagus → crop (thin-walled, food storage) → gizzard/proventriculus (thick-walled, 6 cuticular teeth for grinding, stomodeal valve at posterior end) Lined with cuticle (ectodermal origin)
Salivary Glands	1 pair in thorax; each has gland + reservoir Secrete saliva containing amylase (starch digestion) and chitinase (chitin digestion)
Digestive System — Midgut (Mesenteron)	Ventriculus (stomach); no cuticular lining 6–8 hepatic caecae (finger-like projections at foregut-midgut junction) — secrete digestive enzymes, increase absorptive area Peritrophic membrane: surrounds food, protects midgut lining
Digestive System — Hindgut (Proctodaeum)	Ileum → colon → rectum (with 6 rectal papillae for water + ion reabsorption) → anus Lined with cuticle (ectodermal origin)
Tracheal System (Respiration)	No lungs or gills; breathe through tracheal system Air enters via spiracles → tracheae (reinforced with taenidia — chitinous spiral rings) → tracheoles (finest branches, reach individual cells) Gas exchange by direct diffusion to cells Haemolymph does NOT carry O₂
Spiracles	10 pairs total: 2 thoracic (larger) + 8 abdominal Each has a valve mechanism to control air entry and reduce water loss
Circulatory System	Open type; blood = haemolymph (colourless, no haemoglobin, no O₂ transport) Heart: elongated, 13 funnel-shaped chambers (mid-dorsal, in abdomen), with lateral ostia Anterior aorta: extends from heart to head Heart rate: ~49 beats/min
Blood Sinuses (3)	Pericardial sinus (dorsal, around heart) — separated by dorsal diaphragm Perivisceral sinus (middle, around organs) Perineural/Sternal sinus (ventral, around nerve cord) — separated by ventral diaphragm Alary muscles: fan-shaped, attached to dorsal diaphragm, assist heart in pumping
Excretory System	Malpighian tubules: 100–150 yellow, thread-like tubules at midgut-hindgut junction Absorb waste from haemolymph → convert to uric acid → discharge into hindgut Excretory type: uricotelic (uric acid — insoluble, conserves water) Fat bodies also store uric acid (storage excretion) Additional: urecose glands (in mushroom glands), nephrocytes
Nervous System	Supra-oesophageal ganglion ("brain"): located in head, controls antennae, eyes, mouthparts Connected to sub-oesophageal ganglion via circumoesophageal connectives Ventral nerve cord: double, with 9 ganglia (3 thoracic + 6 abdominal) Peripheral nerves to all body parts
Male Reproductive System	Testes: 1 pair (in 4th–6th abdominal segments), each with lobules Vas deferens → ejaculatory duct → gonopore (opens below anus) Mushroom-shaped gland (= accessory gland): produces spermatophore (sperm packet) Conglobate gland: accessory
Female Reproductive System	Ovaries: 1 pair, each with 8 ovarioles (total 16) Oviducts → vagina → genital pouch (between 7th & 8th sterna) Collateral glands: 1 pair, secrete material for ootheca (egg case) Spermatheca: stores sperm received during mating
Ootheca (Egg Case)	Dark brown, bean-shaped capsule Contains 14–16 eggs arranged in 2 rows Carried by female, then deposited in warm, hidden place
Development	Paurometabolous (incomplete/gradual metamorphosis): egg → nymph (resembles adult but wingless, smaller) → adult Nymph undergoes 13 moults (instars) over ~1 year to become adult No pupal stage (unlike holometabolous insects like butterflies)