Structure and Functions of the Digestive System of Insect | entomology | Competitive exam notes

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🍽️ The Insect Cafeteria: Structure and Functions of the Digestive System

Welcome to the internal workings of an insect! Just like us, insects need to eat, digest, and absorb nutrients to survive. The system responsible for this is the Alimentary Canal, a long, muscular, and tubular structure extending from the mouth to the anus.

Based on the syllabus for Insect Morphology and Systematics, let’s break down the three main regions: the Foregut, Midgut, and Hindgut.

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1. Foregut (Stomodeum) 🥤

The foregut is the first section of the digestive tract. Since it is ectodermal in origin, it is lined with an internal layer of cuticle.

Key Parts & Functions:

• Pre-oral Cavity: The space leading to the mouth.

  • Cibarium: The area between the epipharynx and hypopharynx.

  • Salivarium: The area between the hypopharynx and the salivary duct.

• Pharynx: A well-musculated organ behind the mouth that pushes food into the oesophagus. It acts as a sucking pump in sap feeders.

• Oesophagus: A narrow tube that conducts food into the crop.

• Crop: The dilated distal part of the oesophagus acting as a food reservoir. In bees, this is known as the honey stomach, where nectar conversion occurs.

• Proventriculus (Gizzard): A muscular organ found in solid feeders (absent in fluid/sap feeders). It contains internal cuticular modifications:

  • Teeth-like: In cockroaches to grind and strain food.

  • Plate-like: In honey bees to separate pollen grains from nectar.

  • Spine-like: In fleas to break blood corpuscles.

• Cardiac Valve (Oesophageal Valve): Regulates the flow of food from the foregut to the midgut.

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2. Midgut (Mesenteron) 🧪

The midgut is the primary site for digestion. Unlike the foregut, it is endodermal in origin and contains no cuticular lining.

Key Structures:

• Epithelial Cells: The midgut wall is made of three types of cells:

  1. Secretory (Columnar) cells: For enzyme secretion.

  2. Goblet cells: Aged secretory cells.

  3. Regenerative cells: Replace the secretory cells.

• Peritrophic Membrane: An internal semipermeable lining secreted by epithelial cells. It protects the midgut cells from hard food particles and lubricates food movement. It is present in solid feeders but absent in sap feeders.

• Gastric Caecae: Finger-like outgrowths that increase the functional area of the midgut and shelter symbiotic bacteria.

• Filter Chamber: A complex organ in Homopteran insects (like bugs) where the two ends of the midgut and the beginning of the hindgut are enclosed in a sac. It "short-circuits" excess water to concentrate food and prevent the dilution of digestive enzymes.

• Pyloric Valve (Proctodeal Valve): Regulates food flow from the midgut to the hindgut.

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3. Hindgut (Proctodeum) 🚽

The hindgut is the final section, and like the foregut, it is ectodermal in origin and lined with cuticle.

Key Parts & Functions:

• Permeability: The cuticular lining here is permeable to water, salts, ions, and amino acids.

• Main Function: Absorption of water, salt, and useful substances from faeces and urine.

• Differentiation: It is divided into three regions: Ileum, Colon, and Rectum.

  • Ileum: In termites and scarab larvae, it acts as a fermentation chamber for symbionts.

  • Rectum: Contains rectal pads that help dehydrate faeces before it is expelled through the anus.

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🧠 Gut Physiology: Digestion & Microbes

Digestive Glands 💧

• Salivary Glands: Secretions vary by insect:

  • Cockroach: Contains amylase for starch digestion.

  • Honey Bee: Contains invertase for sucrose digestion.

  • Mosquito: Contains anticoagulin to prevent blood clotting.

  • Gall Midge: Contains Indole Acetic Acid (IAA) to produce plant galls.

Digestive Enzymes 🧬

Insects produce specific enzymes based on their diet:

• Phytophagous larvae: Amylase (Starch), Maltase (Maltose), Invertase (Sucrose).

• Omnivores: Protease (Protein), Lipase (Lipid).

• Wood borers/Termites: Cellulase (Cellulose).

• Meat-eating maggots: Collagenase (Collagen).

• Bird lice: Keratinase (Keratin).

Symbiotic Microbes 🦠

Some insects rely on friendly microbes housed in mycetocytes for digestion:

• Flagellate Protozoa: Produce cellulase for termites.

• Bacteria: Help digest wax in wax moths.

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💩 Excretion & Absorption

• Absorption: Nutrients are absorbed in the midgut; water and ions are reabsorbed in the rectum.

• Honey Dew: Sap feeders (like aphids) excrete liquid faeces rich in sugars and amino acids called honey dew, which attracts ants.

• Solid Feeders: Resorb water in the rectum and expel faeces as dry pellets.

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🧠 Knowledge Check: MCQs

Test your gut feelings with these questions!

1. Which part of the foregut acts as a "food reservoir"?

A. Pharynx

B. Oesophagus

C. Crop

D. Gizzard

2. The "Filter Chamber" is a specialized structure found in which group of insects?

A. Orthoptera (Grasshoppers)

B. Coleoptera (Beetles)

C. Homoptera (Bugs)

D. Hymenoptera (Bees)

3. Which membrane protects the midgut epithelial cells from abrasion by hard food particles?

A. Basement membrane

B. Peritrophic membrane

C. Cell membrane

D. Nuclear membrane

4. In termites, which part of the hindgut acts as a "fermentation chamber"?

A. Colon

B. Rectum

C. Ileum

D. Anus

5. Which enzyme is found in the saliva of mosquitoes to prevent blood clotting?

A. Amylase

B. Invertase

C. Anticoagulin

D. Pectinase

6. What is the liquid faeces of aphids containing soluble sugars called?

A. Frass

B. Meconium

C. Honey dew

D. Pellets

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Answers:

1. C (Crop)

2. C (Homoptera)

3. B (Peritrophic membrane)

4. C (Ileum)

5. C (Anticoagulin)

6. C (Honey dew)