Insect Digestive System
Alimentary Canal Overview
- Definition: A hollow, tubular structure extending from mouth to anus.
- Main Divisions:
- Foregut (Stomodaeum) – Ectodermal origin
- Midgut (Mesenteron / Ventriculus) – Endodermal origin
- Hindgut (Proctodaeum) – Ectodermal origin
Foregut (Stomodaeum)
→ Function: Ingestion, temporary storage, partial digestion.
Parts & Functions:
|
Part |
Function |
Exam Notes |
|
Pharynx |
Sucking pump in fluid feeders |
Prominent in Hemiptera & Diptera |
|
Oesophagus |
Tube carrying food from pharynx to crop |
Short & narrow |
|
Crop |
Storage chamber of food |
Called Honey Stomach in Honeybee |
|
Proventriculus (Gizzard) |
Grinding food |
Well-developed in chewing insects (Orthoptera, Coleoptera) |
Lining: Cuticular (intima) → prevents enzyme absorption.
Midgut (Mesenteron / Ventriculus)
→ Function: Enzyme secretion, digestion, absorption.
→ Origin: Endodermal (no cuticular lining).
Special Features:
- Peritrophic membrane present in most chewing insects – protects midgut epithelium.
- Hepatic / Gastric caeca – finger-like projections at anterior end; secrete digestive enzymes and absorb nutrients.
- No cuticular intima → allows enzyme secretion and absorption.
Absent in: Fluid feeders (Hemiptera, Diptera).
Present in: Solid feeders (Lepidopteran larvae, Beetles).
Hindgut (Proctodaeum)
→ Function: Water & ion reabsorption, feces formation.
→ Origin: Ectodermal (cuticular lining present).
Parts:
- Ileum – receives waste from midgut.
- Colon – reabsorbs water.
- Rectum – stores fecal matter; contains rectal pads (papillae) for water & salt reabsorption.
Peritrophic Membrane
|
Feature |
Description |
|
Origin |
Midgut (since no cuticular lining) |
|
Composition |
Chitin + Protein + Mucopolysaccharides |
|
Function |
Protects epithelium from mechanical injury & infection; allows nutrient passage |
|
Present in |
Solid feeders (Lepidoptera, Coleoptera) |
|
Absent in |
Fluid feeders (Hemiptera, Diptera) |
Filter Chamber (Special Structure in Fluid Feeders)
→ Seen in: Homopteran insects – e.g. Aphids, Leafhoppers, Whiteflies.
→ Function:
- Allows rapid removal of excess water & sugars from liquid diet (plant sap).
- Prevents haemolymph dilution.
- Modifies alimentary canal → anterior & posterior parts closely connected to allow quick filtration.
→ Advantage: Enables continuous feeding on dilute sap and efficient nutrient absorption.
Key Digestive Glands
|
Gland |
Function |
Example |
|
Salivary glands |
Secrete digestive enzymes (amylase, invertase); lubricate food |
Present in most insects |
|
Labial glands (Silk glands) |
Modified salivary glands |
Silkworm, Caddisfly larvae |
|
Gastric caeca |
Secrete enzymes, absorb food |
Grasshopper |
|
Malpighian tubules |
Excretory organ opening at midgut–hindgut junction |
All insects |
Quick Revision Pointers
- Alimentary canal = Mouth → Pharynx → Oesophagus → Crop → Gizzard → Midgut → Hindgut → Anus
- Foregut & Hindgut → Ectodermal origin (with cuticular intima)
- Midgut → Endodermal origin (no cuticular lining)
- Peritrophic membrane → Present in solid feeders, absent in fluid feeders
- Filter chamber → Present in Homopterans (Aphids, Jassids, Whiteflies)
- Honey stomach → Crop of honeybee
- Gizzard → Grinding function (chewing insects)
- Rectal pads → Reabsorption of water & salts
