DIGESTIVE SYSTEM OF INSECTS

• Insects may be phytophagous, entomophagous, wood borers, wool feeders (or) saprophytic.
• Based on the food material ingested, there are structural modifications in the digestive system of insects.
• Insects having the habit of feeding on solid food material, possess biting and chewing type of mouth parts whereas sap-feeding ones have a sucking type. SSapsuckerspossess a filter chamber and solid feeders have well-developed gizzard.
• The alimentary canal in insects extends from mouth to anus which is divided into an anterior stomodaeum (foregut), middle midgut (mesenteron or stomach or ventriculus), and posterior hindgut (proctodaeum).
• The foregut and hindgut are ectodermal in origin whereas the midgut is endodermal in origin. Compared to the carnivores or sap suckers, the phytophagous solid feeders possess longer alimentary canals.

The alimentary canal is divided into 3 parts

A) Foregut :

• It is the anterior part of the alimentary canal which starts with the mouth cavity and ends with the gizzard or proventriculus. It is divided into the pharynx, esophagus, crop, and gizzard.
• It consists of the ollowing parts:

a) Pharynx: It is the region between the mouth and esophagus

b) Oesophagus: It is a narrow part of the foregut through which the food ggetstransported from the pharynx into the crop.

c) Crop: It is a sac-like structure that is a dilated form and mainly serves the purpose of storage of food material.

d) Gizzard: It is a small constricted part of the alimentary canal which consists of the cuticular intima layer modified iintoa teethike denticles that help for grinding the food material.

Internally foregut consists of the following layers.

i. Inner most intima layer

ii. Epithelial cells

iii. Basement membrane

iv. Longitudinal muscles

v. Circular muscles

B) Mid-gut:

• It is also known as mesenteron or stomach. Foregut opens into mthe idgut through the stomodael / cardiac valve.
• The midgut is a short, straight tube in the case of primitive insects or a sac or may be pyriform or fusiform in shape in caterpillars.

Structurally midgut consists of

a) Inner peritrophic membrane

b) A layer of epithelial cells 

c) Basement membrane 

d)Circular muscles 

e) Longitudinal muscles

• The midgut consists of an inner delicate layer called a peritrophic membrane secreted by the epithelial cells.
• The peritrophic membrane protects the tender epithelial cells of the midgut from abrasion by hard food particles as no mucous is secreted in insects that feed on the solid food material
• It is absent in the case of sap-sucking insects.

The epithelial layer of the midgut consists of 3 types of cells

1) Columnar cells: These are columnar  vary in size ,and release enzymes through a series of microvilli arranged in a brush border or honeycomb border.

2) Regenerative cells: These epithelial cells are involved in the production or formation of new cells to replace the whole columnar cells involved in hthe olocrine secretion of enzymes. These regenerative cells may be arranged either in groups or may remain scattered or sometimes singly.

3) Goblet cells: Mainly serve for storage and excretion.

C) Hindgut:

• It is also known as proctodaeum which is lined inside by intima. It is more permeable than that of the foregut.
• Hindgut is divided into 3 regions namely the ileum, colon, and rectum.
• The ileum is a small intestine or tube-like structure and appears as a pouch in carabids.
• The colon may or may not be present and if present, it leads to the rectum.
• The epithelial cells of the rectum may sometimes get differentiated into rectal papillae or pads which vary in number from 3-6.
• These are involved in the reabsorption of water, and salts from the fecal matter.