- Insect digestive system is of complete type which means starting from mouth, the alimentary canal terminates as anus.
- Along its length, alimentary canal is associated with salivary gland, salivary reservoir, hepatic or gastric caeca and Malpighian tubules (act as osmoregulater).
- Alimentary canal vary in size(diameter), anatomically modified structure as per the type of role during the process of digestion.
- Insects possess a diversity of modifications as per their feeding habits. This plays a key role in diversification of class Insecta, although they shared similar basic structure.
Alimentary canal is divisible into 3 major regions:
*Foregut and hindgut has been developed from embryonic ectoderm. Thus lined with exoskeleton of chitin and cuticular glycoprotein. It protects foregut and hind gut epithelial tissue.
*Midgut has developed from embryonic endoderm. It is concerned with catabolic breakdown of macronutrients and their absorption. Thus it is supported with connective tissue.
• General functions performed in this region are conducting food from pre-oral cavity to midgut and temporary food storage.
*Pre-oral cavity: It is enclosed by mouth parts.
Cibarial pump mechanism : Cibarial muscles present just behind clypeus . These muscles are well developed in fluid feeding insects and continuous contraction and relaxation of these muscles created suction mechanism called as cibarial pump mechanism.
Histology of Foregut (from in to out) :
• Intima : It lines the lumen of foregut and has cuticular/ chitinous covering. Hence nutrient absorption is absent in foregut.
• Epidermis : It is a continuous layer of epithelial cells that secrets intima.
• Basement membrane : It forms the base layer for epithelial cells.
• Longitudinal & circular muscles : Contraction and relaxation of these muscles results in peristaltic movement which causes forward movement of food.
Space between mouth and oesophagus.
It receives secretion of salivary glands.
Dorsally this region has dilator muscles, well developed in fluid feeding orders.
Example => Lepidoptera, Hymnoptera, Neuroptera, Dytiscidae.
Pharynx leads into oesophagus.
It acts as conducting tube.
It ends into crop.
It can be said that posteriorly oesophagus dilates and called as crop.
Crop is also known as Diverticulum .
Primarily it acts as food reservoir.
Here digestion also starts as the salivary enzyme (amylase) begins catabolic activities on macronutrients.
In orders Orthoptera and Dictyoptera crop is more capacious and constitutes major portion of foregut.
PROVENTRICULUS or GIZZARD
Another swollen structure present next to the crop
It is well developed in orders Orthoptera and Coleoptera (solid feeding insects).
Inner intima projected like conical teeth structure called denticles, coated with chitinous depositions for grinding of food. It helps in better enzymatic metabolism.
Proventriculus is provided with thick muscles.
It is poorly developed or reduced in orders Isoptera, Hymnoptera, Lepidoptera, Diptera
In Apis : It is useful in engulfing the pollen and other contaminants along with nectar.
Internal projections has filiform hairs. Continuous movement of muscles of proventriculus , inner lumen acts as sieve due to which pollen and other contaminants gets trapped in filiform hairs. These particles gets collected in the pouches of ventricular folds forming boluses that eventually passes to the midgut.
Proventriculus leads to the midgut.
At this junction of foregut and midgut wall invaginated and forms stomodaeal valves.
These are also termed as cardiac valves or oesophageal valves.
It prevents the regurgitation of the food into the foregut.
MODIFICATION OF FOREGUT
Adaptive modification in most of the insect are confined to the foregut region
• Order Lepidoptera : Larvae or caterpillar has unspecialised digestive tract.
Imago/Adult foregut primarily serves the function of food storage (crop) and has primitive midgut and hindgut.
• Order Orthoptera and Coleoptera : has well defined foregut midgut and hindgut.
• Order Diptera : crop is specialised to store blood, midgut is elongated and hindgut has rectal glands
This central region of alimentary canal can also be termed as stomach or ventriculus.
Final digestion occur in this region followed by absorption of micronutrients.
Anteriorly (just after the stomodaeal valves) it has Gastric caecae and posteriorly (before ending into hindgut) it has Malpighian tubules.
HISTOLOGY OF MIDGUT (from outside to inside)
• Longitudinal muscles
• Circular muscles
• Connective tissue
• Basement membrane
• Epithelial membrane : There are three types of cells…
1. Columnar cells -: serves the granular secretions and micronutrients absorption through its brush border or microvilli.
Granular secretions are of two types (i) Holocrine secretion -: That cause complete breakdown of the cells and later replace by regenerative cells.
(ii) Merocrine secretion -: these doesn’t cause complete breakdown and later recovers and starts functioning.
2. Regenerative cells -: Primarily serves the function of regeneration of damaged epithelial cells.
These are present in crypt (fold) of epithelial membrane as bunch called Nidi
3. Goblet cells -: concern with storage.
It may involve in active transport of potassium.
Present in larva of Lepidoptera.
• Peritrophic membrane -: Present next to epithelial membrane preventing it from solid food particles.
It is absent in fluid feeding insects.
MODIFICATION OF MIDGUT
• In May flies – It is modified as aerostatic system.
• In Homoptera (bugs) and some Heteroptera , it is modified into filter chamber. It is the thin walled bladder (anteriorly swollen midgut) which is closely bound to the hindgut and Malpighian tubules. This results in removal of excess water directly into hindgut and concentration of food for effective enzymatic metabolism.
Posterior part of gut .
Externally junction between midgut and hindgut can be marked by Malpighian tubules and internally by pyloric valves.
Due to ectodermal origin, epithelial membrane is coated with cuticular lining.
Hindgut is divisible in three common region -:
1. Ileum (small intestine)
2. Colon (large intestine)
• Ileum : It is short running backward.
In Termites, it is a Symbiotic home of some flagellates that responsible for cellulose digestion.
It forms the fermentation chamber in larvae of Scarbaeoidea.
This region receives the excretory product from malpighian tubules and responsible for osmoregulation of haeomolymph.
Ileum may be long in Dystiscus and Nicrophorous or may be undifferentiated from colon in Orthoptera and Hemiptera.
• Colon :This region is marked by coiling of tube.
• Rectum : Posterior most region of hindgut marked as swollen sac like structure termed as rectum, temporary storage of undigested matter.
Internally it is composed of six projections lined with fine setae called rectal papillae.
Rectal papillae serves the function of reabsorption of water.
Anus : It is slit opens to external beneath 10th tergum.
It is over covered by pair of podial plates.
Glands associated with digestion are Labial glands or salivary gland and Gastric caecae.
Salivary gland : Also termed as labial glands.
Present in pair on either side of thorax hangs freely in haemolymph.
Secretion of the gland is called saliva, constitute water and enzyme (amylase).
Types of salivary glands
1. Acinuous salivary gland : These are glandular or sac like glands present on either side and salivary reservoir .
Gland is composed of two types of cells – (i) Peripheral cells and (ii) Central cells.
Peripheral cells bears micro-villi, absorbs water from haeomolymph and pass it to canaliculi (ducts or small canal structure) that unites to common duct.
Central cells associated with the production of carbohydrate digesting enzyme called amylase.
In Orthoptera (Loftus), Blattodea (Periplaneta) possess acinuous salivary gland.
2. Tubular salivary gland : As the name suggests, these glands are tubular in structure that runs deep into haeomolymph to the abdominal region.
Insects belongs to order Lepidoptera, Diptera possess tubular glands.
Tubular gland is divisible into regions which vary among different insects.
In adult Blowfly, tubular gland is divisible into three regions i.e. Apical region, reabsorptive region, and proximal end.
Apical region is also called secretary region as it produce amylase enzyme and involve in absorption of water from haeomolymph which then pass to canaliculi.
Reabsorption region is specific for reabsorption of ions back into the haeomolymph.
Gastric caecae : Arises from midgut.
It’s secretion contains Proteolytic enzyme i.e. Trypsin for protein catabolism.
Lipolytic enzyme : Lipase for lipid catabolism.
Amylolytic enzyme : i.e. Maltase and Invertase for starch catabolism.