DIGESTIVE SYSTEM
FOOD UPTAKE AND UTILIZATION
S. VISHNU PRIYAA
II – MSc ZOOLOGY
DIGESTIVE SYSTEM:
Digestive system is a network of organs that help to digest and absorb nutrition
from the food.
The digestive system is made up of the digestive tract or the alimentary canal and
other organs that help the body break down and absorb food.
It is a long, twisting tube that starts at the mouth and ends in the anus.
The alimentary canal consists of three regions: the foregut, midgut, and hindgut.
The foregut is also known as the stomodaeum
The midgut is also known as the mesenteron
The hindgut is also known as the proctodaeum
Each of these regions may be divided into two or more sub-regions; and there are
usually valves and sphincters between these regions which regulate the passage of
food from one region to another.
PREORAL CAVITY:
The mouthparts lie close together and form a small cavity often called the mouth
cavity.
Since it lies anterior to the true opening to the stomodaeum(foregut) it is more
properly known as the preoral cavity.
The true mouth is the opening of the stomodaeum(foregut).
This preoral food chamber is also sometimes called as cibarium.
There are usually muscles attached to the cibarium that have originated from the
clypeus these are called the dilators of the cibarium.
These dilators are located anterior or ventral to the frontal ganglion.
This generally helps to masticate the food.
FOREGUT:
The foregut is ectodermal in origin it is lined with a layer of cuticle, known as the
intima, which is shed at each molt, intima protects the foregut from abrasion by food
particles.
The foregut epithelium consists of flattened cells, inside the epithelium is a layer of
longitudinal muscle and a layer of circular muscle.
The circular muscles are not inserted into the epithelium, but are continuous around
the gut so that when they contract the epithelium becomes longitudinally folded.
When the gut is distended with food these folds are flattened out.
In addition, especially in the proventriculus, there may be 6-8 permanent infoldings
of the wall.
The longitudinal muscles may be inserted into the circular muscles or into the
epithelium.
The contraction of circular and longitudinal muscles helps in the passage of food.
Outside the muscle layers is a delicate connective tissue sheath.
The foregut consists of Pharynx, Esophagus, Crop, Proventriculus.
PHARYNX:
The first part of the foregut is the pharynx which connects anteriorly with the buccal
cavity.
In general the pharynx is that part of the foregut that lies within the head.
The pharynx has associated with it a set of muscles called precerebral dorsal dilator
muscles of the pharynx (or) dilators of the pharynx which arise ventrally on the
tentorium and dorsally from the frons.
They insert on the pharynx, posterior to the frontal ganglion.
These are best developed in sucking insects, especially in Lepidoptera and
Hymenoptera which use the pharyngeal pump to draw up fluids.
They are also present in biting and chewing insects and play a part in passing food
from the mouth to the esophagus.
In some insects the pharynx is divided into 2 regions called the anterior pharynx and
the posterior pharynx.
ESOPHAGUS:
The next part of the foregut is the esophagus which is an undifferentiated part of the
foregut serving to pass food from the pharynx to the crop.
In the simplest case it is a simple tube leading from the pharynx to the midgut.
Sometime the esophagus is regionally differentiated, in such case only the tubular
region lying just posterior to the pharynx is called the esophagus.
CROP:
The crop or ingluvies is an enlargement of the foregut in which food is stored.
Usually it represents the posterior part of the esophagus, but in some fluid feeders
(Diptera and Lepidoptera) it is a lateral diverticulum.
Often the crop is folded longitudinally and transversely when it is empty. In general,
secretion and absorption do not occur in the crop, being limited by the impermeable
intima.
Digestion can occur, as a result of salivary enzymes passing along the food and
midgut enzymes being regurgitated from the midgut.
PROVENTRICULUS:
Proventriculus can be quite variable. In fluid feeders it is often absent except a
simple valve at the origin of the midgut.
A valve is also present in many other insects and often the circular muscles form a
sphincter at the entrance to the midgut.
This valve or sphincter is called the esophageal or cardiac valve.
The proventriculus often has on its internal surface sclerotized structures (cuticular
intima modified into tooth like denticles) for the grinding of food.
For example, in the cockroach and cricket the intima is developed into six strong
plates or teeth, which serve to break up the food.
The proventiculus as a whole controls the passage of food from the crop to the
midgut.
The proventriculus is very specialized in bees, it helps to remove pollen from the
nectar.
Writhing movements of the crop keep the pollen dispersed while the lips of the
proventriculus make snapping movements in such a way that the spines strain off
the grains of pollen and retain them.
In this way a bolus of pollen is formed and is then passed back through the
proventriculus to the midgut. Nectar is retained in the crop for regurgitation and
processing to form honey.
MIDGUT:
The middle section of the alimentary canal is the stomach and is commonly called
the ventriculus.
This is the portion of the alimentary canal where digestive juices are secreted, hence
most of the digestion takes place here.
This portion of the alimentary canal begins at the posterior foldings of the
stomodaeal valve, the line being marked by the termination of the stomodaeal
intima.
Posteriorly the midgut ends just anterior to the Malpighian tubules.
The midgut is usually a simple tube, undifferentiated except for the presence of 4, 6,
or 8 caeca at the anterior end called gastric caecae.
Gastric caecae is finger like outgrowths found in the anterior or posterior part of the
midgut, they increase the functional area of the midgut and shelter symbiotic
bacteria in some insects.
In some Diptera, the midgut is differentiated into an anterior cardiac valve
(Proventriculus) and a long ventriculus.
The most characteristic cells of the midgut epithelium are tall and columnar with
microvilli forming a striated border bounding the lumen.
Typically, the basal membrane is very deeply infolded and large numbers of
mitochondria are associated with the folds.
Rough endoplasmic reticulum is also often extensive and it is probable that this is
concerned with enzyme production.
The epithelial layer of midgut consists of three types of cells, they are:
Columnar cells:
They are concerned with enzyme secretion and with absorption, they are columnar
in shape and releases enzymes through the microvilli arranged in a brush border.
Secretion may involve the complete breakdown of the columnar cell.
Secretion that involves the complete breakdown of a cell is called holocrine
secretion.
When the cell does not breakdown completely, but recovers and functions again, the
process is known as merocrine secretion.
Goblet cells:
They probably play a role in secretion, but they appear to also be involved in storage
and excretion.
They are found in some insects like Lepidoptera caterpillars, Ephemeroptera, and
Plecoptera, in addition to the columnar cells.
Regenerative cells:
These epithelial cells are involved in the production of new columnar cells to replace
the old ones which is been broke down during the holocrine secretion process. These
regenerative cells are either scattered or in groups called nidi.
The muscle layers of the midgut are usually poorly developed, but the circular
muscles lie adjacent to the epithelium. The muscle layers are bounded by a delicate
connective tissue sheath.
PERITROPHIC MEMBRANE:
Midgut consists of an inner delicate membrane called peritrophic membrane
secreted by 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.
The peritrophic membrane forms a coat over the food particles and no damage will
occur to epithelial cells of midgut.
This layer is said to be permeable to the digestive enzymes and the products of
digestion.
It is absent in case of sap sucking insects.
FILTER CHAMBER:
This is a characteristic arrangement of the midgut in hemipteran insects (fluid
feeders).
Anterior part of midgut forms a thin-walled bladder (i.e.) filter chamber Which is
closely bound to either posterior part of midgut or the anterior hindgut and
Malpighian tubules.
Filter chamber enables the excess fluids including sugar in the food to pass directly
from the anterior part of the midgut to the hindgut without passing through the
middle portion of midgut.
Thus preventing excessive dilution of haemolymph. Enzymes and facilitate better
enzyme activity.
In aphids, the honey dew (rich in sugars) is the substance that is being excreted after
passing through the filter chamber.
HINDGUT:
The hindgut is lined by a layer of cuticle which is thinner and more permeable than
that of the foregut.
The epithelium generally is thin, but the cells are more cuboid than in the foregut
while those of the rectal pads are tall with a clear cytoplasm.
The musculature is poorly developed except around the rectum, but the longitudinal
muscles are usually external to the circular.
Along the rectum the longitudinal muscles are often collected into strands opposite
to the gaps between adjacent rectal pads.
It consists of pylorus, ileum and rectum.
PYROLUS:
The pylorus is the first part of the hindgut and from it the Malpighian tubules often
arise.
In some insects it forms a valve between the midgut and hindgut.
This valve is called the pyloric valve or the proctodaeal invagination.
It is on the pylorus that the Malpighian tubules open.
ILEUM:
The hindgut in general is divided into two main regions; the first is the anterior
intestine.
Sometimes the anterior intestine may be divided into an anterior ileum and a
posterior colon.
In most insects the ileum is an undifferentiated tube running back to the rectum.
But in some termites it forms a pouch consisting of a specialized cells called the
mycetomes it consists of flagellates concerned with the production of cellulase
enzymes.
In some larval Scarabaeoidea there is a comparable fermentation chamber in which
the intima is produced into spines.
It is believed the ileum is involved in the removal of water from the haemolymph in
Heteroptera.
In some Diptera it is thought to aide in the excretion of ammonia.
RECTUM:
The second main region of the hindgut is the posterior intestine or rectum.
The rectum is the terminal structure and opens externally through the anus.
The rectum is often an enlarged sac and is thin walled except for certain regions, the
rectal pads, which have a columnar epithelium.
There are usually 6 rectal pads and they may extend longitudinally along the rectum
or they may be papilliform.
Each pad may consist of a single layer of cells (Odonata, Orthoptera) or there may be
2 layers (Neuroptera, Lepidoptera, Hymenoptera).
The pads have a good tracheal supply indicating a high level of metabolism.
The rectum, and in particular the rectal pads, are important in the reabsorption of
water, salts and amino acids from the urine.
In addition in some aquatic insects, there are tracheal gills in the rectum.
In larval Anisoptera water is pumped in and out of the rectum so that the water
round the gills is constantly renewed, and by the forcible ejection of water the insect
is able to propel itself forwards rapidly.
PROCESS OF DIGESTION:
The process of digestion involves five steps they are,
Ingestion:
Food is partially digested in the oral cavity with the help of salivary enzymes.
In insects like fluid feeders, carnivorous Hemiptera, blowfly larvae etc., digestion
occurs outside the intestine by a process called extra intestinal (or) extra oral
digestion.
Transportation:
The food material entered in to the esophagus is transported in to the crop by
muscular activity i.e. by peristatic movements.
Food moves continuously from esophagus in to the crop where it is stored.
From the crop food enters in to the gizzard where it is still broken in to very minute
particles with the help of denticles or the cuticular teeth.
Digestion:
From the gizzard through the stomodeal valve, food passes in to the midgut where
actually digestion starts.
The epithelial cells produce enzymes:
Proteases which break proteins in to amino acids
Carbohydrases breaking carbohydrates in to mono &disaccharides
Lipases breaking lipids in to fatty acids and glycerol.
In termites, digestion takes place in colon of hindgut where mycetome cells
containing the protozoans secrete the enzyme cellulase which can digest the wood
material rich in cellulose.
In scarabid beetle larvae, bacteria are involved in digestion.
In wood feeders, keratin digestion is facilitated by alkaline pH of midgut.
In Tineola (cloths moth), keratinase secreted by protozoans.
Absorption:
Midgut epithelial cells absorb the nutrients from the digested food and pass on the
faecal matter and undigested food material in to the hindgut.
The cells of the hindgut are also involved in the reabsorption of water, salts and
other metabolites from the faecal matter.
Egestion:
The waste food material or the faecal matter is discharged through the anus.
REFERENCE:
The Insects Structure and Function, R.F. Chapman, fifth edition.
Insect morphology- Digestive system.
https://www.slideshare.net/sureshjambagi2/digestive-system-in-insects
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