The insect's digestive system is a closed system, with one long enclosed coiled tube called the alimentary canal which runs lengthwise through the body. The alimentary canal only allows food to enter the mouth, and then gets processed as it travels toward the anus.

1. Watentena Amos
April, 2018
wateamos@gmail.com
Insect Digestive System

2. Essential Nutrients for Insects
Water
This is the ultimate challenge
for many terrestrial
insects
 Drinking or moisture in
food
 Oxidative metabolism
 Absorption of water vapor

3. Carbohydrates
 Oxidation of
carbohydrates, fats,
organic acids and suitable
amino acids provide energy
 Requirements can be quite
high; certain insect flight
muscles convert more
energy per unit weight
than any other animal
tissue
 Varies in different insects

4. Essential Amino Acids
 Insects need at least the
same 10 amino acids in
their diet
 Predators have little
problem with this
 Phytophagous insects,
particularly sap-suckers
have problems of amino
acids

5. Essential Lipids
Insects are unable to
synthesize
polyunsaturated fatty
acids; Involved in
formation of phospholipids
of cell membranes
Unlike in mammals, sterols
are required for many
hormones; they are
derived from cholesterols
(animal food) or β-
sitosterol (plant food)

6. Growth factors (Vitamins)
 Vitamin Bs particularly are
important
 Vertebrate blood is
particularly low in these

7. Minerals
Requirements essentially the
same across animal
kingdom

8. The Insect Digestive System
 Foregut: Processing and
storage of food
 Midgut: Digestion and
absorption of food
 Hindgut: Absorption of
water, salts, elimination of
wastes
 the foregut and the
hindgut are lined with
cuticle (derived from
ectoderm)
 Midgut is not, instead lined
with peritrophic membrane
(derived from endoderm)
 The insect’s digestive
system will reflect the
diet in much the same way
that mouthparts do
 Considerable variation is
built around a common
theme
 Many of these functional
differences are analogous
to differences we see
across vertebrate
diversity

9. Functions of the Gut
 Obtaining food
 Mechanically breaks good down into smaller particles that
facilitate the activity of digestive enzymes
 Enzymatic breakdown of larger food molecules into
molecules that can enter the hemolymph
 Produce molecules that coordinate feeding and other
digestive activities

10. FOREGUT MIDGUT HINDGUT

11. pharynx
esophagus
crop
Malpighian tubulessalivary glands
cibarium
salivarium
gastric caeca
proventriculus
pylorus
ileum
rectum
ventriculus
anus

12. Modifications of the Gut

13. The Fore Gut

14. Functions of the Fore Gut
Mouth and oral cavity:
consumption of food
 Ventral glandular
salivarium
 Dorsal muscular cibarium
Mechanical processing of
food in pharynx and
proventriculus (gizzard)
Storage in crop

15. The Salivery Glands Types of Salivery glands
Acinous (alveolar);
multicellular with sac-like
units directly innervated
from CNS
Tubular; not directly
innervated- seratonin
from hemolymph to
regulate enzyme
production

16.  Stomatogastric NS;
Produces seratonin for
enzyme production
 Subesophageal ganglion;
Produces dopamine for
fluid secretion

17. Functions of Salivary Glands
 Moisten food
 Lubricate mouthparts
 Contains digestive enzymes: Amylase – complex sugars to
simple sugars and Salivary enzymes for pre-oral digestion
 Non-digestive functions (production of toxins of
predators that affect CNS of prey, counter toxins of
plant allelochemicals, anti-coagulants and silk production)

18. Mosquito Saliva Composition
 20 polypeptides
 > 70 unique proteins

19. Insect Saliva Composition Changes
(Lutzomyia longipalpis)

20. Location of Salivary Glands
Cicada (Homoptera)

21. Proventriculus of Grasshopper
Intima

22. The Mid Gut
Most digestion of food
occurs here
Two main areas:
 Gastric caeca often house
endosymbionts
 Ventriculus where most
digestion occurs

23. Absorption of Digested Material

24. Absorption of Digested Material

25. Movement of H20
Movement of water, inorganic ions, sugars & amino acids
caecum
Malpighian
tubule

26. Peritrophic Membrane
 Secreted by microvillate columnar epithelial cells
 Made up of an amorphous sheet of polysaccharide, chitin,
glycoprotein, and protein
 Tubular film that surrounds the bolus and within which
considerable digestion occurs
 Numerous insect pathogens center activity on peritrophic
membrane
 Including Bt: genetically derived insecticide from Bacillus
thuringiensis

27. Peritrophic Membrane
Secretion Delamination

28. Peritrophic Membrane
Insect eats Bt crystals and
spores. Enzymes are
activated by proteolytic
enzymes in the insect gut
The toxin binds to specific
receptors in the gut and
the insects stops eating
The crystals cause pores to
open in the peritrophic
membrane, allowing spores
and normal gut bacteria to
enter the body
The insect dies as spores and
gut bacteria proliferate in
the body

29. Midgut Enzymes
Insect Diet Protease Lipase Amylase Invertase Maltase
Cockroach omnivorous √ √ √ √ √
Carausius phytophagous √ √ √ √ √
Lepidoptera
larvae
phytophagous √ √ √ √ √
adults nectar √
adults Non-feeding
Lucilia meat √ √
Glossina blood √

30. Cross section of a mosquito showing
the Fat body (FB)

31. Functions of the Fat Body
 metabolism of carbohydrates, lipids and nitrogenous
compounds
 storage of glycogen, fat and protein
 regulation of blood sugar
 synthesis of major hemolymph (blood) proteins
(haemoglobins, vitellogenins, storage proteins)

32. Hindgut & Malpighian Tubules

33. The Hind Gut
 This is a region of water
conservation and
excretion
 Intimately involved in
osmoregulation and
elimination of wastes
(especially nitrogenous)
 Note: insects have open
circulatory system,
therefore, no kidney or
nephridia
 Aquatic insects also have
chloride cells to actively
pump in ions.
Malpighian Tubules
 Outgrowth of hindgut,
ectodermal in origin
 Dead end tips open into
hemolymph
 Transport epithelium
secretes nitrogenous
wastes and solutes into
tubules: the filtrate
 These are delivered to the
hindgut

34. The Hind Gut
 Parts of rectal epithelium
are thickened to form
rectal pads
 These are specialized for
absorption of water from
feces before defecation
 Active transport of ions
across these cells sets up
osmotic gradient, water is
reabsorbed

35. Cryptonephridia
 Some desert insects have
intimate association
between Malpighian
tubules and hindgut
 Bounded by perinephric
membrane
 Allows extreme
conservation of water,
including absorption of
water from humid air in
the rectum

36. Unusual Insects, Unusual Diets
Wood regularly consumed by some
wood-boring beetles, termites,
wood-feeding roaches, and
silverfish
 Some endogenous production of
cellulases (wood-roaches,
termites)
 Most endosymbiotic interactions
with bacteria or fungi
 Some exogenous consumption of
fungi to obtain cellulases
 Some only consume rare starch,
sugar, or whole cell walls in wood
tissue, not lignin itself

37. Unusual Insects, Unusual Diets
 Beeswax is ordinarily
resistant to digestion; but
wax moths can eat it: have
a highly basic gut
 Clothes moths (Tineidae)
are also known to scavenge
horn, hooves, and even
tortoise shells
 Some moths and beetles
can feed on keratin

38. Thank You