Digestion and Excretion in Insects

Digestion and Excretion

The Alimentary Canal

• Protostomes possess a complete alimentary canal.
• A complete system includes both a mouth and an anus.
• Digestion occurs through enzymatic hydrolysis.

Embryonic Development & Specialization of the Alimentary Canal

• A. Stomodeal and proctodeal inpouching occur.
• B. Primordial mesenteron forms at ends of inpouching.
• C. Mesenteral elements combine to form a blind sac.
• D. Regions between stomodeal, mesenteral, and proctodeal split to form a continuous tube; further specialization occurs.
• The alimentary canal forms from two openings.
• The stomodeal and proctodeal areas are lined with chitin, while the mesenteral area is not.

Basic Pattern of the Alimentary Canal

• Foregut
  • Originates from stomodeal invagination.
  • Lined with chitin.
  • Very little digestion occurs here.
• Midgut
  • Formed from mesenteral growth.
  • Not lined with chitin.
  • Main area of digestion.
• Hindgut
  • Formed from proctodeal invagination.
  • Lined with chitin.
  • Permeable to some substances.

Generalized Insect Alimentary Canal

The Insect Alimentary Canal consists of the following:

• Preoral Cavity
• Cibarium
• Pharynx
• Esophagus
• Crop
• Proventriculus
• Ventriculus
• Gastric Caeca
• Stomach
• Intestine
• Rectum
• Anus

Components and Their Functions

• Preoral Cavity – salivary duct opens here
• Cibarium – important in food ingestion
• Pharynx
• Esophagus – conducts food rearward
• Crop – pre-digestive storage
• Proventriculus – valve; grinding
• Ventriculus – enzyme secretion
• Gastric Caeca – tubes that aid in digestion; house microbes
• Stomach – lined with peritrophic membrane
• Intestine – receives waste from midgut; excretions from the Malphigian Tubules
• Rectum – reabsorbs water and salts from waste products
• Anus – exit for fecal pellet

Digestion of Solid Food

• Grinding of food takes place in the pre-oral cavity.
• Labial and Salivary glands lubricate the food and aid in carbohydrate digestion (very limited).
• Peristalsis moves food from the mouth and down the esophagus and into the crop for storage.
• Proventriculus serves as a valve that controls food movement into the midgut.
• In many insects, especially those that have a diet high in cellulose, the proventriculus has a grinding surface.
• Enzymatic digestion picks up in the ventriculus.
• Gastric caecae extend from the ventriculus to increase surface area.
• Digestion occurs through the action of carbohydrases, lipases, and proteinases.
• Specialized enzymes in many insects:
  • Hemicellulase (leaf-cutter ants)
  • Collagenase (carrion flies)
  • Keratinases (clothes moths)
• Gut microbes:
  • Cellulase from protozoans (termites)
  • Wax enzymes from bacteria (wax worm)
• Absorption of food from the midgut into the hemolymph is either passive or active.
• Pylorus, ileum & colon permeable to water, salt, sugar, amino acids.
• Waste material is dehydrated as it passes through hindgut.
• Fecal material is passed through the rectum.
• The peritrophic membrane is a special membrane lining of the midgut that protects the epithelial cells from damage.
• It is formed by cellular secretions.
• Encloses the food but is permeable to allow for digestion.
• Continually formed as digestion occurs.
• Moves with food along the midgut and into the hindgut.
• Breaks down upon entering the pylorus.

Digestion of Liquid Food

• Special modifications are required to deal with the increase in fluids and potentially a decrease in nutrients.
• Mouthpart morphology:
  • Externally modified into a tube to imbibe fluids.
  • Internally, musculature is modified and/or exaggerated to form a pump.
    • Cibarium – contraction of clypeal muscles cause the cibarial space to increase and draw in fluids.

Gut Morphology

• Modifications that allow for concentration of food:
  • Pouch (diverticulum) off of the esophagus.
    • Allows for liquid storage.
  • Compartmentalization of the ventriculus into multiple chambers.
    • Specialized for particular digestive functions.
  • Filter Chamber
    • Anterior and posterior sections of the alimentary canal are in close contact with each other.
    • Allows for passive diffusion of water out of the gut.

Fat Body

• Present in many insects.
• Amorphous white/yellow tissue.
• Surrounds alimentary canal but present mostly in abdomen.
• Trophocyte:
  • Responsible for all metabolic & storage functions (below).
  • Produced as necessary – little fat body indicates periods of development or starvation.
• Many metabolic functions:
  • Metabolism of carbohydrates, lipids, and proteins.
  • Storage of glycogen, fat and proteins.
  • Synthesis & regulation of blood sugars.
  • Synthesis of major hemolymph proteins.

Symbionts

• Insects ingest many compounds they can’t metabolize
• Symbionts – bacteria, yeast, other fungi, or protozoans
• Extracellular
  • Free in gut/diverticulia or housed in special chambers
  • Termites = bacteria, fungi and protists; digest lignan and fix nitrogen
  • Proctodeal feeding
• Intracellular
  • In gut epithelium or specialized cells (mycetocytes and bacteriocytes)
  • Wolbachia which is notable for significantly altering the reproductive capabilities of its hosts
  • Transovarial transmission

The Excretory System

• Malphigian Tubules
  • Blind tubes, one cell thick
  • Central lumen empties into the midgut/hindgut junction,
  • Usually bathed in hemolymph
  • In close proximity to the fat body
• Functions
  • Elimination & detoxification of metabolic by products
  • Absorb solutes, water, and wastes from the surrounding hemolymph
  • Releasing wastes from the organism in the form of solid nitrogenous compounds
• Mode of Operation
  • Amines, salts (Na, K) and water are removed from the hemolymph and pass into the lumen of the tubules
  • Salts and water are reabsorbed in the proximal portion of the tubules (and/or in rectum), as needed.

Composition of Urine

• Uric Acid – most common nitorgenous product in terrestrial insects; conserves water because
  • Relatively non-toxic – can be concentrated without causing damage, so requires less water to carry it
  • Relatively soluble – easier to concentrate, allowing water removal
  • Low H/N ratio – more H is saved, so more metabolic water is available, but it is more expensive to make
• Ammonia – commonly excreted by aquatic and semi-aquatic insects; does not conserve water
  • Poisonous – requires a lot of water to carry it
  • Highly soluble – easiest to concentrate, allowing for excess water removal
  • High H/N ratio – less H is saved, metabolic water is not an issue, it is cheap to make

Other Excretory Systems

• Labial Glands
  • take the place of Malphigian tubules in Collembola, Diplura and Aphids
• Hidgut epithelium excretion
• Storage excretion
  • Nephrocytes (cells of the hemolymph)
    • sieving the hemolymph of certain compounds;
  • Fat body
    • urate storage cells
  • Epidermal cells and the cuticle
    • store excess compounds