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Nutrition, Digestion, Excretion

• Nutritional Ecology– Essential nutrients

• The Digestive System• The Excretory System

What nutrients are essentialfor insects?

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Water

• This is the ultimatechallenge for manyterrestrial insects.– Drinking or moisture

in food.– Oxidative

metabolism.– Absorption of water

vapor.

Energy• Oxidation of

carbohydrates, fats,organic acids, suitableamino acids.

• Each are variablyavailable to differenttypes of insects.

• Requirements can bequite high: Certaininsect flight musclesconvert more energyper unit weight than anyother animal tissue.

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Essential Amino Acids

• Insects need at leastthe same 10 aminoacids in their diet as wedo.– Predators have little

problem with this.– Phytophagous insects

more of a problem.– Particularly sap-suckers.– Adam will talk more

about this on Friday.

Essential Lipids• Insects are unable to

synthesizepolyunsaturated fattyacids– Involved in formation of

phospholipids of cellmembranes.

• Also sterols (unlikemammals).– Required for many

hormones.– Derived from cholesterols

(animal food) or β-sitosterol (plant food)

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Vitamins and Growth factors

• Vitamin Bsparticularlyimportant.

• Vertebrate blood isparticularly low inthese (which insectscare?)

• How do they get it?

Minerals

• Requirementsessentially the sameacross animalkingdom (e.g. wecannot synthesizethem).

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The Insect Gut• The insect’s digestive

system & excretorysystem will reflect the dietin much the same waythat mouthparts do.

• Considerable variation isbuilt around a commontheme.

• Many of these functionaldifferences areanalogous to differenceswe see across vertebratediversity…

Gross Gut Morphology• Foregut: Processing and storage of food.• Midgut: Digestion and absorption of food.• Hindgut: Absorption of water, salts,

elimination of wastes.• Sound familiar?

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Gross Gut Morphology• Note that the foregut and the hindgut are

lined with cuticle (derived from ectoderm).• Midgut is not, instead lined with peritrophic

membrane (derived from endoderm).

Foregut

• Mouth and oral cavity: consumption of food.– Ventral glandular salivarium– Dorsal muscular cibarium

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Foregut

• Mechanical processing of food in pharynx andproventriculus (gizzard).

• Storage in crop.

Midgut• Most digestion of food occurs here.• Two main areas:

– Gastric caeca often house endosymbionts.– Ventriculus where most digestion occurs.

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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.• Why would insects do this?

Why?

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Peritrophic membrane

• Numerous insectpathogens centeractivity onperitrophicmembrane.

• Including Bt:genetically derivedinsecticide fromBacillusthuringiensis.

Peritrophic membrane1. Insect eats Bt crystals and

spores. Enzymes areactivated by proteolyticenzymes in the insect gut.

2. The toxin binds to specificreceptors in the gut and theinsects stops eating.

3. The crystals cause poresto open in the peritrophicmembrane, allowingspores and normal gutbacteria to enter the body.

4. The insect dies as sporesand gut bacteria proliferatein the body.

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Hindgut & Malpighian Tubules:Water conservation and excretion.

Hindgut & Malpighian Tubules:Water conservation and excretion.

• Intimately involved in osmoregulationand elimination of wastes (especiallynitrogenous).

• Re: insects have open circulatorysystem, therefore, no kidney ornephridia.

• Aquatic insects also have chloride cellsto actively pump in ions.

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Malphigian Tubules• Outgrowth of hindgut,

ectodermal in origin.• Dead end tips open into

hemolymph• Transport epithelium

secretes nitrogenouswastes and solutes intotubules: the filtrate.

• Water follows, how?• These are delivered to

the hindgut.

The Hindgut• Parts of rectal

epithelium arethickened to form rectalpads.

• These are specializedfor absorption of waterfrom feces beforedefecation.

• Active transport of ionsacross these cells setsup osmotic gradient,water is reabsorbed.

• How are nitrogenouswastes excreted?

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Cryptonephridia• Some desert insects have

intimate associationbetween Malpighiantubules and hindgut.

• Bounded by perinephricmembrane.

• Allows extremeconservation of water,including absorption ofwater from humid air in therectum.

Some unusual diets…

• Insects can digest someabundant, yet resistantcompounds.

• Some moths andbeetles can feed onkeratin.– Requires enzyme, low

oxygen environment toreduce sulfur bonds, andreducing agent.

Clothes moths (Tineidae) are also known toscavenge horn, hooves, and even tortoise shells.

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Some unusual diets…

• Beeswax is ordinarilyresistant to digestion.

• But wax moths can eatit: have a highly basicgut.

Wax moths (Pyralidae) are considered apest by beekeepers

Some 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 interactionswith bacteria or fungi.

– Some exogenous consumptionof fungi to obtain cellulases.

– Some only consume rarestarch, sugar, or whole cellwalls in wood tissue, not ligninitself.

Asian longhorn beetle house an endosymbioticfungus that produces cellulolytic enzymes

Termites and wood-roaches are the onlyinsects known to convincingly produce their

own cellulotyic enzymes