Insect Nutrition and Digestion

Introduction

• Insect evolutionary success and diversity are due to their ability to occupy ecological niches and use different food sources.
• Newly hatched insects need to find food fast or they will die.
• With sufficient food, larvae can grow, molt, and become adults.
• Processing food and the alimentary canal function are critical activities for their survival.
• Insect food and feeding habits are diverse, resulting in high gut structure and function diversity.
• Insects use their digestive system to extract nutrients from the food they consume.
• Most food is ingested as macromolecules and complex substances such as proteins, polysaccharides, fats, and nucleic acids.
• Digestion is the process of breaking down macromolecules into smaller molecules via catabolic reactions for energy, growth, or reproduction.
• All insects possess a complete digestive system.
• A complete digestive system means that food processing happens inside a tube-like compartment, also known as the alimentary canal.
• The alimentary canal runs lengthwise from the mouth to the anus.
• Normally ingested food travels in one direction.

Solid vs Liquid Diet

• The most likely ancestral feeding behavior was that of a general scavenger, like modern-day cockroaches.
• Ancestral feeding behavior was followed by the evolution to phytophagous or carnivorous feeding.
• Generalist feeder guts are simple, not convoluted, and around the same length as their bodies.
• Evolution and adaption to new foods led to the evolution of gut structure and function.
• Insect food can be separated into categories of solid vs. liquid, and plant vs. animal origin.
• Solid foods are broken up mechanically with mandibles and grinding via a muscular proventriculus.
• Solid feeder guts tend to be straighter and not much longer than their bodies because the food cannot pass easily through a convoluted gut.
• Liquid diets can pass easily through a convoluted gut.
• Diets can be dilute, which causes problems like getting rid of excess water and sugar in the diet.
• Insects that consume dilute liquid food have adapted to deal with the excess water and other components such as salts or sugars.

Plant vs Animal Origin

• Plant food has a lower nutrient quality than animal sources, it requires a large intake and results in steady elimination such as frass, frass droppings, phloem, or xylem sap.
• Plants give insects sufficient carbohydrates, lipids, and phytosterols for proper nutrition.
• Plant tissues have lower levels of amino acids compared to animal tissues. Some amino acids can be critically low or absent.
• Some amino acids, vitamins, and dietary components are supplied by symbionts and do not always need to come from food.
• Animal feeders feed less frequently and at irregular intervals, so they have guts specialized for storage.
• Praying mantises have large crops to feast when food is available to hold a large meal suitable for digestion over time.
• Insects that eat animal based foods get a better balance of amino acids than those that eat plants.
• Additionally, animal tissues are rich in carbohydrates, cholesterol, and other lipids.

Major Structural Regions of the Gut

• A long, muscular, and enclosed tube that extends from the mouth to the anus is also called the Digestive gut or Alimentary Canal.

Foregut

• A foregut develops from invaginating ectodermal tissue at the anterior end of the body during an insect's embryonic development
• Foregut epithelial cells secrete a cuticular lining attached to the surface of cells, also known as the lumen side.
• This lining contains chitin and proteins, which is the same as the epicuticle and endocuticle on the body surface.
• Heavily sclerotized regions of the foregut lining contains a hard exocuticle.
• The foregut consists of the buccal cavity (mouth), pharynx, esophagus, crop, proventriculus, and esophageal invagination. Any of these features can be highly modified.
• The mouth, or buccal cavity, is an enlarged opening that receives slightly chewed food in mandibulate insects or fluid ingested by insects with piercing and sucking mouthparts.
• Salivary glands secrete fluid and carbohydrate-digesting enzymes, mostly amylases, into the mouth.
• Salivary secretions contain amylase, lubricate food, and contribute to digestion in the crop.
• The pharynx passes food to the esophagus, which may be a simple tube that continues to the proventriculus.
• The foregut can expand into an enlarged crop, or the crop can be a diverticulum from the main part of the foregut, like in some diptera.
• The crop releases contents periodically through the proventriculus before entering the midgut.
• At the end of the foregut, the proventriculus can be muscular, with heavily sclerotized teeth, ridges, and spines used for grinding food. It can also be a simple valve.
• Esophageal valves, or cardiac sphincters, are flaplike or valve like extensions of the proventriculus.

Midgut

• The midgut is the primary area that digestive enzymes are secreted for digestion and absorption in most insects.
• Gastric caeca arise at or near the origin of the midgut, or at various points.
• Gastric caeca are a major site for absorption and digestive enzyme production.
• Midgut cells secrete a chitin and protein-containing membrane called the peritrophic matrix that surrounds the food and protects the cells from rough food particles.

Hindgut

• The hindgut develops from ectodermal tissue during the embryonic stage, and also like the foregut, hindgut cells have a cuticular lining on their surface.
• Malpighian tubules usually mark the beginning of the hindgut.
• The junction between the mid- and hindgut is called the pylorus.
• The part of the hindgut after the Malpighian tubules is called the ileum.
• Sometimes the ileum grades into the rectum, but in some insects, there is a distinct middle region called the colon.
• The terminal part of the hindgut is the rectum.
• The entire hindgut has a chitinous lining on the lumen surface of the cells, sometimes referred to as an intima.
• Contents of the hindgut are usually fluid as they pass into the rectum.
• The rectum reabsorbs water, ions, and dissolved substances from the primary urine flushed into the hindgut by the Malpighian tubules.
• Water recovery by the rectum causes dry frass or fecal pellets that are characteristic of many terrestrial insects.

Microvilli

• Microvilli at the apical surface of midgut cells increase the surface area for enzyme secretion and absorption.
• Microvilli have been described as a brush border or a striated border.
• A brush border gives the appearance of many closely spaced, very fine, and short hairs.
• A striated border gives the appearance of less numerous stick-like extensions from the apical surface.

Glycocalyx

• Most insects do not have a mucus lining comparable to vertebrates in the midgut.
• Glycocalyx is a viscous secretion consisting of proteins and carbohydrates.
• It is present on the surface of and between microvilli.
• The viscous glycocalyx traps and concentrates secreted enzymes and products during digestion.

Peritrophic Matrix

• The peritrophic matrix (PM), also called the peritrophic membrane, surrounds the food in the midgut.
• The PM serves as a shield to protect microvilli from physical contact with food particles.
• Type I PM is secreted as a continuous delamination along the length of the midgut and most insects that produce PM produce this type.
• Type II PM is secreted from a ring of cells at the anterior margin of the midgut, similar to a stocking, and it is continuously secreted as food is pushed into it from the foregut.

Digestive Enzymes

• Constitutive secretion occurs when enzymes are released from the cells as soon as they are synthesized.
• Regulated secretion occurs when the enzyme is synthesized and stored, often as a zymogen.
• A zymogen is a protein that contains a peptide sequence that prevents enzymatic activity until the sequence is removed and a signal is received to release it.
• Most insects use constitutive secretion instead of regulated.

Gut PH

• The gut pH is highly variable in different insects.
• The pH of a gut segment helps to dictate the actions of enzymes secreted into the segment.
• Gut pH can affect the solubility of ingested components, toxicity of toxins, and the microorganisms present.
• Carbohydrate-digesting enzymes work best at near neutral pH or slightly acidic conditions. Lipases work best at alkaline pHs near 8.
• Termites, crickets, and possibly other insects that have hindgut fauna to digest cellulose have very acidic conditions prevail in the special hindgut regions
• Anaerobic fermentation of glucose from cellulose digestion causes these acid conditions.
• The process of anaerobic fermentation results in production of short-chain fatty acids including acetic, propionic, and butyric acids.

Hematophagy

• Blood feeding has evolved independently in the course of insect evolution.
• The transmission of disease from vector to humans is of particular concern to humans.
• Nearly 14,000 species of arthropods in 400 different genera evolved the ability to feed upon vertebrate blood. But considering that there is over a million known species of insects, the number that feed on vertebrate blood is actually very small.