In this lesson, we'll switch our attention to what goes on inside an insect's body starting with the digestive and excretory system. As with many animals, insects have a complete digestive tract. This means that the digestive system consists of a single complete tube known as the alimentary canal. The alimentary canal is only a single cell layer thick and rests on a thin fibrous extracellular matrix of tissue called the basement membrane surrounded by the gut muscles. The structure of the alimentary canal varies with the insect's diet. Insects that feed on solid food typically have a wide, short, and straight alimentary canal. Strong muscles and specialized membranes line the inner cavity to protect the gut from abrasions. Insects that feed on liquid diets have alimentary canals that are long, narrow, and convoluted to maximize the surface area in contact with the liquid. Liquid diets have a high water content, so fluid feeding insects have mechanisms to remove excess water and concentrate the nutrients. The nutritional properties of an insect's diet also require modifications to the alimentary canal. For example, land feeding insects need to process large amounts of nutrient poor food in the form of leaves and stems, which are typically continuously available. There is therefore little need for the alimentary canal to have high storage capacities, and so the digestive tracks of plant feeding insects are usually short. Conversely, a diet of animal tissue is nutrient rich, but its availability is intermittent. Insects that feed on other animals tend to require a large food storage capacity in a long alimentary canal to sustain themselves when resources are scarce. The insect digestive system is divided into three easy to remember regions; foregut, midgut, and hindgut. These regions are separated by valves and sphincter muscles that control the movement of food. Each region of the alimentary canal has specialized functions. The foregut is where food is ingested and physically broken down, and may also be the site of food storage. Most digestion and absorption of nutrients occurs in the midgut, while absorption of water, salts, and remaining nutrients occurs in the hindgut. The function and structure of the three gut regions differ. The foregut and hindgut have a cuticular lining that must be shed with each molt, while in the midgut where nutrient absorption occurs, there is no cuticular lining. To see how the insect digestive system works, we follow the journey of a piece of solid food through the alimentary canal of a typical insect starting at the foregut. The foregut is the anterior portion of the alimentary canal and is where ingestion, storage, transport, and the physical breakdown of food occurs. It has a protective cuticular lining that is shed at each molt. Food manipulated or chewed by the mouthparts is passed into the mouth where digestion begins with the aid of enzymes in the saliva produced by salivary glands. The food forms a bolus within the muscular pharynx after which it moves through the esophagus and into the crop where it can be stored temporarily. The food then moves into the proventriculus where it can be mechanically broken down even further by rows of cuticular teeth. This is especially important for insects that feed on tough or fibrous materials such as leaves. Not all insects have a proventriculus lined with tooth-like structures, especially if the diet consists solely of a liquid food such as nectar or blood. After being shredded in the proventriculus, bits of food then pass on into the midgut. This is the major site of digestion and nutrient absorption in insects, where carbohydrate and protein polymers in insect food are broken down into simple constituents by enzymes. For optimal enzymatic activities, conditions in the midgut are usually maintained at a pH between 6.0 and 7.5. Nutrients from digested food are selectively absorbed by the midgut epithelium and transported around the insect's body by the insect's blood. Nutrient absorption is maximized by an increase in midgut surface area caused by outpockets at the anterior end of the midgut known as the gastric caeca. The midgut has no cuticular lining, but the midgut epithelial cells secrete a cocktail of digestive enzymes and also a structure called the peritrophic membrane. This structure protects the delicate midgut epithelium from damage and also compartmentalizes the digestive process. This structure generally moves with the food bolus and is excreted with the solid waste material. Therefore the midgut epithelial cells need to secrete the peritrophic membrane continuously while the insect is actively feeding. Some liquid feeders such as the Hemiptera and Thysanoptera or Thrips lack a protective peritrophic membrane in the midgut because a liquid diet is less likely to damage the gut lining. Nutrient absorption of liquid diets is facilitated by an extracellular lipoprotein membrane called the perimicrovillar membrane. The insect body often hosts a number of beneficial microorganisms known as symbionts. They can be found all over the insect; on the surface of the body, within specialized cells in the body cavity, and especially within the alimentary canal. Symbionts that live in the alimentary canal help with metabolic processes and can include bacteria, fungi including yeasts, and tiny single-celled organisms called protists. Some symbionts help digest material that insects are unable to handle alone such as cellulose, whereas others synthesize essential vitamins and amino acids, and can even detoxify poisons. These beneficial symbionts can be transferred between individual insects via ingestion or from mother to offspring through vertical or transovarial transmission. Gastric symbionts are not found in predatory insects. At the junction between the midgut and the hindgut of most insects are the malpighian tubules, tubes that are extensions of the hindgut that permeate into the insect's body cavity. The primary function of these tubules is to remove nitrogenous waste products and maintain osmoregulatory balance in the insect. The distal ends of the malpighian tubules indiscriminately remove substances dissolved in the insect's blood. Any waters, sugars, or salts initially removed will then be reabsorbed either at the proximal end of the malpighian tubules or in the hindgut before excretion. Osmoregulation is important in all insects as desiccation or drying out is a constant risk. Terrestrial insects excrete nitrogenous wastes in a highly concentrated form called uric acid. This nitrogenous waste has low toxicity and can be excreted with very little water. The last region of the alimentary canal is the hindgut, which concentrates the waste products, and absorbs water, salts, and essential nutrients from the remainder of the food bolus. After resorption, everything left in the hindgut is excreted from the rectum via the anus as frass. Like in the foregut, the hindgut has a cuticular lining that must be replaced when the insect molts. Digestion is also aided by a structure located outside of the alimentary canal called the fat body. The fat body is essentially a network of fatty tissue located throughout the insect's abdominal cavity. It helps to metabolize large macromolecules like fats, proteins, and carbohydrates, and is also an essential storage organ from which the insect draws nutrients during non-feeding parts of its life cycle, such as when a butterfly is in its cocoon. The fat body can contribute up to 65 percent of an insect's entire weight and is often present in large amounts in reproducing females, migrating individuals, and diapausing insects. With that, we have come to the end of the lesson on the insect digestion and excretory systems. We hope our journey through the gut has left you well-acquainted with the structures of the insect's alimentary canal, and how they vary in different types of insects. Up next, the circulatory and gas exchange systems.
