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1 INTERNAL ANATOMY AND PHYSIOLOGY NOTES PART II. DIGESTION & EXCRETION An insect uses its digestive system to extract nutrients andd other substances from the food it consumes. Most food is ingested in the form of macromolecules and other complex substances (suchh as proteins, polysaccharides, fats, nucleic acids, etc.) which must be broken down into smaller, usable molecules (e.g. amino acids, simple sugars, etc). The processs of breaking down this food is digestion. Alll insects have a complete digestive system. This means that food processing occurs within a tube-like enclosure, the alimentary canal that runs from mouth to anus. Food travels from the mouth to the anus. An incomplete digestive system (see figure on the left; e.g. as found in hydra and starfish), on the other hand, has a single, pouch-like opening that serves as both mouth and anus. A complete digestive system, unlike an incomplete digestive system, allows for functional specializationn differentt parts of the system may be specially adapted for various functions of digestion, absorption, and excretion. In insects, the alimentary canal is divided into three functional regions: the foregut, midgut, and hindgut. Insects of different groups have a lot of ways of feeding and eat a lot of differentt kinds of foods. Cicadas feed on watery plant sap with their straw-like mouthparts. Bed bugs and female mosquitoes feed on vertebrate blood. Some ermites can feed on dry wood. Caddisfly larvae can feed on bacterial and algae. Insects have a diverse range of mouthpart types that are useful for their different diets. Gut structure and function also are matched well with the requirements of the different insect diets. Insects that take solid food usually have a wide, straight, and short gutt with strong muscles and protection from rough foods such as grasss and leaves. Insects feeding on liquids usually have long, narrow, and convoluted guts to allow maximal contact with the liquid food. The insect gut is formed from a one-cell-thick major regions in the gut: Foregut, Midgut, and Hindgut. layer. This sheet of cells is continuous with the epidermis. There are threee Includes material from the intro. entomology course taughtt by Dr. Douglas Emlen and from Thee Insects, Authors: Gullan & Cranston.

2 The foregut and hindgut are formed by invaginations of the epidermis so in some ways, items inside the gut in these sections are still outside of the animal. This is ectodermal tissue. These two regions are lined with cuticle which is thick and protective, but not sclerotized (hard). This soft protective lining is called the intima and must be shed at each molt! The midgut is not ectodermal in origin and not lined with cuticle. Instead, the midgut cells are protected by the peritrophic membrane, a papery matrix of protein-carbohydrates and chitin fibrils. It looks clear like saran wrap. The three main regions of the gut each have sphincters (muscular valves) controlling food movement between the regions. Below I describe a typical insect gut, though some species may be missing some of the components within each section. The foregut ingests, stores, grinds, and transports food. It includes the mouth and pharynx. The pharynx varies a lot across insects, but sucking insects have visceral muscles and pumps that pump liquid from the mouth into the pharynx. Outside the alimentary canal, insects have paired salivary glands that reside in the thorax, adjacent to the foregut. Salivary ducts lead from the glands to the reservoirs and then forward, through the head, to dump into the preoral cavity. The pharynx opens into the esophagus which is a storage area that widens into a crop at the posterior end. In some species, such as mosquitoes, the crop has several extensions blind sacs which store food called diverticula. One rare modification of diverticula: some insects shunt toxic plant resins out of the mainstream gut, where they are stored, and regurgitate them for defense (e.g. sawfly larvae). Highlight: Sawfly larvae Rothschild (1972) listed 43 species of insects in six orders that sequester toxins from their host plants. Sawfly larvae of Perga affinis feed exclusively on Eucalyptus leaves, and they have a diverticular pouch off the foregut in which they store an oily fluid that they regurgitate when attached, a habit that has earned them the name spitfires. The fluid consists of oils essentially identical to those in the Eucalyptus leaves on which the larvae feed. How the oils are extracted from the food and collected in the diverticular pouch is not known. When tested against birds, mice, and three species of ants, the fluid was an effective deterrent. The defensive effect of the secretion is enhanced by the spitfires habit of forming dense rosette clusters in Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston.

3 the daytime (when they are not feeding). With their heads directed outward and secreting the offensive fluid synchronou usly (Morrow, Bellas, and Eisner 1976) They group together by tapping. Thus, they use vibrational communication. The crop often contains symbionts, as do other parts of thee gut. See below. The crop then opens into the proventriculus (the gizzard) which is often hard with sclerotized teeth forr grinding. The stomodael valve sparates the foregut from the midgut. The midgut is the principal site for the production and secretion of digestivee enzymes, digestion, and absorption. The midgut consists of several diverticula called gastric caeca. These gastric caeca increasee the surface area for digestion and absorption. The rest of the midgut is the ventriculus (the stomach of the insect). Peritrophic membranee 1) helps protect the delicate, absorptive midgut epithelial cells, and 2) subdivides midgut space in a way that facilitates counter-currents a circular backflow of food and enzymes that greatly facilitates digestion. The midgut is derived from embryonic endoderm so it is NOT protected by an intima. Instead, the midgut is lined with a semipermeablee membrane secreted by a cluster of cells (the cardial epithelium) ) that lie just behind the stomodeal valve. This peritrophic membrane consists of chitin fibrils embeddedd in a protein-carbohydrate matrix. It protects the delicate digestive cells without inhibiting absorption off nutrient molecules. The hindgut commences with the pylorus, where the Malpighian tubules feed in. The pylorus has a muscular valve, the pyloric sphincter. The hindgut continues ileum, colon, and rectum. The hindgut function is absorption of water, salts from faeces and urine. Once digested, food must be transported to body tissuess using the circulatory system (hemolymph, etc.) This system works to remove metabolic wastess from the insect body.. The excretory wastess of insectss are removed from the hemocoel by the Malpighian tubuless and dumped into the hindgut where they Includes material from the intro. entomology course taughtt by Dr. Douglas Emlen and from Thee Insects, Authors: Gullan & Cranston.

4 mix with frass. Aquatic insects excrete wastes freely into the water, however, terrestrial insects must conserve water and are often very efficient at extracting water from waste beforee it is removed from the body. Insects must also conservee ions like sodium, potassium, and chloride which may be limitingg in their diet. Such ions are water soluble and easily lost. Thus, the production of insect waste (urine/feces) is actually the result of both osmoregulation and excretion. The primary excretory organ is the network of Malpighian tubules floating in the hemocoel. They are long, slender tubes, closed at their distal end. They feed into the hindgut at the juncture with the midgut. These are free-flowing, which means that they aree bathed by hemolymph. They have a large surface area that is effective at up-taking metabolic byproducts (principally ammonium ions) from the blood. The toxic NH + 4 is quickly converted to urea and then to uric acid by a series of chemical reactions within the Malpighian tubules. The uric acid, a semi-solid, accumulates inside each tubule and is eventually emptied into the hindgut for elimination as part of the fecal pellet. The rectum is often very specialized for the uptake of water. It has thick rectal pads the increase the surface area. More than 90% of water is absorbed or reabsorbed by these pads. Details of what is absorbed and how much varies with the insect diet. Includes material from the intro. entomology course taughtt by Dr. Douglas Emlen and from Thee Insects, Authors: Gullan & Cranston.

5 Cryptonephric system (means hidden kidney ) this interesting modification of the excretory system occurs in most Lepidoptera and Coleoptera. The distal ends of the Malpighian tubules wrap back around posteriorly and are actually embedded in the rectal pads. This set-up is extremely efficient and resorbing water from the hindgut and/or at regulating ions. SYMBIOSIS in ecology is described as two or more different species with a close, often interdependent relationship (they might need each other!). In entomology, symbionts generally refer to HARBORED microorganisms that contribute to the nutrition of their insect hosts. They can be bacteria, yeasts, protists, etc. Can be extracellular or intracellular (actually inside insect cells!) Examples of species harboring symbionts include many hemipterans, lice, termites, seed or grain-feeding beetles, and cockroaches. For some reason predatory insects rarely have symbionts. So, it s probably the case that microorganisms are required by insects on suboptimal diets. TERMITES: hindguts contain a veritable fermenter with many bacteria, fungi, and protists that breakdown lignocellulose, and fix nitrogen from the air. Termites produce a lot of methane (greenhouse gas)! Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston.

6 INTERNAL ANATOMY AND PHYSIOLOGY STUDY GUIDE, PART II. Study Questions and Objectives What tissues need to be shed when an insect molts? Explain where a sawfly larva sequesters toxins and what it does with them when disturbed. Be able to label the diagram below and describe the function of each component. Describe the components of the insect excretory system and explain how the waste products are handled. Foregut (Stomodeum) Midgut (Mesenteron) Hindgut (Proctodeum) pharynx crop esophagus gastric caecum proventriculus ventriculus intima ileum intima rectum anus preoral cavity salivary gland Malpighian tubule peritrophic membrane colon Study Terms Foregut, midgut, hindgut, alimentary canal Ectoderm, endoderm Complete versus incomplete digestive systems Crop, pharynx, esophagus, proventriculus, diverticula Functional specialization of the alimentary canal Ventriculus, peritrophic membrane, gastric caecae Symbionts (in entomology) Ileum, colon, rectum, intima Malpighian tubules Osmoregulation, rectal pads, Uric acid, frass, Cryptonephric system Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston.