the foregut, or stomodaeum 

the midgut, or mesenteron 

the hindgut, or proctodaeum 

•are derived from ectodermal tissue

•are lined internally by a thin layer of cuticle, the intima 

•The intima is shed at each molt along with the outer exoskeleton

•Most insects have a pair of glands lying below the anterior part of the alimentary canal

othe ducts from these glands extend forward and unite into a common duct that opens near the base of the labium or hypopharynx 

othese labial glands generally function as salivary glands

othere is often an enlargement of the duct from each gland that serves as a reservoir for the salivary secretion

oThe labial glands in the larvae of Lepidoptera, Trichoptera, and Hymenoptera secrete silk

•The silk is used in making cocoons and shelters

•Use in food gathering by net-spinning caddisflies

•a pharynx (immediately behind the mouth)

•esophagus (a slender tube extending posteriorly from the pharynx)

•crop (an enlargement of the posterior portion of the foregut) 

•a proventriculus 

oAt the posterior end of the foregut is a stomodaeal valve, which regulates the passage of food between the foregut and the midgut 

oIt often bears diverticula, the gastric caeca (cm), near its anterior end

oThe midgut is not lined by cuticle

oThe epithelial layer of the midgut is involved in:

othe secretion of digestive enzymes

othe absorption of the products of digestion into the body of the insect

oIndividual midgut epithelial cells are short-lived and are constantly being replaced

oThe midgut is the primary area of digestion and absorption in the alimentary canal

oIn many species, the midgut epithelium and the food are separated by a peritrophic membrane – a nonliving, permeable network of chitin and protein that is secreted by the epithelium

•to limit abrasion of the epithelium

•to inhibit the movement of pathogens from the food to the insects tissues

•or to serve as a means of separating endo- and ectoperitrophic spaces within which digestive specialization can occur

extends from the pyloric valve, which lies between the midgut and hindgut, to the anus

•Posteriorly, the hindgut is supported by muscles extending to the abdominal wall

•The hindgut is generally differentiated into at least two regions, the anterior intestine and the posterior rectum 

•The anterior intestine may be simple tube, or it may be divided into an anterior ileum and a posterior colon

•The Malpighian tubules, which are excretory organs, arise at the anterior end of the hindgut, and the contents of these tubules empty into the hindgut

othe final site for resorption of water, salts, and any nutrients from the feces and the urine

oThe rectum in some species has thick, large rectal pads for removing water from the feces

a great variety of living, dead, and decomposing animals, plants, fungi, and microorganisms, and on their products

oIn some cases, liquid foods, such as blood or plant juices, may constitute their entire food supply

oThe digestive system varies considerably with the type of food consumed

oFood habits may even vary greatly in a single species

oLarvae and adults often differ; some adults do not feed at all

a modification of the alimentary canal in which two normally distant parts are held together by connective tissue

Found in Hemipterans, midgut divided into 3 ventriculi

Third coils around and connects with the first to form filter chamber

Passes honeydew, excess water and sugar

The primary excretory system of an insect consists of a group of hollow tubules

which arise as evaginations at the anterior end of the hindgut

The principle nitrogenous waste 

•Relatively nontoxic; can be tolerated in higher concentrations than ammonia

•Insoluble in water; why is this important? Conserves water

oMalpighian tubules are bound very closely to the hindgut

•In species such as the mealworm, Tenebrio molitor, that live in conditions of high moisture stress, this arrangement of the tubules is apparently involved in extracting water from the fecal pellets

•The fat body is a large, somewhat amorphous organ housed in the abdomen and the thorax

•In many ways, the fat body is analogous to the liver of vertebrates

oThe fat body serves as a food reservoir and it is an important site of intermediate metabolism

•In some species of insects, it is important in storage excretion (e.g., cockroaches)

oThe cells of the fat body (adipocytes) synthesize and store lipids, proteins, and carbohydrates

oThe fat body is usually best developed in late nymphal or larval instars

•By the end of metamorphosis, it is often depleted

oSome insects that do not feed as adults retain their fat body in their adult life

stationary cells that occur either singly or in groups and are usually suspended in the hemolymph by strands of connective tissue

•Their main function is to help maintain hemolymph homeostatis by taking in, via endocytosis, toxic molecules, breaking these molecules down or storing them, and releasing other molecules into the hemolymph by exocytosis 

•Their function is kidneylike 

conduct synthesis of proteins, which are released into the hemolymph 

oThese cells also contribute to the synthesis of hemolymph solutes and possibly hemolymph pigments 

•Those in which the factors do not require de novo synthesis

•Those that are inducible and require de novo synthesis of RNA and proteins

•Inducible factors in the hemolymph include antimicrobial proteins (e.g., cecropin), and lysozymes

•The noninducible (constitutive) factors are the lectins (hemagglutins) and the phenyloxidases 

•The phenyloxidase system, once activated, produces a cascade of chemical events that lead to various biochemical pathways

oThe main, and often only, blood vessel is located dorsal to the alimentary canal and extends through the thorax and the abdomen

oElsewhere, the hemolymph flows unrestricted through the hemocoel (the body cavity)

The slender anterior portion of the dorsal blood vessel

oThe aorta extends anteriorly from the heart and opens behind or beneath the brain

•It usually lacks any valvular openings

•In some insects, it may be looped, arched, or coiled

The heart is usually closed at the posterior end

but has valvular openings which allow hemolymph to enter the heart

oExtending from the lower surface of the heart to the lateral portions of the terga are pairs of sheetlike muscle bands

•more or less separates the region around the heart (the pericardial sinus) from the main body cavity (the perivisceral sinus)

a second septum

contains muscle fibers, like the dorsal diaphragm

separates the perivisceral sinus from the perineural sinus

tracheae terminate in very small diameter branches

othat permeate and actually penetrate the tissues

•It is across the walls of the tracheoles that gas exchange actually takes place

helical rings of the tracheae

ogive the tracheae strength against collapse, and flexibility to bend and twist

located laterally in the pleural wall

oThese vary in number from 1 to 10 pairs; some species have no functional spiracles

oThere is typically a pair on the anterior margin of both the meso- and metathorax, and a pair on each of the first eight (or fewer) abdominal segments

oThey vary in size and shape and usually have some sort of valvelike closing device

connect the tracheae on opposite sides of the body

oThus, the entire system is interconnected

by simple diffusion in many small insects

oIn most larger insects, however, this movement is augmented by active ventilation, chiefly by the abdominal muscles

generally enters through the anterior spiracles and exits by the posterior ones

flow of air through the tracheal system is achieved by controlling which spiracles are open and when

oSections of the main tracheal trunks are often dilated to form air sacs, which help in ventilation

•Closed tracheal systems 

oThere is a network of tracheae just under the integument, widely over the body, or especially under certain surfaces, the gills

oSome aquatic and parasitic insect species have closed tracheal systems

•The gills in mayfly nymphs are in the form of leaflike structures on the sides of the first seven abdominal segments

•In dragonfly nymphs, the gills are folds in the rectum, and water is moved in and out over these folds

•In damselfly nymphs, the gills are three leaflike structures at the end of the abdomen as well as folds in the rectum

•In stonefly nymphs, the gills are fingerlike or branched structures located around the bases of the legs or on the basal abdominal segments

oFrom air spaces in submerged parts of certain aquatic plants;

oThrough spiracles placed at the water surface (with the body of the insect submerged);

oOr from a film of air held somewhere on the surface of the body while the insect is submerged

oMany aquatic insects, such as waterscorpions, rattailed maggots, and the larvae of culicine mosquitoes, have a breathing tube at the posterior end of the body, which is extended to the surface

oHydrophobic hairs around the end of this tube prevent water from enter the breathing tube, and allow the larvae to hang from the surface film

oOther aquatic insects that are surface breathers, such as backswimmers and anopheline mosquitoes, get air through posterior spiracles

oThese insects do not have an extended breathing tube

oThe insects that obtain their oxygen from atmospheric air at the water surface do not spend all of their time at the surface

oThey can submerge and remain underwater for considerable periods of time

oMany aquatic bugs and beetles carry a thin film of air somewhere on the body surface when they submerge

oThis air film is usually under the wings or on the ventral side of the body

oThe air film acts like a physical gill, with dissolved oxygen in the water diffusing into the bubble when the partial pressure of oxygen in the film falls below that of the water

oThe insect may get several times as much oxygen from this temporary structure as was originally in it