Entomology 3rd edition - C.Gillott - Chapter 16 potx

The gut normally occurs as a continuous tube between the mouth and anus, and its length is broadly correlated with feeding habits, being short in carnivorous forms where digestion and ab

1 Introduction

Insects feed on a wide range of organic materials About 75% of all species are tophagous, and these form an important link in the transfer of energy from primary produc-ers to second-order consumers Others are carnivorous, omnivorous, or parasitic on otheranimals In accord with the diversity of feeding habits, the means by which insects locatetheir food, the structure and physiology of their digestive system, and their metabolism arehighly varied

phy-The feeding habits of insects take on special significance for humans, on the one hand,because of the enormous damage that feeding insects do to our food, clothing, and health,and, on the other, because of the massive benefits that insects provide as plant pollinatorsduring their search for food (see also Chapter 24) In addition, because many species areeasily and cheaply mass-cultured in the laboratory, they have been used widely in research

on digestion and absorption, as well as in the elucidation of basic biochemical pathways,the role of specific nutrients, and other aspects of animal metabolism

2 Food Selection and Feeding

Distinct visual, chemical, and mechanical cues act at each step of the food locationand ingestion process These steps include attraction to food, arrest of movement, tasting,biting, further tasting as ingestion begins, continued ingestion, and termination of feeding.The sensitivity of the insect to these cues varies with its physiological state For example,

a starved insect may become highly sensitive to odors or tastes associated with its normalfood, and in extreme cases may become quite indiscriminate in terms of what it ingests

On the other hand, a female whose abdomen is full of eggs is normally “uninterested” infeeding

In some plant-feeding (phytophagous) species, visual stimuli such as particular terns (especially stripes) or colors may serve to initially attract an insect to a potential foodsource Usually, however, the initial orientation, where this occurs, is dependent on olfac-tory stimuli In many larval forms there appear to be no specific orienting stimuli because,under normal circumstances, larvae remain on the food plant selected by the mother prior

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to oviposition In the migratory locust, on which much work has been done, olfaction is ofprimary importance in food location Once the insect makes contact with the vegetation,tarsal chemosensilla initiate a reflex that results in the stoppage of movement Sensilla onthe labial and maxillary palps then taste the surface waxes of the plant, after which the locusttakes a small bite Whether feeding continues is sometimes determined by mechanosensillarresponses to physical stimuli such as the hardness, toughness, shape, and hairiness of thefood More commonly, it is substances in the released sap that, by stimulating chemosensilla

in the cibarial cavity, regulate the continuation or arrest of feeding (Chapman, 2003) Thesesubstances are called “phagostimulants” or “deterrents,” respectively The substances mayhave nutritional value to the insect or may be nutritionally unimportant (“token stimuli”).Nutritional factors are almost always stimulating in effect Sugars, especially sucrose, areimportant phagostimulants for most phytophagous insects Amino acids, in contrast, are gen-erally by themselves weakly stimulating or non-stimulating, though may act synergisticallywith certain sugars or token stimuli For example, Heron (1965) showed in the spruce bud-

worm (Choristoneura fumiferana) that, whereas sucrose and l-proline in low concentration

were individually only weak phagostimulants, a mixture of the two substances was highlystimulating In addition to sugars and amino acids, other specific nutrients may stimulatefeeding in a given species Such nutrients include vitamins, phospholipids, and steroids.Token stimuli may either stimulate or inhibit feeding Thus, derivatives of mustard oil,produced by cruciferous plants, including cabbage and its relatives, are important phagos-timulants for a variety of insects that normally feed on these plants, for example, larvae

of the diamondback moth (Plutella xylostella), the cabbage aphid (Brevicoryne brassicae) , and the mustard beetle (Phaedon cochleariae) Indeed, Plutella will feed naturally only on

plants that contain mustard oil compounds Many secondary plant metabolites, includingalkaloids, terpenoids, phenolics, and glycosides, are feeding deterrents for phytophagousinsects In a given food source there will probably be a mixture of phagostimulants anddeterrents, and the balance of this sensory input, integrated through the central nervoussystem, determines the overall palatability of the food

Species whose choice of food is limited are said to be oligophagous In extreme cases, aninsect may be restricted to feeding on a single plant species and is described as monophagous.Species that may feed on a wide variety of plants are polyphagous, though it must be notedthat even these exhibit selectivity when given a choice Not surprisingly, monophagousand oligophagous species are especially sensitive to the presence of deterrents in non-hostplants

In many predaceous insects, especially those that actively pursue prey, vision is ofprimary importance in locating and capturing food As noted in Chapter 12 (Section 7.1.2),some predaceous insects have binocular vision that enables them to determine when prey iswithin catching distance Carnivorous species, especially larval forms, whose visual sense

is less well developed, depend on chemical or tactile stimuli to find prey For example,many beetle larvae that live on or in the ground locate prey by their scent Species parasitic

on other animals usually locate a host by its scent, though tsetse flies may initially orient

by visual means to a potential host For many species that feed on the blood of birds andmammals, temperature and/or humidity gradients are important in determining the preciselocation at which an insect alights on a host and begins to feed

The extent of food specificity for carnivorous insects is varied Many insects are quitenon-specific and will attempt to capture and eat any organism that falls within a given sizerange (even to the extent of being cannibalistic) Others are more selective; for example,spider wasps (Pompilidae), as their name indicates, capture only spiders for provisioning

and a change in the osmotic pressure or composition of the hemolymph as absorption of

digested materials occurs (see chapters in Chapman and de Boer, 1995)

Apart from the specific cues outlined above that facilitate location and selection of

food, there are other factors that influence feeding activity Typically, insects do not feed

shortly before and after a molt, or when there are mature eggs in the abdomen In addition,

a diurnal rhythm of feeding activity may occur, in response to a specific light, temperature,

or humidity stimulus For example, the red locust (Nomadacris septemfasciata) feeds in

the morning and evening, and many mosquitoes feed during the early evening (though

this may change in different habitats) Pupae, most diapausing insects, and some adult

Ephemeroptera, Lepidoptera, and Diptera do not feed

3 The Alimentary System

The gut and its associated glands (Figure 16.1) triturate, lubricate, store, digest, and

absorb food material and expel the undigested remains Structural differences throughout the

system reflect regional specialization for performance of these functions and are correlated

also with feeding habits and the nature of normal food material The structure of the system

may vary at different stages of the life history because of the different feeding habits of

the larva and adult of a species The gut normally occurs as a continuous tube between

the mouth and anus, and its length is broadly correlated with feeding habits, being short

in carnivorous forms where digestion and absorption occur relatively rapidly, and longer

(often convoluted) in phytophagous forms In a few species that feed on fluids, such as

larvae of Neuroptera and Hymenoptera-Apocrita, and some adult Heteroptera there is little

or no solid waste in the food, and the junction between the midgut and hindgut is occluded

As Figure 16.1 indicates, food first enters the buccal cavity, which is enclosed by the

mouthparts and is not strictly part of the gut It is into the buccal cavity that the salivary

glands release their products The gut proper comprises three main regions: the foregut,

in which the food may be stored, filtered, and partially digested; the midgut, which is the

primary site for digestion and absorption of food; and the hindgut, where some absorption

and feces formation occur

3.1 Salivary Glands

Salivary glands are present in most insects, though their form and function are extremely

varied, and they may or may not be innervated (Ribeiro, 1995) Frequently they are known

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FIGURE 16.1. Alimentary canal and associated structures of a locust [After C Hodge, 1939, The anatomy and

histology of the alimentary tract of Locusta migratoria L (Orthoptera: Acrididae), J Morphol 64:375–399 By

permission of the Wistar Press.]

by other names according to either the site at which their duct enters the buccal cavity, forexample, labial glands and mandibular glands, or their function, for example, silk glandsand venom glands

Typically, saliva is a watery, enzyme-containing fluid that serves to lubricate thefood and initiate its digestion Like that of humans, the saliva generally contains onlycarbohydrate-digesting enzymes (amylase and invertase), though there are exceptions tothis statement For example, the saliva of some carnivorous species contains protein- and/orfat-digesting enzymes only; that of bloodsucking species has no enzymes In termite salivathere are cellulose-digesting enzymes: aβ-1-4-glucanase that brings about the initial split-ting of the polymer, andβ-glucosidase that degrades the resulting cellobiose to glucose

(Nakashima et al., 2002; Tokuda et al, 2002) (See also Section 4.2.4.)

In the innervated glands of cockroaches and locusts, release of saliva is induced whenfood stimulates mechano- and chemosensilla on the mouthparts and antennae The informa-tion travels to the subesophageal ganglion and then along aminergic or peptidergic neurons

to the glands where it induces relaxation of the muscles that normally close off the opening

of the salivary gland duct (Ali, 1997) In contrast, the non-innervated glands of Calliphora erythrocephala are stimulated to release saliva by a hemolymph factor, possibly serotonin

(Trimmer, 1985)

also strongly bind histamine, released by the host to induce wound healing (Weichsel et al.,

1998) Toxins (venoms), which paralyze or kill the prey, occur in the saliva of some assassin

bugs (Reduviidae) and robber flies (Asilidae) It is also reported that substances that induce

gall formation by stimulating cell division and elongation are present in the saliva of some

gall-inhabiting species Larvae of black flies and chironomid midges secrete large amounts

of viscous saliva, forming nets that capture food particles

In some species the glands have taken on functions quite unrelated to feeding, for

example, production of cocoon silk by the labial glands of caterpillars and caddisfly larvae,

and pheromone production by the mandibular glands of the queen honeybee

3.2 Foregut

The foregut, formed during embryogenesis by invagination of the integument, is lined

with cuticle (the intima) that is shed at each molt Surrounding the intima, which may

be folded to enable the gut to stretch when filled, is a thin epidermis, small bundles of

longitudinal muscle, a thick layer of circular muscle, and a layer of connective tissue through

which run nerves and tracheae (Figure 16.2) The foregut is generally differentiated into

pharynx, esophagus, crop, and proventriculus Attached to the pharyngeal intima are dilator

muscles These are especially well developed in sucking insects and form the pharyngeal

pump (Chapter 3, Section 3.2.2) The esophagus is usually narrow but posteriorly may be

dilated to form the crop where food is stored In Diptera and Lepidoptera, however, the crop

is actually a diverticulum off the esophagus During storage the food may undergo some

digestion in insects whose saliva contains enzymes or that regurgitate digestive fluid from

the midgut In some species the intima of the crop forms spines or ridges that probably aid in

breaking up solid food into smaller particles and mixing in the digestive fluid (Figure 16.2A)

The hindmost region of the foregut is the proventriculus, which may serve as a valve

regulating the rate at which food enters the midgut, as a filter separating liquid and solid

components, or as a grinder to further break up solid material Its structure is, accordingly,

quite varied In species where it acts as a valve the intima of the proventriculus may form

longitudinal folds and the circular muscle layer is thickened to form a sphincter When a

filter, the proventriculus contains spines that hold back the solid material, permitting only

liquids to move posteriorly Where the proventriculus acts as a gizzard, grinding up food,

the intima is formed into strong, radially arranged teeth, and a thick layer of circular muscle

covers the entire structure (Figure 16.2B)

Posteriorly the foregut is invaginated slightly into the midgut to form the esophageal

(= stomodeal) invagination (Figure 16.3) Its function is to ensure that food enters the

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FIGURE 16.2. Transverse sections through (A) crop and (B) proventriculus of a locust [After C Hodge, 1939,

The anatomy and histology of the alimentary tract of Locusta migratoria L (Orthoptera: Acrididae), J Morphol.

64:375–399 By permission of Wistar Press.]

midgut within the peritrophic matrix It also appears to assist in molding the peritrophicmatrix into the correct shape in some insects

3.3 Midgut

The midgut (= ventriculus = mesenteron) is of endodermal origin and, therefore,has no cuticular lining In most insects, however, it is lined by a thin peritrophic matrix(PM) composed of proteins bound to a meshwork of chitin fibrils (Figure 16.4) Some PMproteins, the peritrophins, are heavily glycosylated like mucus in the intestine of vertebrates.The functions of the PM are to prevent mechanical damage to the midgut epithelium, toprevent entry of microorganisms into the body cavity, to bind potential toxins and otherdamaging chemicals, and to compartmentalize the midgut lumen, that is, to divide it into

an endoperitrophic space (within the matrix) and an ectoperitrophic space (adjacent to the

FIGURE 16.3. Longitudinal section through crop,

proventriculus, and anterior midgut of a cockroach.

[From R E Snodgrass, Principles of Insect

Morphol-ogy Copyright 1935 by McGraw-Hill, Inc Used with

permission of McGraw-Hill Book Company.]

midgut epithelium) (Terra, 1996; Lehane, 1997) This separation of the epithelium from the

food improves digestive efficiency by segregating enzymes between the spaces and enabling

some enzymes to be recycled (Section 4.2.1)

The PM is generally absent in fluid-feeding insects, for example, Hemiptera, adult

Lepidoptera, and bloodsucking Diptera However, some insects produce the PM only at

certain times (e.g., female mosquitoes after a blood meal) Further, as described below, the

FIGURE 16.4. Transverse section through midgut of a locust [After C Hodge, 1939, The anatomy and histology

of the alimentary tract of Locusta migratoria L (Orthoptera: Acrididae), J Morphol 64:375–399 By permission

of Wistar Press.]

of the PM-producing cells, as it hardens, is squeezed to form the tubular membrane In tyoptera, other Orthoptera and Lepidoptera, Hymenoptera, and Neuroptera, a combination

Dic-of both methods seems to be used In mosquitoes, larvae produce a Type II PM, whereasthe adults have a Type I PM

The PM is made up of a meshwork of microfibrils between which is a thin proteinaceousfilm The microfibrils have a constant 60◦orientation to each other in Type I PM, thought

to result from their secretion by the hexagonally close-packed microvilli of the epithelialcells In Type II PM the orientation of the microfibrils is random The PM is permeable tothe products of digestion and to certain digestive enzymes released from the epithelial cells(Section 4.2.1) However, it is not permeable to other large molecules, such as undigestedproteins and polysaccharides, indicating that the PM has a distinct polarity and is not merely

an ultrafilter (Richards and Richards, 1977; Lehane, 1997)

The midgut is usually not differentiated into structurally distinct regions apart from thedevelopment, at the anterior end, of a varied number of blindly ending ceca, which serve toincrease the surface area available for enzyme secretion and absorption of digested material

In many Heteroptera, however, the midgut is divided into three or four easily visible regions

In the chinch bug (Blissus leucopterus) four such regions occur (Figure 16.5) The anterior

region is large and saclike, and serves as a storage region (no crop is present) The secondregion serves as a valve to regulate the flow of material into the third region where digestion

FIGURE 16.5. Alimentary canal of chinch bug (Blissus

leucopterus) showing regional differentiation of midgut.

[After H Glasgow, 1914, The gastric caeca and the caecal

bacteria of the Heteroptera, Biol Bull 26:101–170.]

FIGURE 16.6. Alimentary canal of

cercopid (Cercopoidea) showing filter

chamber arrangement [From R E

Snod-grass, Principles of Insect Morphology.

Copyright 1935 by McGraw-Hill, Inc.

Used with permission of McGraw-Hill

Book Company.]

probably occurs Ten fingerlike ceca filled with bacteria are attached to the fourth region,

which may be absorptive in function The role of the bacteria is not known

In many homopterans, which feed on plant sap, the midgut is modified both

morpho-logically and anatomically so that excess water present in the food can be removed, thus

preventing dilution of the hemolymph Though details vary among different groups of

ho-mopterans, the anterior end of the midgut (or, in some species, the posterior part of the

esophagus) is brought into close contact with the posterior region of the midgut (or anterior

hindgut), and the region of contact becomes enclosed within a sac called the “filter

cham-ber” (Figure 16.6) Such an arrangement facilitates rapid movement of water by osmosis

from the lumen of the anterior midgut across the wall of the posterior midgut and possibly

also the Malpighian tubules Thus, relatively little of the original water in the food actually

passes along the full length of the midgut

The lack of morphological differentiation within the midgut of most species is reflected

in its uniform histology Throughout its length, the mature cells lining the lumen are identical

and serve to produce digestive enzymes, to absorb the products of digestion, and in some

insects secrete the Type I PM Replacement of degenerate cells occurs with the maturation

and differentiation of regenerative cells found singly or in groups (nidi) near the base of

the epithelium (Figure 16.4) Numerous peptide hormone-containing cells also occur in the

midgut, which may play a role in modulating midgut contraction (Lange and Orchard, 1998)

In some species histological differentiation is found For example, specialization of

cer-tain anterior cells for Type I PM production was noted earlier In addition, differentiation

into digestive and absorptive regions occurs in some species In tsetse flies the cells of the

an-terior midgut are small and are concerned with absorption of water from the ingested blood

They produce no enzymes and digestion does not begin until food reaches the middle region

where the cells are large, rich in ribonucleic acid, and produce enzymes In the posterior

midgut the cells are smaller, closely packed, and probably concerned with absorption of

digested food In some species different regions of the midgut are apparently adapted to the

absorption of particular food materials In Aedes larvae the anterior midgut is concerned

CHAPTER 16

with fat absorption and storage, whereas the posterior portion absorbs carbohydrates andstores them as glycogen In larval Lepidoptera goblet cells, with a large flask-shaped centralcavity, are scattered among the regular epithelial cells They are thought to play a role inthe regulation of the potassium level within the hemolymph (Chapter 18, Section 2.2)

3.4 Hindgut

The hindgut is an ectodermal derivative and, as such, is lined with cuticle, though this

is thinner than that of the foregut, a feature related to the absorptive function of this region.The epithelial cells that surround the cuticle are flattened except in the rectal pads (seebelow) where they become highly columnar and filled with mitochondria Muscles are onlyweakly developed and, usually, the longitudinal strands lie outside the sheet of circularmuscle

The hindgut usually has the following regions: pylorus, ileum, and rectum The pylorusmay have a well-developed circular muscle layer (pyloric sphincter) and regulate the move-ment of material from midgut to hindgut Also, the Malpighian tubules characteristicallyenter the gut in this region The ileum (Figure 16.7A) is generally a narrow tube that serves

to conduct undigested food to the rectum for final processing In some insects, however,some absorption of ions and/or water may occur in this region In a few species productionand excretion of nitrogenous wastes occur in the ileum (Chapter 18, Section 2.2) In manywood-eating insects, for example, species of termites and beetles, the ileum is dilated toform a fermentation pouch housing bacteria or protozoa that digest wood particles Theproducts of digestion, when liberated by the microorganisms, are absorbed across the wall

of the ileum The most posterior part of the gut, the rectum, is frequently dilated Though forthe most part thin-walled, the rectum includes six to eight thick-walled rectal pads (Figure16.7B) whose function is to absorb ions, water, and small organic molecules (Chapter 18,Section 4) As a result, the feces of terrestrial insects are expelled as a more or less dry pellet.Frequently, the pellets are ensheathed within the PM, which continues into the hindgut

4 Gut Physiology

The primary functions of the alimentary canal are digestion and absorption For theseprocesses to occur efficiently, food is moved along the canal In some species, enzymesecretions are moved anteriorly so that digestion can begin some time before food reachesthe region of absorption

4.1 Gut Movements

Though the alimentary canal is innervated, neural control is principally associatedwith the opening/closing of valves that occur within the canal (see below) The rhythmicperistaltic muscle contractions that move food posteriorly through the gut are myogenic; that

is, they originate within the muscles themselves rather than occurring as a result of nervousstimuli Myogenic centers have been located in the esophagus, crop, and proventriculus, in

Galleria, for example In insects that form a Type II PM, backward movement of food is

aided by growth of the membrane Antiperistaltic movements also occur in some speciesand serve to move digestive fluid forward from the midgut into the crop

FIGURE 16.7. Transverse sections through (A) ileum and (B) rectum of a locust [After C Hodge, 1939, The

anatomy and histology of the alimentary tract of Locusta migratoria L (Orthoptera: Acrididae), J Morphol.

64:375–399 By permission of Wistar Press.]

The rate at which food moves through the gut is not uniform It varies according

to the physiological state of an insect; for example, it is greater when an insect has been

starved previously or is active The rate may also differ between sexes and with age Another

important variable is the nature of the food Some insects are able to move some components

of the diet rapidly through the gut while retaining others for considerable periods Within

the gut, food moves at variable rates in different regions

The proventricular and pyloric valves are important regulators of food movement,

though little is known about how their opening and closing are controlled In Periplaneta

opening of the proventriculus was shown to depend on the osmotic pressure of ingested fluid

(Davey and Treherne, 1963) As the concentration is increased, the proventriculus opens

less often and less widely, and vice versa Davey and Treherne suggested that osmoreceptors

in the pharynx provide information on the osmotic pressure of the food and this information

travels via the frontal ganglion to the ingluvial ganglion that controls the proventriculus

However, no osmoreceptor has been located and it may be that the osmotic feedback comes

CHAPTER 16

from the hemolymph rather than directly from the gut as seems to be the case in Locusta.

Distension of the foregut or, in blood-feeding species, the abdomen, is known to causerelease of neurosecretion from the corpora cardiaca which enhances gut peristalsis and,hence, the rate of food passage Localized enhancement of peristalsis may be induced byrelease of peptide hormones from cells in the wall of the midgut (Lange and Orchard, 1998)

4.2 Digestion

As noted above, digestion may be initiated by enzymes present in the saliva either mixedwith the food as it enters the buccal cavity or secreted onto the food prior to ingestion Mostdigestion is dependent, however, on enzymes secreted by the midgut epithelium Digestionmostly occurs in the lumen of the midgut, though regurgitation of digestive fluid into thecrop is important in some species In wood-eating forms, much of the digestion is carriedout by microorganisms in the hindgut (Section 4.2.4)

4.2.1 Digestive Enzymes

A wide variety and large number of digestive enzymes have been reported for insects

In many instances, however, enzymes have been characterized (and named) on the basis oftheir activity on unnatural substrates, that is, materials that do not occur in the normal diet ofthe insect This is because many digestive enzymes, especially carbohydrases, are “group-specific”; that is, they hydrolyze any substrate that includes a particular bond betweentwo parts of the molecule For example,α-glucosidase splits all α-glucosides, includingsucrose, maltose, furanose, trehalose, and melezitose Further, in preparing enzyme extractsfor analysis, either gut contents or midgut tissue homogenates are typically used As House

(1974) noted, the former may include enzymes derived from the food per se, while the

latter contains endoenzymes (intracellular enzymes) that have no digestive function Thus,reports on digestive enzyme activity must be examined cautiously

As would be expected, the enzymes produced reflect both qualitatively and titatively the normal constituents of the diet Omnivorous species produce enzymes fordigesting proteins, fats, and carbohydrates Carnivorous species produce mainly lipasesand proteases; in some species these may be highly specific in action Blow fly larvae

quan-(Lucilia cuprina), for example, produce large amounts of collagenase The nature of the

enzymes produced may change at different stages of the life history as the diet of an insectchanges For example, caterpillars feeding on plant tissue secrete a spectrum of enzymes,whereas nectar-feeding adult Lepidoptera produce only invertase Interestingly, however,even in those endopterygotes in which the larvae and adults utilize the same food the prop-

erties of the enzymes change at metamorphosis In Tenebrio, for example, the larval and

adult trypsins and chymotrypsins differ in molecular size, substrate specificity, and kinetics,though why this should be is not clear

Insects can digest a wide range of carbohydrates, even though only a few distinct zymes may be produced As noted earlier,α-glucosidase will hydrolyze all α-glucosides.Likewise,β-glucosidase facilitates splitting of cellobiose, gentiobiose, and phenylgluco-sides;β-galactosidase hydrolyzes β-galactosides such as lactose In some species, however,there appear to be carbohydrate-digesting enzymes that exhibit absolute specificity Thus,

en-adult Lucilia cuprina produce anα-glucosidase, trehalase, that splits only trehalose Thenormal polysaccharide-digesting enzyme produced is amylase for hydrolysis of starch,though particular species may produce enzymes for digestion of other polysaccharides For

boxylic end of a polypeptide, and aminopeptidases, which cause hydrolysis at the amino end

of a molecule A dipeptidase also is frequently present In some species only endopeptidases

occur in the midgut lumen (specifically within the endoperitrophic space), the

exopepti-dases being found outside the PM or even attached to the apical plasma membrane of the

epithelial cells Some insects produce specific enzymes for the digestion of particularly

resistant structural proteins Collagenase has been mentioned already Keratin, the primary

constituent of wool, hair, and feathers, is a fibrous protein whose polypeptide components

lie side by side linked by highly stable disulfide bonds between adjacent sulfur-containing

amino acids, such as cystine and methionine A keratinase has been identified in clothes

moth larvae (Tineola) and may also occur in other keratin-digesting species, such as

der-mestid beetles and Mallophaga The keratinase is active only under anaerobic (reducing)

conditions and, in this context, it is interesting to note that the midgut of Tineola is poorly

tracheated

Dietary fats of either animal or plant origin are almost always triglycerides, that is,

glycerol in combination with three fatty acid molecules The latter may range from

unsat-urated to fully satunsat-urated Lipases, which hydrolyze fats to the constituent fatty acids and

glycerol, have low specificity Therefore, the presence of one such enzyme will normally

satisfy an insect’s needs In a few species, however, at least two lipases have been identified,

having different pH optima and acting on triglycerides of different sizes Fat digestion is

generally somewhat slow as insects lack anything comparable to the bile salts of vertebrates

that would emulsify and stabilize lipid droplets

4.2.2 Factors Affecting Enzyme Activity

According to House (1974), three factors markedly affect digestion in insects: pH,

buffering capacity, and redox potential of the gut.ff

The pH determines not only the activity of digestive enzymes, but also the nature and

extent of microorganisms in the gut and the solubility of certain materials in the gut lumen

The latter affects the osmotic pressure of the gut contents and, in turn, the rate of absorption

of molecules across the gut wall Analyses of the pH in various regions of the gut have been

made for a wide range of species, and various authors have attempted to correlate these

with the feeding habits or phylogenetic position of an insect At best, these correlations are

only broadly correct, and many exceptions are known In most insects the gut is slightly

acid or slightly alkaline throughout its length Further, the pH generally increases from

foregut to midgut, then decreases from midgut to hindgut Though the latter is true for most