Digestive morphophysiology of Gryllodes sigillatus (Orthoptera: Gryllidae)

The evolution of the digestive system in the Order Orthoptera is disclosed from the study of the morphophysiology of the digestive process in its major taxa. This paper deals with a cricket representing the less known suborder Ensifera. Most amylase and trypsin activities occur in crop and caeca, respectively. Maltase and aminopeptidase are found in soluble and membrane-bound forms in caeca, with aminopeptidase also occurring in ventriculus. Amaranth was orally fed to Gryllodes sigillatus adults or injected into their haemolymph. The experiments were performed with starving and feeding insects with identical results. Following feeding of the dye the luminal side of the most anterior ventriculus (and in lesser amounts the midgut caeca) became heavily stained. In injected insects, the haemal side of the most posterior ventriculus was stained. This suggested that the anterior ventriculus is the main site of water absorption (the caeca is a secondary one), whereas the posterior ventriculus secretes water into the gut. Thus, a putative counter-current flux of fluid from posterior to anterior ventriculus may propel digestive enzyme recycling. This was confirmed by the finding that digestive enzymes are excreted at a low rate. The fine structure of midgut caeca and ventriculus cells revealed that they have morphological features that may be related to their involvement in secretion (movement from cell to lumen) and absorption (movement from lumen to cell) of fluids. Furthermore, morphological data showed that both merocrine and apocrine secretory mechanisms occur in midgut cells. The results showed that cricket digestion differs from that in grasshopper in having: (1) more membrane-bound digestive enzymes; (2) protein digestion slightly displaced toward the ventriculus; (3) midgut fluxes, and hence digestive enzyme recycling, in both starved and fed insects.     

Fine structure of the larval midgut of the fly Rhynchosciara and its physiological implications

The midgut of Rhynchosciara americana larvae consists of a cylindrical ventriculus from which protrudes two gastric caeca formed by polyhedral cells with microvilli covering their apical faces. The basal plasma membrane of these cells is infolded and displays associated mitochondria which are, nevertheless, more conspicuous in the apical cytoplasm. The anterior ventricular cells possess elaborate mitochondria-associated basal plasma membrane infoldings extending almost to the tips of the cells, and small microvilli disposed in the cell apexes. Distal posterior ventricular cells with long apical microvilli are grouped into major epithelial foldings forming multicellular crypts. In these cells the majority of the mitochondria are dispersed in the apical cytoplasm, minor amounts being associated with moderately-developed basal plasma membrane infoldings. The proximal posterior ventriculus represents a transition region between the anterior ventriculus and the distal posterior ventriculus. The resemblance between the gastric caeca and distal posterior ventricular cells is stressed by the finding that their microvilli preparations display similar alkaline phosphatase-specific activities. The results lend support to the proposal, based mainly on previous data on enzyme excretion rates, that the endo-ectoperitrophic circulation of digestive enzymes is a consequence of fluid fluxes caused by the transport of water into the first two thirds of midgut lumen, and its transference back to the haemolymph in the gastric caeca and in the distal posterior ventriculus.     

臭腹腺蝗（直翅目：锥头蝗科）消化道对食物储存、消化和吸收相适应的显微结构

显微观察发现臭腹腺蝗Zonocerus variegatus（直翅目：锥头蝗科）嗉囊、中肠和后肠的肠壁结构有所不同。嗉囊为空时纵向折叠。中肠上皮层的厚度随龄期有明显变化,1龄和2龄时明显大于3龄、4龄和5龄。后肠具有帮助消化和吸收的功能。     

Feeding, nutrient flow, and functional gut morphology in the cricket Gryllus bimaculatus

The flow of nutrients through the digestive tract of Gryllus bimaculatus is regulated by the proventriculus, which effectively triturates the partially digested food coming from the crop and shoves the mushy nutrient mass into the space between the paired caeca. The many folds at the base of the caeca form a sieve, and only fine food particles (4-10 microm) and fluids in the mush are filtered under pressure (produced by proventricular peristalsis) into the caeca. Combined with the release of enzymes in the caeca and the influx of water, the caeca are rapidly inflated on day 1 after the terminal molt. The remaining, mostly undigested food is shoved into a tube formed by the peritrophic membrane, which is first formed at the anterior end of the ventriculus. A mucous membrane (peritrophic gel) covers the caecal epithelium, and seems to merge with the true peritrophic membrane at the beginning of the ventriculus. The Type I peritrophic membrane is dragged posteriorly through the entire ventriculus and ileum by the posterior movement of the food bolus, which is shoved posteriorly at a rate of 6 mm/h by proventricular pressure. The growth rate of the peritrophic membrane is about 3 mm/h. Peristalsis does not occur in the midgut or ileum; the muscles in these regions function solely to counteract the internal pressure produced by the proventriculus. The exo- and endoperitrophic space in newly molted animals is open and fluids can flow in both directions. The endoperitrophic space becomes filled on day 1, and leads to a great reduction of the exoperitrophic space. In the ileal pouch (exoperitrophic space) the peritrophic membrane separates the mass of bacteria from the waste bolus within the endoperitrophic space. Feathery bristles arising from the cuticular covering of the finger-like invaginations of the ileal wall hold most of the bacterial mass in place. The crop weight decreases from day 1 to day 3 as the weight of caeca, ventriculus, and ileum increases. After day 3, food uptake and the weight of the entire gut system decrease in female crickets, partly in response to space restrictions in the abdomen caused by rapid ovarial growth.     

Ultrastructure of the digestive tract in Acarus siro (Acari: Acaridida)

The gut of the mite Acarus siro is characterized on the ultrastructural level. It consists of the foregut (pharynx, esophagus), midgut (ventriculus, caeca, colon, intercolon, postcolonic diverticula, postcolon), and hindgut (anal atrium). The gut wall is formed by a single-layered epithelium; only regenerative cells are located basally and these have no contact with the lumen. Eight cell types form the whole gut: (i) simple epithelial cells forming fore- and hindgut; (ii) cells that probably produce the peritrophic membrane; (iii) regenerative cells occurring in the ventriculus, caeca, colon, and intercolon; (iv) spherite cells and (v) digestive cells forming the ventriculus and caeca; (vi) colonic cells and (vii) intercolonic cells; and (viii) cells forming the walls of postcolonic diverticula and postcolon. Spherite and digestive cells change in structure during secretory cycles, which are described and discussed. The cycle of spherite, colonic, and intercolonic cells is terminated by apoptosis. Ingested food is packed into a food bolus surrounded by a single homogeneous peritrophic membrane formed by addition of lamellae that subsequently fuse together. The postcolonic diverticula serve as a shelter for filamentous bacteria, which also are abundant in the intercolon.     

Distribution of digestive enzymes among the endo- and ectoperitrophic spaces and midgut cells of Rhynchosciara and its physiological significance

Determinations of carbohydrases, proteases, carboxylesterases and phosphatases in the midgut cells and in the luminal spaces outside and inside the peritrophic membrane of Rhynchosciara americana larvae have been carried out. The data show that alpha-amylase, cellulase and proteinases are present in cells, ecto- and endoperitrophic spaces; aminopeptidases and trehalase in cells and ectoperitrophic space; and finally disaccharidases (except trehalase), carboxypeptidases, dipeptidases, carboxylesterases and phosphatases only in cells. The results support the conclusion that digestion takes place in three spatially organized steps. The first one occurs inside the peritrophic membrane and comprises the dispersion and/or decrease in molecular weight of the food molecules. The second is the hydrolysis of the polymeric food molecules in the ectoperitrophic space to dimers and/or small oligomers. Finally, terminal digestion occurs in the midgut caeca and posterior ventriculus cells by enzymes presumed to be plasma membrane bound. The existence of two extracellular sites for digestion in R. americana is considered to be an adaptation to conserve secreted enzymes, since only those penetrating the endoperitrophic space are lost quickly in the faeces.     

The digestive system of the “stick bug” Cladomorphus phyllinus (Phasmida,Phasmatidae): A morphological,physiological and biochemical analysis

This work presents a detailed morphofunctional study of the digestive system of a phasmid representative, Cladomorphus phyllinus. Cells from anterior midgut exhibit a merocrine secretion, whereas posterior midgut cells show a microapocrine secretion. A complex system of midgut tubules is observed in the posterior midgut which is probably related to the luminal alkalization of this region. Amaranth dye injection into the haemolymph and orally feeding insects with dye indicated that the anterior midgut is water-absorbing, whereas the Malpighian tubules are the main site of water secretion. Thus, a putative counter-current flux of fluid from posterior to anterior midgut may propel enzyme digestive recycling, confirmed by the low rate of enzyme excretion. The foregut and anterior midgut present an acidic pH (5.3 and 5.6, respectively), whereas the posterior midgut is highly alkaline (9.1) which may be related to the digestion of hemicelluloses. Most amylase, trypsin and chymotrypsin activities occur in the foregut and anterior midgut. Maltase is found along the midgut associated with the microvillar glycocalix, while aminopeptidase occurs in the middle and posterior midgut in membrane bound forms. Both amylase and trypsin are secreted mainly by the anterior midgut through an exocytic process as revealed by immunocytochemical data.     

The functional morphology of the alimentary tract of barnacles (Cirripedia: Thoracica)

The alimentary tract of barnacles is made up of cuticle-lined foregut and hindgut with an intervening U-shaped midgut associated anteriorly with a pair of pancreatic glands and perhaps midgut caeca. Epithelial salivary glands secrete acid mucopolysaccharide, glycoprotein or both. Cells of all the midgut regions are capable of absorption which is carried out mainly by the anterior midgut and caeca. Midgut cells of Balanus balanoides (L.) show a seasonal variation in the distribution of intracellular lipid droplets. Midgut cells rest on an elastic basal lamina and secrete a peritrophic membrane which contains mucopolysaccharide and protein. Cells of the stratum perintestinale connect with the midgut epithelial cells via cell processes which probably translocate absorbed materials. Glycoprotein globules and lipid droplets accumulate in the body parenchyma of B. balanoides and are transported to the ovaries to form yolk (glycolipovitellin). The pancreatic gland cells of all barnacles are active secretory cells secreting proteinaceous material (probably digestive enzymes).     

The ultrastructural investigation of the midgut in the quill mite Syringophilopsis fringilla (Acari,Trombidiformes: Syringophilidae)

The midgut of the females of Syringophilopsis fringilla (Fritsch) composed of anterior midgut and excretory organ (=posterior midgut) was investigated by means of light and transmission electron microscopy. The anterior midgut includes the ventriculus and two pairs of midgut caeca. These organs are lined by a similar epithelium except for the region adjacent to the coxal glands. Four cell subtypes were distinguished in the epithelium of the anterior midgut. All of them evidently represent physiological states of a single cell type. The digestive cells are most abundant. These cells are rich in rough endoplasmic reticulum and participate both in secretion and intracellular digestion. They form macropinocytotic vesicles in the apical region and a lot of secondary lysosomes in the central cytoplasm. After accumulating various residual bodies and spherites, the digestive cells transform into the excretory cells. The latter can be either extruded into the gut lumen or bud off their apical region and enter a new digestive cycle. The secretory cells were not found in all specimens examined. They are characterized by the presence of dense membrane-bounded granules, 2–4 μm in diameter, as well as by an extensive rough endoplasmic reticulum and Golgi bodies. The ventricular wall adjacent to the coxal glands demonstrates features of transporting epithelia. The cells are characterized by irregularly branched apical processes and a high concentration of mitochondria. The main function of the excretory organ (posterior midgut) is the elimination of nitrogenous waste. Formation of guanine-containing granules in the cytoplasm of the epithelial cells was shown to be associated with Golgi activity. The excretory granules are released into the gut lumen by means of eccrine or apocrine secretion. Evacuation of the fecal masses occurs periodically. Mitotic figures have been observed occasionally in the epithelial cells of the anterior midgut.     

Ultrastructural and biochemical aspects of digestion in the imagoes of the flyRhynchosciara americana

The midgut of adultRhynchosciara americana Wiedemann (Diptera: Sciaridae) displays, in contrast to the midguts of other adult Diptera, two caeca connected to a ventriculus. All midgut cells exhibit long apical microvilli, and narrow and ramified basal channels with openings to the underlying space. These morphological features are thought to be involved in the absorption of nutrients from food. Enzymatic assays inR. americana adults revealed that amylase occurs in salivary glands and midgut, whereas aminopeptidase, α-glucosidases and trypsin occur only in the midgut, mainly in the ventriculus. There is a soluble (Mr 105000) and a membrane-bound aminopeptidase (solubilized form, Mr 110000). Soluble α-glucosidase inactivates easily and could not be characterized, whereas membrane-bound α-glucosidases were resolved after solubilization into three molecular species (Mr 186000, 105000 and 84000) with different substrate specificities. The activities of trypsin (pH optimum 9.0), which was inhibited completely by soybean trypsin inhibitor, and of amylase (pH optimum 5.5), were not sufficiently high to be further characterized. The data support the assertion thatR. americana adults are able, to a limited extent, to digest and absorb starch and proteins, in addition to nectar sugars. The results, supported by published data, suggest that there is an inverse correlation between the digestive enzyme activities and midgut absorptive surface in insects which has nectar as a major food.     

Digestive physiology and characterization of digestive cathepsin L-like proteinase from the sugarcane weevil Sphenophorus levis

Sugarcane is an important crop that has recently become subject to attacks from the weevil Sphenophorus levis, which is not efficiently controlled with chemical insecticides. This demands the development of new control devices for which digestive physiology data are needed. In the present study, ion-exchange chromatography of S. levis whole midgut homogenates, together with enzyme assays with natural and synthetic substrates and specific inhibitors, demonstrated that a cysteine proteinase is a major proteinase, trypsin is a minor one and chymotrypsin is probably negligible. Amylase, maltase and the cysteine proteinase occur in the gut contents and decrease throughout the midgut; trypsin is constant in the entire midgut, whereas a membrane-bound aminopeptidase predominates in the posterior midgut. The cysteine proteinase was purified to homogeneity through ion-exchange chromatography. The purified enzyme had a mass of 37 kDa and was able to hydrolyze Z-Phe-Arg-MCA and Z-Leu-Arg-MCA with k_cat/K_m values of 20.0 ± 1.1 μM⁻¹ s⁻¹ and 30.0 ± 0.5 μM⁻¹ s⁻¹, respectively, but not Z-Arg-Arg-MCA. The combined results suggest that protein digestion starts in the anterior midgut under the action of a cathepsin L-like proteinase and ends on the surface of posterior midgut cells. All starch digestion takes place in anterior midgut. These data will be instrumental to developing S. levis-resistant sugarcane.     

Digestive enzymes in midgut cells,endo-and ectoperitrophic contents,and peritrophic membranes of Spodoptera frugiperda (lepidoptera) larvae

In the midgut of Spodoptera frugiperda larvae, subcellular fractionation data suggest that aminopeptidase and part of amylase, carboxypeptidase A, dipeptidase, and trypsin are bound to the microvillar membranes; that major amounts of soluble dipeptidase, cellobiase, and maltase are trapped in the cell glycocalyx; and finally that soluble carboxypeptidase, amylase, and trypsin occur in intracellular vesicles. Most luminal acetylglucosaminidase is soluble and restricted to the ectoperitrophic contents. Aminopeptidase occurs in minor amounts bound to membranes both in the ectoperitrophic contents and incorporated in the peritrophic membrane. Amylase, carboxypeptidase A, and trypsin are found in minor amounts in the ectoperitrophic contents (both soluble and membrane-bound) and in major amounts in the peritrophic membrane with contents. Part of the activities recovered in the last mentioned contents corresponds to enzyme molecules incorporated in the peritrophic membrane. The results suggest that initial digestion is carried out in major amounts by enzymes in the endoperitrophic space and, in minor amounts, by enzymes immobilized in the peritrophic membrane. Intermediate and final digestion occur at the ectoperitrophic space or at the surface of midgut cells. The results also lend support to the hypothesis that amylase and trypsin are derived from membrane-bound forms, are released in soluble form by a microapocrine mechanism, and are partly incorporated into the peritrophic membrane. © 1994 Wiley-Liss, Inc.     

Peritrophic membrane role in enhancing digestive efficiency. Theoretical and experimental models

The peritrophic membrane (PM) is an anatomical structure surrounding the food bolus in most insects. Rejecting the idea that PM has evolved from coating mucus to play the same protective role as it, novel functions were proposed and experimentally tested. The theoretical principles underlying the digestive enzyme recycling mechanism were described and used to develop an algorithm to calculate enzyme distributions along the midgut and to infer secretory and absorptive sites. The activity of a Spodoptera frugiperda microvillar aminopeptidase decreases by 50% if placed in the presence of midgut contents. S. frugiperda trypsin preparations placed into dialysis bags in stirred and unstirred media have activities of 210 and 160%, respectively, over the activities of samples in a test tube. The ectoperitrophic fluid (EF) present in the midgut caeca of Rhynchosciara americana may be collected. If the enzymes restricted to this fluid are assayed in the presence of PM contents (PMC) their activities decrease by at least 58%. The lack of PM caused by calcofluor feeding impairs growth due to an increase in the metabolic cost associated with the conversion of food into body mass. This probably results from an increase in digestive enzyme excretion and useless homeostatic attempt to reestablish destroyed midgut gradients. The experimental models support the view that PM enhances digestive efficiency by: (a) prevention of non-specific binding of undigested material onto cell surface; (b) prevention of excretion by allowing enzyme recycling powered by an ectoperitrophic counterflux of fluid; (c) removal from inside PM of the oligomeric molecules that may inhibit the enzymes involved in initial digestion; (d) restriction of oligomer hydrolases to ectoperitrophic space (ECS) to avoid probable partial inhibition by non-dispersed undigested food. Finally, PM functions are discussed regarding insects feeding on any diet.     

Compartmentalization of the digestive process in Abracris flavolineata (Orthoptera: Acrididae) adults

Carbohydrases predominate in the crop and their pH optima agree with pH prevailing in crop contents. Major amounts are also found in caecal contents. Aminopeptidase and trypsin are active mainly in the caeca, where they predominate in cells and contents, respectively. Aminopeptidase is partly membrane-bound. Except for trehalase, salivary glands display negligible amounts of digestive enzymes. The specific activity of digestive enzymes is high in all midgut cells and the enzyme molecules do not differ among gut compartments, as judged by polyacrylamide gel electrophoresis. Thus, it is probable that digestive enzymes are synthesized and secreted by all midgut cells (mainly in caeca) and then passed forward into the crop. Digestive enzymes are found in hindgut in concentrations similar to those in ventricular contents and, since they are stable in gut contents, they are likely excreted at a rate similar to undigested food. The data support the hypothesis that carbohydrate and protein are digested mainly in crop and caecal lumina, respectively, with part of the final digestion of proteins occurring at the surface of caecal cells. The peculiar features of the digestion of A. flavolineata grasshoppers, including the lack of midgut countercurrent fluxes, are thought to be derived from putative Polyneoptera ancestors.     

The anatomy and histology of the midgut and hindgut of Charybdis (Goniohellenus) truncata (Fabricius, 1798) (Decapoda: Brachyura)

The midgut of C. (G.) truncata accounts for half of the postgastric intestinal tract. The paired anterior midgut caeca arise just behind the pyloric stomach, on either side of the midgut. The unpaired posterior midgut caecum arises dorsally at the rear end of the midgut, where this joins the hindgut. The midgut and its caeca help in the digestive absorption of food. The hindgut is of ectodermal origin and is lined with chitin of a collagenous nature. The connective tissue of the anterior part of the hindgut is packed with tegumental glands whose secretion contains both sulphated and weakly acidic mucosubstances, which facilitate the passage of faecal matter and help to bind food particles. The digestive gland - the hepatopancreas - opens into the anterior part of the midgut, below the anterior midgut caeca. Histologically, its tubules contain three different types of cells - "F", "R" and "B" cells.     

The alimentary canal of <Emphasis Type="Italic">Blomia tropicalis</Emphasis> (Acari: Astigmata: Echymopodidae): the application of three-dimensional reconstruction technology

The domestic mite species Blomia tropicalis is an important indoor allergen source related to asthma and other allergic diseases in tropical and subtropical regions. Here, we describe the alimentary canal of B. tropicalis with the particular application of three-dimensional reconstruction technology. The alimentary canal of B. tropicalis resembles the typical acarid form consisting of the cuticle-lined foregut and hindgut separated by a cuticle-free midgut. The foregut is divided into a muscular pharynx and an esophagus. The midgut is composed of a central ventriculus, two lateral caeca, a globular colon and a postcolon with two tubiform postcolonic diverticula. The most common cells forming the epithelium of ventriculus and caeca are squamous and cuboidal. The globular cells contain a big central vacuole in the posterior region of the caeca. The epithelium of the colon and postcolon has significantly longer microvilli. The anal atrium is a simple tube with flattened epithelial cells. The spatial measurements of the three-dimensional model suggest that the paired caeca and central ventriculus occupy 55.1 and 34.6%, respectively, of the total volume of the alimentary canal and may play the key role in food digestion. J. Wu and F. Yang contributed equally.     

The ultrastructure of the digestive cells of Argulus japonicus, Thiele 1900 (Crustacea: Branchiura)

The ultrastructure of the cells of the digestive system of Argulus japonicus is described with the use of transmission electron microscopy. Specimens of Argulus japonicus were collected from the Vaal Dam in South Africa and fixed in Todd's fixative. The samples were post fixed in osmium tetroxide and embedded in resin. The anterior midgut is composed mostly of R cells while the enteral diverticula are composed mainly of R cells in the proximal diverticules and of F cells in the distal diverticula. The posterior midgut is composed of very large papilliform B cells and of R cells. The R cells in the anterior midgut probably absorb nutrients including lipids. The F cells are filled mostly with rough endoplasmic reticulum, suggesting enzyme synthesis, while the B cells portrayed endocytotic vesicles, indicating intracellular digestion of predigested food. The R cells of the posterior midgut are less active than cells present in the anterior midgut. E cells and peritrophic membrane were not observed.     

The fine structure of the midgut in the mite Anystis baccarum (L.) (Acari, Actinedida: Anystidae)

The ventriculus and the midgut caeca of the fed females of Anystis baccarum (L.) were investigated by using light and electron microscopy. In addition to the main type of polyfunctional digestive cells, special secretory cells were detected in the anterior region of the ventriculus. The shape and the ultrastructure of the digestive cells vary depending on their physiological state. Intracellular digestion, absorption or excretion processes prevail at different stages of the cell cycle. The secretory cells are characterized by the presence of extensive rough endoplasmic reticulum, filling whole space of the cell. These cells do not contain the apical network of pinocytotic canals, which are typical for the digestive cells. Three types of secretory granules were found in the cytoplasm of the secretory cells that probably correspond to three sequential stages of granulogenesis. The primary secretory granules are formed by the fusion of Golgi vesicles. The primary granules fuse to form complex vesicles with heterogeneous contents. These secondary granules aggregate to form very large inclusions of high electron density (tertiary secretory granules), which probably represent the storage of the secretory product. All types of secretory granules were observed close to the apical plasmalemma.     

The ultrastructure of the midgut and the formation of peritrophic membranes in a harvestman,Phalangium opilio (Chelicerata Phalangida)

Summary The ultrastructure of the midgut epithelium of Phalangium opilio was examined. In the anterior part of the midgut the epithelium consists of three different types of cells, called resorption, digestion, and excretion cells according to their presumed functions. Excretion cells may represent old digestion cells. The relation between resorption and digestion cells needs further investigation. The epithelium of the posterior part of the midgut consists of two types, transport and secretion cells, which seem to serve mainly for the resorption of water and the secretion of peritrophic membranes, respectively.Peritrophic membranes are secreted by the anterior midgut epithelium mainly in a period between 2 and 4 h after feeding. Chitin or chitin precursors could be localized in vesicles and in the brush border of midgut cells, and in the peritrophic membranes, using colloidal gold labelled with wheat germ agglutinin. Two different textures of chitin-containing microfibrils were found in the peritrophic membranes, either a random or a hexagonal texture. The latter results if the microfibrils polymerize between the basal parts of the microvilli. Irregularities of the hexagonal texture can be correlated with an irregular pattern of the microvilli. In the posterior midgut peritrophic membranes with a random texture, chitin-containing microfibrils are continuously secreted in the form of patches.     

The physiological role of the peritrophic membrane and trehalase: Digestive enzymes in the midgut and excreta of starved larvae of Rhynchosciara

Amylase, cellulase, trehalase, aminopeptidase and trypsin were determined using the midgut and trehalose using the haemolymph of starved and of subsequently fed larvae of Rhynchosciara americana. Midgut trehalase activity decreases steadily during starvation and increases again on feeding, whereas haemolymph trehalose titres remain constant, suggesting that trehalase is a true digestive enzyme. The decrease in amylase, cellulase and trypsin activity in the midgut during starvation is of the same order as that recovered from the excreta. Since this finding is exactly what one would expect if enzyme production stops in response to starvation, this supports the hypothesis that synthesis that synthesis of these enzymes is controlled. The excretion rate of amylase, cellulase and trypsin is very low in comparison to their activity inside the peritrophic membrane and the travel time of the food bolus through the gut. It is proposed that the peritrophic membrane separates two extracellular sites for digestion as an adaptation to conserve secreted enzymes. This could be accomplished by the existence of an endo-ectoperitrophic circulation of the enzymes involved in the initial attack on the food and by restricting to the ectoperitrophic fluid the enzymes which participate only in intermediary digestion of food.