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

16 F ood Uptake and Utilizatio n 1 . Intr oduc t ion I nsects feed on a wide ran g eofor g anic materials. About 7 5 % of all species are ph y- t opha g ous, and these form an important link in the transfer of ener gy from primar y produc- ers to second-order consumers. Others are carnivorous, omnivorous, or p arasitic on othe r an i ma l s. In accor d w i t h t h e di vers i ty o ff ee di ng h a bi ts, t h e means b yw hi c hi nsects l ocate th e i r f oo d ,t h e structure an d p h ys i o l ogy o f t h e i r di gest i ve system, an d t h e i r meta b o li sm ar e highly var i e d . The feedin g habits of insects take on special si g nificance for humans, on the one hand , b ecause of the enormous dama g e that feedin g insects do to our food, clothin g , and health, an d ,ont h eot h er, b ecause o f t h e mass i ve b ene fi ts t h at i nsects p rov id eas pl ant p o lli nators d ur i ng t h e i r searc hf or f oo d (see a l so C h apter 24). In a ddi t i on, b ecause many spec i es ar e eas ily an d c h eap ly mass-cu l ture di nt h e l a b orator y ,t h e yh ave b een use d w id e ly i n researc h on di g estion and absorption, as well as in the elucidation of basic biochemical pathwa y s, t he role of s p ecific nutrients, and other as p ects of animal metabolism . 2. Food S election and Feedin g D i st i nct v i sua l ,c h em i ca l ,an d mec h an i ca l cues act at eac h step o f t h e f oo dl ocat i on and in g estion process. These steps include attraction to food, arrest of movement, tastin g , b itin g , further tastin g as in g estion be g ins, continued in g estion, and termination of feedin g . T he sensitivity of the insect to these cues varies with its physiological state. For example , a starve di nsect may b ecome hi g hl y sens i t i ve to o d ors or tastes assoc i ate d w i t hi ts norma l f oo d ,an di n extreme cases may b ecome qu i te i n di scr i m i nate i n terms o f w h at i t i ngests. On the other hand, a female whose abdomen is full of e gg s is normall y “uninterested” in feedin g. I n some plant-feeding (phytophagous) species, visual stimuli such as particular pat - t erns (espec i a ll y str i pes) or co l ors may serve to i n i t i a ll y attract an i nsect to a potent i a lf oo d source. Usua ll y, h owever, t h e i n i t i a l or i entat i on, w h ere t hi s occurs, i s d epen d ent on o lf ac- t or y stimuli. In man y larval forms there appear to be no specific orientin g stimuli because, u nder normal circumstances, larvae remain on the food plant selected b y the mother prior 4 8 7 488 CHAPTER 1 6 to oviposition. In the mi g rator y locust, on which much work has been done, olfaction is of primar y importance in food location. Once the insect makes contact with the ve g etation, tarsal chemosensilla initiate a reflex that results in the stoppa g e of movement. Sensilla on t h e l a bi a l an d max ill ary pa l ps t h en taste t h e sur f ace waxes o f t h ep l ant, a f ter w hi c h t h e l ocus t ta k es a sma ll bi te. W h et h er f ee di ng cont i nues i s somet i mes d eterm i ne db y mec h anosens ill ar responses to p hy s i ca l st i mu li suc h as t h e h ar d ness, tou gh ness, s h ape, an dh a i r i ness o f t he f ood. More commonl y , it is substances in the released sap that, b y stimulatin g chemosensilla i n the cibarial cavit y ,re g ulate the continuation or arrest of feedin g (Chapman, 2003). These su b stances are ca ll e d “p h agost i mu l ants” or “ d eterrents,” respect i ve l y. T h esu b stances may h ave nutr i t i ona l va l ue to t h e i nsect or may b e nutr i t i ona ll yun i mportant (“to k en st i mu li ”). Nutritional factors are almost alwa y s stimulatin g in effect. Su g ars, especiall y sucrose, ar e i mportant pha g ostimulants for most ph y topha g ous insects. Amino acids, in contrast, are g en - e rall y b y themselves weakl y stimulatin g or non-stimulatin g , thou g hma y act s y ner g isticall y w ith certain sugars or token stimuli. For example, Heron (1965) showed in the spruce bud - w orm ( C h oristoneura f umi f eran a )t h at, w h ereas sucrose an d l -pro li ne i n l ow concentrat i on w ere i n di v id ua lly on ly wea k p h a g ost i mu l ants, a m i xture o f t h etwosu b stances was highly stimulatin g . In addition to su g ars and amino acids, other specific nutrients ma y stimulat e f eedin g in a g iven species. Such nutrients include vitamins, phospholipids, and steroids. T o ke nst i mu li may e i t h er st i mu l ate or i n hibi t f ee di ng. T h us, d er i vat i ves o f mustar d o il, p ro d uce db y cruc if erous p l ants, i nc l u di ng ca bb age an di ts re l at i ves, are i mportant p h agos- t i mu l ants f oravar i et y o fi nsects t h at norma lly f ee d on t h ese p l ants, f or examp l e, l arva e o f the diamondback moth ( Plutella xylostella), the cabba g e aphid ( B revicoryne brassicae ) , and the mustard beetle ( Phaedon cochlearia e ) . Indeed , Plutella w ill feed naturall y onl y on p lants that contain mustard oil compounds. Many secondary plant metabolites, includin g a lk a l o id s, terpeno id s, p h eno li cs, an d g l ycos id es, are f ee di ng d eterrents f or p h ytop h agou s i nsects. In a g i ven f oo d source t h ere w ill pro b a bl y b eam i xture o f p h agost i mu l ants an d deterrents, and the balance of this sensor y input, inte g rated throu g h the central nervou s sy stem, determines the overall palatabilit y of the food . Species whose choice of food is limited are said to be oligophagous. In extreme cases, an i nsect may b e restr i cte d to f ee di ng on a s i ng l ep l ant spec i es an di s d escr ib e d as monop h agous. S pec i es t h at may f ee d onaw id evar i ety o f p l ants are po l yp h agous, t h oug hi t must b e note d that even these exhibit selectivit y when g iven a choice. Not surprisin g l y , monopha g ous and oli g opha g ous species are especiall y sensitive to the presence of deterrents in non-host p lants. In many pre d aceous i nsects, espec i a ll yt h ose t h at act i ve l y pursue prey, v i s i on i so f p r i mary i mportance i n l ocat i ng an d captur i ng f oo d . As note di nC h apter 12 (Sect i on 7.1.2) , s ome pre d aceous i nsects h ave bi nocu l ar v i s i on t h at ena bl es t h em to d eterm i ne w h en pre yi s within catchin g distance. Carnivorous species, especiall y larval forms, whose visual sens e i s less well developed, depend on chemical or tactile stimuli to find pre y . For example , m any b eet l e l arvae t h at li ve on or i nt h e groun dl ocate prey b yt h e i r scent. Spec i es paras i t i c o not h er an i ma l s usua ll y l ocate a h ost b y i ts scent, t h oug h tsetse fli es may i n i t i a ll yor i ent by v i sua l means to a potent i a lh ost. For man y spec i es t h at f ee d on t h e bl oo d o fbi r d san d m ammals, temperature and/or humidit yg radients are important in determinin g the precis e l ocation at which an insect ali g hts on a host and be g ins to feed . T he extent of food specificity for carnivorous insects is varied. Many insects are quite n on-spec ifi can d w ill attempt to capture an d eat any organ i sm t h at f a ll sw i t hi nag i ven s i ze r ange (even to t h e extent o fb e i ng cann ib a li st i c). Ot h ers are more se l ect i ve; f or examp l e , s pider wasps (Pompilidae), as their name indicates, capture onl y spiders for provisionin g 489 FOO D U PT A KE A ND UTILIZATION t heir nest. Parasitic insects, too, exhibit various de g rees of host specificit y . Thus, cer - t ain sarcopha g id flies parasitize a ran g eof g rasshopper species; the common cattle g rub ( H ypoderma lineatum)ist y picall y found on cattle or bison, rarel y on horses and humans; li ce are extreme l y h ost-spec ifi c, as wou ld b e expecte d o f se d entary spec i es . T h e term i nat i on o ff ee di ng (assum i ng t h at f oo d supp l y i s not li m i t i ng) i s l arge l yre l ate d t o the amount of food in g ested and the stimulation of strate g icall y located stretch receptors. I n locusts, for example, the fillin g of the crop is measured b y receptors at the anterio r end that send si g nals to the brain via the stomato g astric s y stem. In flies both crop- an d esop h agus- filli ng are i mportant i n b r i ng i ng f ee di ng to a c l ose, w hil e i n f ema l e mosqu i toe s an d , pro b a bl y , Rh o d nius p ro l ixu s th es i gna l sar i se f rom stretc h receptors l ocate di nt he abdominal wall, which is g reatl y distended after a blood meal. Other factors that ma y pla y a minor role in terminatin g feedin g are adaptation of the chemosensilla on the mouthpart s and a chan g e in the osmotic pressure or composition of the hemol y mph as absorption of d igested materials occurs (see chapters in Chapman and de Boer, 199 5 ). Apart f rom t h e spec ifi c cues out li ne d a b ove t h at f ac ili tate l ocat i on an d se l ect i on o f f oo d ,t h ere are ot h er f actors t h at i n fl uence f ee di n g act i v i t y .T y p i ca lly , i nsects d o not f ee d shortl y before and after a molt, or when there are mature e gg s in the abdomen. In addition , a diurnal rh y thm of feedin g activit y ma y occur, in response to a specific li g ht, temperature , or h um idi ty st i mu l us. For examp l e, t h ere dl ocust ( N oma d acris septem f asciata) f ee d s i n th e morn i ng an d even i ng, an d many mosqu i toes f ee dd ur i ng t h e ear l yeven i ng (t h oug h thi sma y c h an g e i n diff erent h a bi tats). Pupae, most di apaus i n gi nsects, an d some a d u l t E phemeroptera, Lepidoptera, and Diptera do not feed . 3 . The Alimentar y S y stem The g ut and its associated g lands (Fi g ure 1 6 .1) triturate, lubricate, store, di g est, and absorb food material and expel the undi g ested remains. Structural differences throu g hout the system reflect regional specialization for performance of these functions and are correlate d a l so w i t hf ee di ng h a bi ts an d t h e nature o f norma lf oo d mater i a l .T h e structure o f t h e system ma y var y at diff erent sta g es o f t h e lif e hi stor yb ecause o f t h e diff erent f ee di n gh a bi ts o f t he larva and adult of a species. The g ut normall y occurs as a continuous tube betwee n t he mouth and anus, and its len g th is broadl y correlated with feedin g habits, bein g short in carnivorous forms where digestion and absorption occur relatively rapidly, and longer (o f ten convo l ute d ) i np h ytop h agous f orms. In a f ew spec i es t h at f ee d on fl u id s, suc h a s l arvae o f Neuroptera an d Hymenoptera-Apocr i ta, an d some a d u l t Heteroptera t h ere i s li tt le or no solid waste in the food, and the j unction between the mid g ut and hind g ut is occluded. As Fi g ure 16.1 indicates, food first enters the buccal cavit y , which is enclosed b y the mouthparts and is not strictly part of the gut. It is into the buccal cavity that the salivar y g l an d sre l ease t h e i r pro d ucts. T h e gut proper compr i ses t h ree ma i nreg i ons: t h e f oregut, i nw hi c h t h e f oo d may b e store d , fil tere d ,an d part i a ll y di geste d ;t h em id gut, w hi c hi st h e pr i mar y s i te f or dig est i on an d a b sorpt i on o ff oo d ;an d t h e hi n dg ut, w h ere some a b sorpt i o n and feces formation occur . 3.1. Salivar y Glands S a li var ygl an d s are present i n most i nsects, t h ou gh t h e i r f orm an df unct i on are extreme ly varied, and the y ma y or ma y not be innervated (Ribeiro, 1995). Frequentl y the y are known 490 CHAPTER 1 6 F I GU RE 16.1. Alimentary canal and associated structures of a locust. [After C. Hodge, 1939, The anatomy an d hi sto l ogy o f t h ea li mentary tract o f Locusta migratoria L. (Ort h optera: Acr idid ae) , J. Morp h o l . 64 : 37 5 –399. By p erm i ss i on o f t h eW i star Press. ] b yot h er names accor di ng to e i t h er t h es i te at w hi c h t h e i r d uct enters t h e b ucca l cav i ty, f o r e xample, labial g lands and mandibular g lands, or their function, for example, silk g lands and venom g lands . T ypically, saliva is a watery, enzyme-containing fluid that serves to lubricate the f oo d an di n i t i ate i ts di gest i on. L ik et h at o fh umans, t h esa li va genera ll y conta i ns on l y c ar b o h y d rate- di gest i ng enzymes (amy l ase an di nvertase), t h oug h t h ere are except i ons t o this statement. For example, the saliva of some carnivorous species contains protein- and/o r f at-di g estin g enz y mes onl y ; that of bloodsuckin g species has no enz y mes. In termite saliva there are cellulose-di g estin g enz y mes: a β -1-4- g lucanase that brin g s about the initial split- t i ng o f t h epo l ymer, an d β -g l ucos id ase t h at d egra d es t h e resu l t i ng ce ll o bi ose to g l ucose ( Na k as hi ma et a l. , 2002 ; To k u d a e ta l , 2002) . (See a l so Sect i on 4.2.4. ) In t h e i nnervate dgl an d so f coc k roac h es an dl ocusts, re l ease o f sa li va i s i n d uce d w h e n f ood stimulates mechano- and chemosensilla on the mouth p arts and antennae. The informa- tion travels to the subesopha g eal g an g lion and then alon g aminer g ic or peptider g ic neuron s to t h eg l an d sw h ere i t i n d uces re l axat i on o f t h e musc l es t h at norma ll yc l ose o ff t h e open i ng of t h esa li vary g l an dd uct (A li , 1997). In contrast, t h e non- i nnervate d g l an d so f Ca ll i ph ora er y t h rocep h a la a re st i mu l ate d to re l ease sa li va by a h emo ly mp hf actor, poss ibly seroton i n ( Trimmer, 1985 ). 491 FOO D U PT A KE A ND UTILIZ A TION Other substances that ma y occur in saliva, thou g h havin g no direct role in di g estion, ar e im p ortant in food ac q uisition. For exam p le, the saliva of a p hids has a viscous com p onent, released durin g penetration of the st y lets, which hardens to form a leakproof seal around th e mout hp arts. A phid sa li va a l so conta i ns p ect i nase an dp erox id ase. T h e f ormer f ac ili tate s penetrat i on o f t h e sty l ets t h roug h t h e i nterce ll u l ar spaces o f p l ant t i ssues w hil et h e l atter ma yi nact i vate tox i cp hy toc h em i ca l s(M il es, 1999). H y a l uron id ase, w hi c hb rea k s d ow n connective tissue, is secreted b y some insects that suck animal tissue fluids. A spectrum of compounds that assist feedin g is present in the saliva of bloodsuckin g species. These i nc l u d e ant i coagu l ants, i n hibi tors o f p l ate l et di s i ntegrat i on, pyrase (an enzyme t h at b rea ks d own ADP, to prevent p l ate l et aggregat i on), an d vaso dil ators suc h as n i tr i cox id e(R ib e i ro , 199 5 ; Ribeiro and Francischetti, 2003 ) . The nitric oxide is carried to the host’s skin o n h eme-containin g proteins (nitrophorins) (Valenzuela and Ribeiro, 1998). The nitrophorins also stron g l y bind histamine, released b y the host to induce wound healin g (Weichsel et al. , 1998). Tox i ns (venoms), w hi c h para l yze or kill t h e prey, occur i nt h esa li va o f some assass in b ugs (Re d uv iid ae) an d ro bb er fli es (As ilid ae). It i sa l so reporte d t h at su b stances t h at i n d uce g a ll f ormat i on by st i mu l at i n g ce ll di v i s i on an d e l on g at i on are present i nt h esa li va o f som e g all-inhabitin g species. Larvae of black flies and chironomid mid g es secrete lar g e amount s of viscous saliva, formin g nets that capture food particles. I n some spec i es t h eg l an d s h ave ta k en on f unct i ons qu i te unre l ate d to f ee di ng, f or examp l e, pro d uct i on o f cocoon s ilk b yt h e l a bi a l g l an d so f caterp ill ars an d ca ddi s fl y l arvae, an d p h eromone pro d uct i on by t h e man dib u l ar gl an d so f t h e queen h one yb ee. 3 . 2 . Fore g u t T h e f oregut, f orme dd ur i ng em b ryogenes i s b y i nvag i nat i on o f t h e i ntegument, i s li ne d wi t h cut i c l e(t h e i nt i ma) t h at i ss h e d at eac h mo l t. Surroun di ng t h e i nt i ma, w hi c h may b e folded to enable the g ut to stretch when filled, is a thin epidermis, small bundles o f lon g itudinal muscle, a thick la y er of circular muscle, and a la y er of connective tissue throu g h w hich run nerves and tracheae (Figure 16.2). The foregut is generally differentiated into p h arynx, esop h agus, crop, an d proventr i cu l us. Attac h e d to t h ep h aryngea li nt i ma are dil ato r musc l es. T h ese are espec i a ll ywe ll d eve l ope di n suc ki ng i nsects an df orm t h ep h aryngea l pump (Chapter 3, Section 3.2.2). The esopha g us is usuall y narrow but posteriorl y ma y b e d ilated to form the cro p where food is stored. In Di p tera and Le p ido p tera, however, the cro p is actuall y a diverticulum off the esopha g us. Durin g stora g e the food ma y under g o some di gest i on i n i nsects w h ose sa li va conta i ns enzymes or t h at regurg i tate di gest i ve fl u id f rom th em id gut. In some spec i es t h e i nt i ma o f t h e crop f orms sp i nes or r id ges t h at pro b a bl ya id i n b reakin g up solid food into smaller particles and mixin g in the di g estive fluid (Fi g ure 1 6 .2A). T he hindmost re g ion of the fore g ut is the proventriculus, which ma y serve as a valve re g ulatin g the rate at which food enters the mid g ut, as a filter separatin g liquid and soli d components, or as a gr i n d er to f urt h er b rea k up so lid mater i a l . Its structure i s, accor di ng l y, qu i te var i e d . In spec i es w h ere i t acts as a va l ve t h e i nt i ma o f t h e proventr i cu l us may f orm l ong i tu di na lf o ld san d t h ec i rcu l ar musc l e l ayer i st hi c k ene d to f ormasp hi ncter. W h en a fi lter, the proventriculus contains spines that hold back the solid material, permittin g onl y liquids to move posteriorl y . Where the proventriculus acts as a g izzard, g rindin g up food, t he intima is formed into strong, radially arranged teeth, and a thick layer of circular muscle covers the entire structure (Figure 16.2B) . Poster i or l yt h e f oregut i s i nvag i nate d s li g h t l y i nto t h em id gut to f orm t h e esop h agea l ( = s tomodeal) inva g ination (Fi g ure 1 6 .3). Its function is to ensure that food enters the 492 CHAPTER 1 6 F I GU RE 16.2 . T ransverse sections throu g h (A) crop and (B) proventriculus of a locust. [After C. Hod g e, 1939, T he anatomy and histology of the alimentary tract o f L ocusta mi g ratoria L. (Orthoptera: Acrididae) , J . M or p hol . 64 :37 5 –399. B y permission of Wistar Press. ] m idgut within the peritrophic matrix. It also appears to assist in molding the peritrophic m atr i x i nto t h e correct s h ape i n some i nsects. 3 .3. Midgut Th em id gut (= ventr i cu l u s = m esenteron) i so f en d o d erma l or i g i nan d ,t h ere f ore, has no cuticular linin g . In most insects, however, it is lined b y a thin peritrophic matrix ( PM) composed of proteins bound to a meshwork of chitin fibrils (Fi g ure 1 6 .4). Some PM proteins, the peritrophins, are heavil yg l y cos y lated like mucus in the intestine of vertebrates. T h e f unct i ons o f t h e PM are to prevent mec h an i ca ld amage to t h em id gut ep i t h e li um, t o prevent entry o f m i croorgan i sms i nto t h e b o d ycav i ty, to bi n d potent i a l tox i ns an d ot h e r d ama gi n g c h em i ca l s, an d to compartmenta li ze t h em idg ut l umen, t h at i s, to di v id e i t i nt o an endoperitrophic space (within the matrix) and an ectoperitrophic space (ad j acent to the 493 FOO D U PT A KE A ND UTILIZ A TION F IGURE 16.3. L ong i tu di na l sect i on t h roug h crop, p roventr i cu l us, an d anter i or m idg ut o f a coc k roac h. [ From R. E. Snodgrass , P rinciples o f Insect Morphol- ogy . C opyright 1935 by McGraw-Hill, Inc. Used wit h p erm i ss i on o f McGraw-H ill Boo k Compan y . ] midgut epithelium) (Terra, 199 6 ; Lehane, 1997). This separation of the epithelium from th e f oo di mproves dig est i ve e ffi c i enc yby se g re g at i n g enz y mes b etween t h e spaces an d ena bli n g some enz y mes to be rec y cled (Section 4.2.1). The PM is g enerall y absent in fluid-feedin g insects, for example, Hemiptera, adult L epidoptera, and bloodsucking Diptera. However, some insects produce the PM only at certa i nt i mes (e.g., f ema l e mosqu i toes a f ter a bl oo d mea l ). Furt h er, as d escr ib e db e l ow , t h e F I G URE 16.4. Transverse sect i on t h rou gh m idg ut o f a l ocust. [A f ter C. Ho dg e, 1939, T h e anatom y an dhi sto l o gy of the alimentar y tract o f L ocusta m ig rator i a L. (Ortho p tera: Acrididae), J. Mor p hol . 64 : 37 5 –399. B y permission of Wistar Press. ] 494 CHAPTER 1 6 t y pe of PM, whether or not a PM is produced, and the manner in which it is produced, ma y v ar y between life sta g es (Lehane, 1997). T he PM is formed in two principal wa y s. In T y pe I PM delamination of successiv e c oncentr i c l ame ll ae occurs a l ong t h em id gut ( i nO d onata, Ep h emeroptera, P h asm id a, som e O rt h optera, some Co l eoptera, an dl arva l Lep id optera). T h e Type II PM f orms b y secret i o n f rom a spec i a l zone o f ce ll s (car di a) at t h e anter i or en d o f t h em idg ut ( i nD i ptera, Dermaptera , Isoptera, Embioptera, and some Lepidoptera). In this method the esopha g eal inva g ination presses firml y a g ainst the anterior wall of the mid g ut so that the ori g inall y viscous secretio n of t h e PM-pro d uc i ng ce ll s, as i t h ar d ens, i s squeeze d to f orm t h etu b u l ar mem b rane. In D i c- tyoptera, ot h er Ort h optera an d Lep id optera, Hymenoptera, an d Neuroptera, a com bi nat i on o f both methods seems to be used. In mosquitoes, larvae produce a T y pe II PM, whereas the adults have a T y peIPM . T he PM is made u p of a meshwork of microfibrils between which is a thin p roteinaceous film. The microfibrils ha v e a constant 60 ◦ or i entat i on to eac h ot h er i n Type I PM, t h oug h t to resu l t f rom t h e i r secret i on b yt h e h exagona ll yc l ose-pac k e d m i crov illi o f t h eep i t h e li a l c e ll s. In T y pe II PM t h eor i entat i on o f t h em i cro fib r il s i s ran d om. T h ePM i s permea bl eto the products of di g estion and to certain di g estive enz y mes released from the epithelial cells ( Section 4.2.1). However, it is not permeable to other lar g e molecules, such as undi g ested p rote i ns an d po l ysacc h ar id es, i n di cat i ng t h at t h ePM h as a di st i nct po l ar i ty an di s not mere ly a nu l tra fil ter (R i c h ar d san d R i c h ar d s, 1977; Le h ane, 1997). Th em id gut i s usua ll y not diff erent i ate di nto structura ll y di st i nct reg i ons apart f rom t h e development, at the anterior end, of a varied number of blindl y endin g ceca, which serve to i ncrease the surface area available for enz y me secretion and absorption of di g ested material. In many Heteroptera, however, the midgut is divided into three or four easily visible regions. In t h ec hi nc hb ug ( Bl issus l eucopterus) four such regions occur (Figure 16. 5 ). The anterior r eg i on i s l arge an d sac lik e, an d serves as a storage reg i on (no crop i s present). T h e secon d r e g ion serves as a valve to re g ulate the flow of material into the third re g ion where di g estio n F IGURE 16.5. A li mentar y cana l o f c hi nc hb u g ( Bl issus l euco p terus ) showin g re g ional differentiation of mid g ut. [ After H. Glasgow, 1914, The gastric caeca and the caeca l b acter i ao f t h e Heteroptera , B io l .Bu ll . 26 : 101 – 1 7 0 . ] 495 FOO D U PT A KE A ND UTILIZ A TION F IGURE 16.6 . Ali mentar y cana l o f cercopid (Cercopoidea) showin g filter chamber arrangement. [From R. E. Snod- g rass, P rincip l es of Insect Morp h o l ogy . Cop y ri g ht 193 5 b y McGraw-Hill, Inc . U sed with permission of McGraw-Hil l B oo k Compan y . ] pro b a bly occurs. Ten fi n g er lik e ceca fill e d w i t hb acter i a are attac h e d to t h e f ourt h re gi on , w hich ma y be absorptive in function. The role of the bacteria is not known. I n man y homopterans, which feed on plant sap, the mid g ut is modified both morpho- logically and anatomically so that excess water present in the food can be removed, thus prevent i ng dil ut i on o f t h e h emo l ymp h .T h oug hd eta il s vary among diff erent groups o fh o- mopterans, t h e anter i or en d o f t h em id gut (or, i n some spec i es, t h e poster i or part o f t he esopha g us) is brou g ht into close contact with the posterior re g ion of the mid g ut (or anterior h ind g ut), and the re g ion of contact becomes enclosed within a sac called the “filter cham- b er” (Figure 16.6). Such an arrangement facilitates rapid movement of water by osmosi s f rom t h e l umen o f t h e anter i or m id gut across t h ewa ll o f t h e poster i or m id gut an d poss ibl y a l so t h eMa l p i g hi an tu b u l es. T h us, re l at i ve l y li tt l eo f t h eor i g i na l water i nt h e f oo d actua ll y passes alon g the full len g th of the mid g ut. The lack of morpholo g ical differentiation within the mid g ut of most species is reflecte d in its uniform histology. Throughout its length, the mature cells lining the lumen are identical an d serve to pro d uce di gest i ve enzymes, to a b sor b t h e pro d ucts o fdi gest i on, an di n some i nsects secrete t h e Type I PM. Rep l acement o fd egenerate ce ll s occurs w i t h t h e maturat i on and differentiation of re g enerative cells found sin g l y or in g roups (nidi) near the base o f t he epithelium (Fi g ure 1 6 .4). Numerous peptide hormone-containin g cells also occur in the mid g ut, which ma y pla y a role in modulatin g mid g ut contraction (Lan g e and Orchard, 1998) . I n some spec i es hi sto l og i ca l diff erent i at i on i s f oun d . For examp l e, spec i a li zat i on o f cer - t a i n anter i or ce ll s f or Type I PM pro d uct i on was note d ear li er.Ina ddi t i on, diff erent i at i on i nto dig est i ve an d a b sorpt i ve re gi ons occurs i n some spec i es. In tsetse fli es t h ece ll so f t h ean - t erior mid g ut are small and are concerned with absorption of water from the in g ested blood. T he y produce no enz y mes and di g estion does not be g in until food reaches the middle re g ion wh ere t h ece ll s are l arge, r i c hi nr ib onuc l e i cac id ,an d pro d uce enzymes. In t h e poster i o r m id gut t h ece ll s are sma ll er, c l ose l y pac k e d ,an d pro b a bl y concerne d w i t h a b sorpt i on o f dig este df oo d . In some spec i es diff erent re gi ons o f t h em idg ut are apparent ly a d apte d to t h e absorption of particular food materials. I n Aedes l arvae the anterior mid g ut is concerne d 49 6 CHAPTER 1 6 w ith fat absorption and stora g e, whereas the posterior portion absorbs carboh y drates an d stores them as g l y co g en. In larval Lepidoptera g oblet cells, with a lar g e flask-shaped central c avit y , are scattered amon g the re g ular epithelial cells. The y are thou g ht to pla y a role in t h eregu l at i on o f t h e potass i um l eve l w i t hi nt h e h emo l ymp h (C h apter 18, Sect i on 2.2). 3 .4. H i nd g ut T he hindgut is an ectodermal derivative and, as such, is lined with cuticle, though this i st hi nner t h an t h at o f t h e f oregut, a f eature re l ate d to t h ea b sorpt i ve f unct i on o f t hi sreg i on. T h eep i t h e li a l ce ll st h at surroun d t h e cut i c l e are fl attene d except i nt h e recta l pa d s (se e below) where the y become hi g hl y columnar and filled with mitochondria. Muscles are onl y w eakl y developed and, usuall y , the lon g itudinal strands lie outside the sheet of circular m uscle . Th e hi n d gut usua ll y h as t h e f o ll ow i ng reg i ons: py l orus, il eum, an d rectum. T h epy l orus m ay h aveawe ll - d eve l ope d c i rcu l ar musc l e l ayer (py l or i csp hi ncter) an d regu l ate t h e move - m ent o f mater i a lf rom m idg ut to hi n dg ut. A l so, t h eMa l p ighi an tu b u l es c h aracter i st i ca lly e nter the g ut in this re g ion. The ileum (Fi g ure 1 6 .7A) is g enerall y a narrow tube that serve s to conduct undi g ested food to the rectum for final processin g . In some insects, however , some a b sorpt i on o fi ons an d /or water may occur i nt hi sreg i on. In a f ew spec i es pro d uct i on an d excret i on o f n i trogenous wastes occur i nt h e il eum (C h apter 18, Sect i on 2.2). In many w oo d -eat i n gi nsects, f or examp l e, spec i es o f term i tes an db eet l es, t h e il eum i s dil ate d t o f orm a fermentation pouch housin g bacteria or protozoa that di g est wood particles. The products of di g estion, when liberated b y the microor g anisms, are absorbed across the wal l of t h e il eum. T h e most poster i or part o f t h e gut, t h e rectum, i s f requent l y dil ate d .T h oug hf o r t h e most part t hi n-wa ll e d ,t h e rectum i nc l u d es s i xtoe i g h tt hi c k -wa ll e d recta l pa d s(F i gure 16 .7B) whose function is to absorb ions, water, and small or g anic molecules (Chapter 18 , S ection 4). As a result, the feces of terrestrial insects are expelled as a more or less dr y pellet . Frequentl y , the pellets are ensheathed within the PM, which continues into the hind g ut. 4 . G ut Phys i ology T he primar y functions of the alimentar y canal are di g estion and absorption. For these processes to occur e ffi c i ent l y, f oo di s move d a l ong t h e cana l . In some spec i es, enzyme secret i ons are move d anter i or l ysot h at di gest i on can b eg i n some t i me b e f ore f oo d reac h e s t h ere gi on o f a b sorpt i on . 4 .1. Gut Movement s Th oug h t h ea li mentary cana li s i nnervate d , neura l contro li spr i nc i pa ll y assoc i ate d wi t h t h e open i n g /c l os i n g o f va l ves t h at occur w i t hi nt h e cana l (see b e l ow). T h er hy t h m ic peristaltic muscle contractions that move food posteriorl y throu g h the g ut are m y o g enic; that i s, the y ori g inate within the muscles themselves rather than occurrin g as a result of nervou s st i mu li . Myogen i c centers h ave b een l ocate di nt h e esop h agus, crop, an d proventr i cu l us, in G a ll eria, f or examp l e. In i nsects t h at f orm a Type II PM, b ac k war d movement o ff oo di s a id e dbyg rowt h o f t h e mem b rane. Ant i per i sta l t i c movements a l so occur i n some spec i es and serve to move di g estive fluid forward from the mid g ut into the crop. [...]... produced As noted earlier, α-glucosidase will hydrolyze all α-glucosides Likewise, β-glucosidase facilitates splitting of cellobiose, gentiobiose, and phenylglucosides; β-galactosidase hydrolyzes β-galactosides such as lactose In some species, however, there appear to be carbohydrate-digesting enzymes that exhibit absolute specificity Thus, adult Lucilia cuprina produce an α-glucosidase, trehalase, that... at metamorphosis it may make up between one-third and one-half of the dry weight of an insect Large amounts of fat also accumulate in the egg during vitellogenesis Fats are used as an energy source during “long-term” energy-requiring events, for example, embryogenesis, metamorphosis, starvation, and sustained flight (Chapter 14, Section 3.3.5) On a weight-for-weight basis, fats contain twice as much... Physiol B 109:1–62 Tokuda, G., Saito, H., and Watanabe, H., 2002, A digestive β-glucosidase from the salivary glands of the termite, Neotermes koshuensis (Shiraki): Distribution, characterization and isolation of its precursor cDNA by 5 - and 3 -RACE amplifications with degenerate primers, Insect Biochem Molec Biol 32 :168 1 168 9 Treherne, J E., 1967, Gut absorption, Annu Rev Entomol 12:43–58 Trimmer,... Hall, London Vol Locke, M., 1998, The fat body, in: Microscopic Anatomy of Invertebrates, V 11A (Insecta), Wiley-Liss, New York Miles, P W., 1999, Aphid saliva, Biol Rev 74:41–85 Nakashima, K, Watanabe, H., Saitoh, H., Tokuda, G., and Azuma, J.-I., 2002, Dual cellulose-digesting system of the wood-feeding termite, Coptotermes formosanus Shiraki, Insect Biochem Molec Biol 32:777– 784 Perry, A S., and Agosin,... g1ucose, glucuronic acid, or phosphate are examples of the methods employed (Perry and Agosin, 1974; Wilkinson, 507 FOOD UPTAKE AND UTILIZATION 508 CHAPTER 16 1976) Hydroxylation and conjugation are also important in making the normally fat-soluble insecticides water-soluble so that they can be excreted Each of these processes is enzymatically controlled, and it is not surprising to find, therefore, that metabolic... the adult emerges, the concentration has fallen to about one-third the value at pupation During sexual maturation in females of many species, the fat body produces vitellogenins (“female-specific” proteins) These proteins, whose synthesis is regulated by juvenile hormone or ecdysone, are accumulated in large amounts by the developing oocytes (Chapter 19, Section 3.1.1) In female insects that do not feed... is not always obligate, but may be facultative or even accidental Bacteria are important cellulose-digesting agents in many phytophagous insects, especially wood-eating species whose hindgut may include a fermentation pouch in which the microorganisms are housed In other species, for example, the wood-eating cockroach Panesthia, bacteria in the crop are essential for cellulose digestion In larvae of... esterases bring about hydrolysis, mixed-function oxidases commonly induce hydroxylation, and glutathione S-transferase is responsible for promoting conjugation with this tripeptide In fact, the involvement of these enzymes in insecticide detoxication appears to be an extension of a more general function Thus, insects that encounter a broad range of naturally occurring plant-derived toxicants, for example,... effective synergist for DDT in DDT-resistant houseflies However, perhaps not surprisingly, by 1955 the flies had developed resistance to the combination! 6 Summary Visual, tactile, or chemical cues stimulate food location and/or selection in most insects The stimuli may be general, for example, color, pattern, and size, or highly specific 509 FOOD UPTAKE AND UTILIZATION 510 CHAPTER 16 such as the particular odor... UPTAKE AND UTILIZATION 502 CHAPTER 16 transport the fungi to new locations Some ants and higher termites, for example, culture ascomycete or basidiomycete fungi in special regions of the nest called fungus gardens Chewed wood or other vegetation is brought to the fungus garden and becomes the substrate on which the fungi grow, forming hyphae to be eaten by the insects Certain wood-boring insects, for example, . at which the y are converted to trehalose. Apparentl y n o m ec h an i sm f or act i ve upta k eo f monosacc h ar id es occurs (or i s necessary) i n S c h istocerca b ecause i ts pr i nc i pa l “ bl oo d sugar” i s. Heteroptera , B io l .Bu ll . 26 : 101 – 1 7 0 . ] 495 FOO D U PT A KE A ND UTILIZ A TION F IGURE 16. 6 . Ali mentar y cana l o f cercopid (Cercopoidea) showin g filter chamber arrangement. [From R. E. Snod- g rass, P rincip l es of Insect Morp h o l ogy . Cop y ri g ht 193 5 b y McGraw-Hill,. m idg ut o f a coc k roac h. [ From R. E. Snodgrass , P rinciples o f Insect Morphol- ogy . C opyright 1935 by McGraw-Hill, Inc. Used wit h p erm i ss i on o f McGraw-H ill Boo k Compan y . ] midgut