Entomology 3rd edition - C.Gillott - Chapter 21 doc

21 P ostembr y onic Development 1 . Intr oduc t ion Durin g their postembr y onic g rowth period insects pass throu g h a series of sta g es (instars) u ntil the y become adult, the time interval (stadium) occupied b y each instar bein g terminate d by a molt. Apter yg otes continue to g row and molt as adults, periods of g rowth alternat- i ng w i t h per i o d so f repro d uct i ve act i v i ty.Int h ese i nsects structura l diff erences b etwee n j uven il ean d a d u l t i nstars are s li g h t, an d t h e i r met h o d o fd eve l opment i st h us d escr ib e d a s ameta b o l ous. Amon g t h e Pter yg ota, w hi c h w i t h rare except i ons d o not mo l t i nt h ea d u lt sta g e, two forms of development can be distin g uished. In almost all exopter yg otes the late r j uvenile instars broadl y resemble the adult, except for their lack of win g s and incompletel y f orme d gen i ta li a. Suc hi nsects, i nw hi c h t h ere i s some d egree o f c h ange i nt h emo l t f rom j uven il etoa d u l t, are sa id to un d ergo part i a l ( i ncomp l ete) metamorp h os i s, an d t h e i r d e- ve l opment i s d escr ib e d as h em i meta b o l ous. En d opter yg otes an d a f ew exopter yg otes h ave larvae whose form and habits, b y and lar g e, are ver y different from those of the adults. As a result, the y under g o strikin g chan g es (complete metamorphosis), spread over two molts , i nt h e f ormat i on o f t h ea d u l t( h o l ometa b o l ous d eve l opment). T h e fi na lj uven il e i nstar h a s b ecome spec i a li ze d to f ac ili tate t h ese c h anges an di s k nown as t h e pupa (see a l so C h apter 2, Sect i on 3.3 ). I n insect evolution increasin g functional separation has occurred between the larva l phase, which is concerned with g rowth and accumulation of reserves, and the adult sta g e , wh ose f unct i ons are repro d uct i on an ddi spersa l . Assoc i ate d w i t h t hi s tren di s a ten d ency f or an i nsect to spen d a greater part o fi ts lif easa j uven il e, w hi c h contrasts w i t h t h es i tuat i o n i n man y ot h er an i ma l s. T h us, i n apter yg otes, t h ea d u l t sta g ema yb e cons id era bly l on g er t han the j uvenile sta g e. Furthermore, feedin g (in the adult) serves to provide raw material s b oth for reproduction and for g rowth. In exopter yg otes and primitive endopter yg otes adults may live for a reasonable period, but this is not usually as long as the larval phase. Feeding i nt h ea d u l t stage i spr i mar il y assoc i ate d w i t h repro d uct i ve requ i rements, t h oug hi n som e i nsects i t prov id es nutr i ents f or an i n i t i a l ,s h ort “somat i c growt h p h ase” i nw hi c h t h e fli g h t muscles, g ut, and cuticle become full y developed. Man y endopter yg otes live for a relativel y short time as adults and ma y feed little or not at all because sufficient reserves have been acquired during larval life to satisfy the needs of reproduction. 62 3 6 24 CHAPTER 21 2 . Growth 2.1. Ph y sical As p ects Growt hi n i nsects an d ot h er art h ropo d s diff ers f rom t h at o f mamma l s i nvar i ous respects . In i nsects growt hi sa l most ent i re l y restr i cte d to t h e l arva li nstars, t h oug hi n some spec i e s there is a short period of somatic g rowth in newl y enclosed adults when additional cuticle m a y be deposited, and g rowth of fli g ht muscles and the alimentar y canal ma y occur. As a c onsequence, the length of the juvenile stage is considerably longer than that of the adult. An e xtreme examp l eo f t hi s i s seen i n some may fl y spec i es w h ose aquat i c j uven il e stage may requ i re 2 or 3 years f or comp l et i on, yet g i ve r i se to an a d u l tt h at li ves f or on l ya f ew h ours or da y s. Growth in man y animals is discontinuous or c y clic; that is, periods of active g rowth ar e separated b y periods when littleor no g rowth occurs. Nowhereis discontinuous g rowth better seen than in arthropods, which must periodically molt their generally inextensible cuticle in o r d er to s i gn ifi cant l y i ncrease t h e i rs i ze (vo l ume). It s h ou ld b e apprec i ate d , h owever, t h at, t h oug hi ncreases i nvo l ume may b e di scont i nuous, i ncreases i nwe i g h t are not (F i gure 21.1). A s an insect feeds durin g each stadium, reserves are deposited in the fat bod y , whose wei g h t and volume increase. In a hard-bodied insect this increase in volume ma y be compensated f or by a decrease in the volume occupied by the tracheal system or by extension of the a bd omen as a resu l to f t h eun f o ldi ng o fi ntersegmenta l mem b ranes. In many en d opterygote l arvae, o f course, t h e ent i re b o d y i s l arge l y covere d w i t h extens ibl e cut i c l e, an db o d ys i ze i ncreases almost continuousl y (but see below). Fo r m an y insects g rown under standard conditions the amount of g rowth that occurs is predictable from one instar to the next; that is, it obeys certain “growth laws.” For example , D yar’s l aw, b ase d on measurements o f t h ec h ange i nw id t h o f t h e h ea d capsu l ew hi c h occurs at eac h mo l t, states t h at growt hf o ll ows a geometr i c progress i on; t h at i s, t h e proport i onate i ncrease in size for a g iven structure is constant from one instar to the next. Mathematicall y e x p ressed, the law state s x / y = c onstant (value usuall y 1.2–1.4), wher e x = size in a g ive n i n sta r a n d y = s ize in previous instar (Fi g ure 21.1). Thus, when the size of a structure is pl otte dl ogar i t h m i ca ll y aga i nst i nstar num b er, a stra i g h t li ne i so b ta i ne d ,w h ose gra di ent is c onstant f orag i ven spec i es (F i gure 21.2). In t h ose i nsects w h ere i t app li es Dyar’s l aw can b e use d to d eterm i ne h ow man yi nstars t h ere are i nt h e lif e hi stor y .However,soman yf actors FIGURE 21.1 . Ch an g e i n h ea d w id t h w i t h t i me to ill ustrate D y ar’s l aw . 6 2 5 P OS TEMBRY O NI C DEVEL O PMEN T F I GU RE 21.2 . Hea d w id t h p l otte dl o g ar i t h m i ca lly a g ainst instar number in various species. [After V. B . W igglesworth, 1965 , T h e Princip l es of Insect P h ysio l ogy , 6 th ed., Methuen and Co. B y permission of the author.] a ff ect growt h rates an d t h e f requency o f ec d ys i st h at t h e l aw i s f requent l y i napp li ca bl e. I n an y event, the law requires that the interval between molts remains constant, but this i s rarel y the case . As winged insects grow, they change shape; that is, the relative proportions of differen t parts o f t h e b o d yc h ange. T hi s di sproport i onate growt h ,w hi c hi s not un i que to i nsects, i s d escr ib e d as “a ll ometr i c” ( h eterogon i c, di s h armon i c). In ot h er wor d s, eac h part h as i ts own g rowth rate, expressed b y the equation y = bx k ( y = l inear size of the part, x = linea r size of the standard (e. g ., bod y len g th), b = i nitial g rowth index ( y interce p t), and k = allometric coefficient). Normally, allometric growth is expressed as a log-log plot, whe n k i st h e gra di ent o f t h es l ope (F i gure 21.3). G rowt hl aws d o not app l y i ns i tuat i ons w h ere t h e num b er o fi nstars i svar i a bl e. T hi s variabilit y ma y be a natural occurrence, especiall y in primitive insects such as ma y flie s t hat have man y instars. In addition, females that are t y picall y lar g er than males ma y have a g reater number of instars than males. Variabilit y ma y also be induced b y environmenta l c on di t i ons. For examp l e, rear i ng i nsects at a b norma ll y hi g h temperature o f ten i ncreases t h e n um b er o fi nstars, as d oes sem i starvat i on. In contrast, i n some caterp ill ars crow di ng l ea d s t oa d ecrease i nt h e num b er o f mo l ts . C aterpillars and probabl y other larvae whose c y lindrical bod y is covered with a thin i nte g ument rel y on h y drostatic (hemol y mph) pressure to maintain the ri g idit y necessar y f or l ocomot i on. However, t hi s presents a pro bl em w i t h respect to t h e i r b o d y f orm d ur i n g growt h . Mec h an i ca ll y, i nacy li n d er un d er i nterna l pressure t h e h oop stress (aroun d t h e b o dy ) i stw i ce t h eax i a l ( l en g t h w i se) stress. T h us, a caterp ill ar t h eoret i ca lly s h ou ld b ecome proportionatel y fatter as it g rows, much like a balloon when inflated. That it does not d o 6 2 6 CHAPTER 21 FIGURE 21.3 . A ll ometr i c growt hin Carausius (Phasmida). [After V. B. Wi gg lesworth, 196 5, T h e P rinciples o f Insect Physiology, 6th ed., Methuen a n d Co. By perm i ss i on o f t h e aut h or. ] so is due to the occurrence of axial p leats (transverse cuticular folds) that reduce the axial stress b yun f o ldi ng as t h e i nsect en l arges (Carter an d Loc k e, 1993). 2.2. B i ochem i cal C han g es dur i n gG rowt h L ike the physical changes noted above, biochemical changes that occur during postem - b ryon i c d eve l opment may a l so b e d escr ib e d as a ll ometr i c. T h at i s, t h ere l at i ve proport i ons of t h evar i ous bi oc h em i ca l components c h ange as growt h ta k es p l ace. T h ese c h anges ar e e speciall y noticeable in endopter yg otes durin g the final larval and pupal sta g es. At hatchin g , the fat content of a larva is t y picall y low (less than 1% in the caterpilla r M alacosoma,fo r e xample) and remains at about this level until the final larval stadium when fat is s y nthesize d an d store di n l arge quant i ty, reac hi ng a b out 30% o f t h e d ry b o d ywe i g h t. T h oug hf at i st h e typ i ca l reserve su b stance i n most i nsects, mem b ers o f some spec i es store g l ycogen. Aga i n , t hi s usua lly occurs i n sma ll amounts i nnew ly h atc h e di nsects, b ut i ts proport i on i ncrease s steadil y throu g h larval development, and at pupation g l y co g en ma y beasi g nificant com - ponent of the dr y wei g ht (one-third in the hone y bee). Like fat, g l y co g en is stored in the fa t b o d y. In contrast, t h e proport i ons o f water, prote i n, an d nuc l e i cac id s genera ll y d ec li ne d ur i ng l arva ld eve l opment. However, t hi s i so f ten not t h es i tuat i on i n l arvae t h at requ i re l ar ge amounts of protein for specific purposes, for example, spinnin g a cocoon. In B omybyx m o r i , for example, the hemol y mph protein concentration increases sixfold in late larva l develo p ment, and about 50% of the total p rotein content of a mature larva is used in c ocoon f ormat i on. T h e great i ncrease i n concentrat i on o fh emo l ymp h prote i no f ten can b e accounte df or a l most ent i re l y b y synt h es i s, i nt h e f at b o d y, o f a f ew spec ifi c prote i ns. In t h e fl y Calliphora stygi a , for example, the protein “calliphorin” makes up 7 5 % (about 7 m g )o f 6 2 7 P OS TEMBRY O NI C DEVEL O PMEN T t he hemol y mph protein b y the time a mature larva stops feedin g . The calliphorin is used in t he pupa as a ma j or source of nitro g en (in the form of amino acids) for formation of adul t t issues and as a source of the ener gy required in bios y nthesis. Thus, at eclosion (emer g ence o f t h ea d u l t), t h e h emo l ymp h ca lli p h or i n content h as f a ll en to 0.03 mg, an d , 1 wee k a f te r emergence, t h e prote i n h as ent i re l y di sappeare d . Durin g metamorphosis some of the above trends ma y be reversed. The proportions of fat and/or g l y co g en decline as these molecules are utilized in ener gy production. I n C alli p hora t he fat content decreases from 7% to 3% of the dr y wei g ht throu g h the pupal period. In th e h oney b ee, w hi c h ma i n l y uses g l ycogen as an energy source, t h eg l ycogen content d rops t o l ess t h an 10% o fi ts i n i t i a l va l ue as metamorp h os i s procee d s. For most i nsects t h ere i s li tt l ec h an g e i nt h e net prote i n content d ur i n g pupat i on, t h ou gh ma j or qua li tat i ve c h an g es occur as adult tissues develop. In members of a few species a si g nificant decline in total protein content occurs durin g metamorphosis as protein is used as an ener gy source. Th e m oth C e l erio, f or examp l e, o b ta i ns on l y 20% o fi ts energy requ i rements i n metamorp h os is f rom f at, t h e rema i n i ng 80% com i ng l arge l y f rom prote i n . S uper i mpose d on t h e overa ll bi oc h em i ca l c h an g es f rom h atc hi n g to a d u l t h oo d are chan g es that occur in each stadium, related to the c y clic nature of g rowth and moltin g .Fac - t ors to be considered include the phasic pattern of feedin g activit y throu g hout the stadium , synt h es i so f new an dd egra d at i on o f o ld cut i c l e, an d net pro d uct i on o f new t i ssues (t h oug h some hi sto l ys i sa l so occurs i n eac hi nstar). Measurement o f ox yg en consumpt i on s h ows t h at i t f o ll ows a U-s h ape d curve t h rou gh each stadium with maximum values bein g obtained at the time of moltin g . The maxima ar e correlated with the g reat increase in metabolic activit y at this time, associated especiall y wi t h t h e synt h es i so f new cut i c l ean df ormat i on o f new t i ssues. I n L ocusta l ar v ae t h ere are s i gn ifi cant d ecreases i nt h e car b o h y d rate an dli p id contents o f t h e f at b o d yan dh emo l ymp h at ec d ys i s, pro b a bl y corre l ate d w i t h t h e use o f t h ese su b strates to supp l y energy (H ill an d G oldsworth y ,19 6 8). Conversel y , as feedin g restarts after a molt, these materials are a g ai n accumulated . C hanges in the amount of protein in the fat body and hemolymph of L ocust a are a l so cyc li ca l ,w i t h max i mum va l ues occurr i ng i nt h e secon dh a lf o f eac h sta di um (H ill and Goldsworthy, 19 6 8). The early increase in protein content is related to renewed feedin g activit y after the molt. Feedin g activit y reaches a peak in the middle of the stadium, providin g materials for g rowth of muscles (and presumabl y other tissues, thou g h these were not studie d by Hill and Goldsworth y ) and for the s y nthesis of cuticle. Excess material is stored in th e fat b o d yan dh emo l ymp h .Int h e secon dh a lf o f t h e sta di um f ee di ng act i v i ty d ec li nes, an d thi s i s f o ll owe db ya d ecrease i nt h e l eve l o f prote i n i nt h e h emo l ymp h an df at b o d y. H ill and Goldsworth y (19 6 8) su gg ested that the latter probabl y reflects the use of protein in th e s y nthesis of new cuticle. However, rec y cled protein from the old cuticle ma y account fo r most (about 80% in L ocusta ) o f the p rotein content of the new cuticle . 3 . Forms o f Develo p men t Throu g h insect evolution there has been a trend toward increasin g functional and struc- t ura ldi vergence b etween j uven il ean d a d u l t stages. Juven il e i nsects h ave b ecome mor e concerne d w i t hf ee di ng an d growt h ,w h ereas a d u l ts f orm t h e repro d uct i ve an ddi spersa l phase. This specialization of different sta g es in the life histor y became possible with the introduction into the life histor y of a pupal instar, thou g h the latter’s ori g inal function was 6 2 8 CHAPTER 21 probabl y related specificall y to eva g ination of the win g s and development of the win g m usculature (Cha p ter 2, Section 3.3). In modern insects three basic forms of postembr y onic development can be reco g nized , d escr ib e d as ameta b o l ous, h em i meta b o l ous, an dh o l ometa b o l ous, accor di ng to t h e extent of metamorp h os i s f rom j uven il etoa d u l t(F i gure 21.4). 3 .1. Ametabolous Develo p ment In T h ysanura (an d ot h er pr i m i t i ve h exapo d s), w hi c h as a d u l ts rema i nw i ng l ess, t he d egree o f c h ange f rom j uven il etoa d u l t f orm i ss li g h tan di s man if est pr i mar il y i n i ncrease d b o dy s i ze an dd eve l opment o ff unct i ona lg en i ta li a. Juven il ean d a d u l t apter yg otes i n h a bi t the same ecolo g ical niche, and the insects continue to g row and molt after reachin g sexua l m aturit y . The number of molts throu g h which an insect passes is ver y hi g h and variable. F o r e xam p le, in the firebrat, Thermobia domestica , bet w een 4 5 and 60 molts ha v e been recor d e d. 3 .2. Hem i metabolous Developmen t E xopterygotes usually molt a fixed number of times, but, with the exception of Ep h emeroptera, w hi c h pass t h roug h aw i nge d su bi mago stage, never as a d u l ts. In spec i es wh ere t h e f ema l e i s muc hl arger t h an t h ema l e, s h e may un d ergo an a ddi t i ona ll arva l mo l t . The number of molts is t y picall y 4or 5 , thou g h in some Odonata and Ephemeroptera whose l arval life ma y last2or3 y ears a much g reater and more variable number of molts occur s ( e. g ., 10–15 in species of Odonata, 15–30 in most Ephemeroptera). In a l most a ll exopterygotes t h e l ater j uven il e i nstars b roa dl y resem bl et h ea d u l t, excep t t h at t h e i rw i ngs an d externa l gen i ta li a are not f u ll y d eve l ope d . Ear l y i nstars s h ow no trace o f wi n g s, b ut, l ater, externa l w i n gb u d sar i se as sc l erot i ze d , non-art i cu l ate d eva gi nat i ons o f t h e ter g opleural area of the win g -bearin g se g ments. Win g s develop within the buds durin g the final larval stadium and are expanded after the last molt. Other, less obvious, chan g es that o ccur d ur i ng t h e growt h o f exopterygotes i nc l u d et h ea ddi t i on o f neurons, Ma l p i g hi an tu b u l es, ommat idi a, an d tarsa l segments, p l us t h e diff erent i at i on o f a ddi t i ona l sens ill a i n t h e i ntegument. T hi smo d eo fd eve l opment i s d escr ib e d as h em i meta b o l ous an di nc l u d es a partial (incomplete) metamorphosis from larva to adult . 3 . 3 . Holometabolous Develo p men t Ho l ometa b o l ous d eve l opment, i nw hi c h t h ere i s a mar k e d c h ange o ff orm f rom l arv a to adult (complete metamorphosis), occurs in endopter yg otes and a few exopter yg otes, fo r e xample, whiteflies (Aleurodidae: Hemiptera), thrips (Th y sanoptera), and male scale insect s ( Coccidae: Hemi p tera). Perha p s the most obvious structural difference between the larval an d a d u l t stages o f en d opterygotes i st h ea b sence o f any externa l s i gn o f w i ng d eve l opment i nt h e l arva l stages. T h ew i ng ru di ments d eve l op i nterna ll y f rom i mag i na ldi scs t h at i n most l arvae lie at the base of the peripodial cavit y ,aninva g ination of the epidermis beneath th e l arval cuticle, and are eva g inated at the larval-pupal molt (see Section 4.2 and Fi g ure 21.11) . As noted above, the evolution of a pupal sta g e in the life histor y has mad e h o l ometa b o l ous d eve l opment poss ibl e. T h e pupa i s pro b a bl ya hi g hl ymo difi e dfi na lj u - v en il e i nstar (C h apter 2, Sect i on 3.3) w hi c h ,t h roug h evo l ut i on, b ecame l ess concerne d wi t hf ee di n g an db u ildi n g up reserves (t hi s f unct i on b e i n gl e f ttoear li er i nstars) an d mor e 6 2 9 P OS TEMBRY O NI C DEVELOPMEN T FI G URE 21.4. Bas i ct y pes o fd eve l opment i n i nsects. Bro k en arrow i n di cates severa l mo l ts. 630 CHAPTER 21 specialized for the breakdown of larval structures and construction of adult features. I n o ther words, the pupa has become a non-feedin g sta g e; it is g enerall y immobile as a result o f histol y sis of larval muscles; it broadl y resembles the adult and thereb y serves as a mold f or t h e f ormat i on o f a d u l tt i ssues, espec i a ll y musc l es . 3 .3.1. The Larval Stage Amon g en d opter yg otes t h e extent to w hi c h t h e l arva l an d a d u l t h a bi ts an d structur e differ [and therefore the extent of metamorphosis (Section 4.2)] is varied. Broadl y speakin g , i n members of more p rimitive orders the extent of these differences is small, whereas the o pposite is true, for example, in the Hymenoptera and Diptera. Endopterygote larvae can be arranged in a number of basic types (Figure 21. 5 ). The most primitive larval form is t h eo li gopo d . Larvae o f t hi s type h ave t h ree pa i rs o f t h orac i c l egs an d awe ll - d eve l ope d head with chewin g mouthparts and simple e y es. Oli g opod larvae can be further subdivide d i nto (1) scarabaeiform larvae (Fi g ure 21.5A), which are round-bodied and have short le gs and a weakly sclerotized thorax and abdomen, features associated with the habit of bur- rowing into the substrate, and (2) campodeiform larvae (Figure 21. 5 B), which are active, pre d aceous sur f ace- d we ll ers w i t h a d orsoventra ll y fl attene db o d y, l ong l egs, strong l ysc l e - rotized thorax and abdomen, and pro g nathous mouthparts. Scarabaeiform larvae are t y pical F I GU RE 21.5 . Larval t y pes. (A) Scarabaeiform ( P opillia j aponica , Coleo p tera); (B) cam p odeiform ( Hipp o - damia conver g ens , Coleoptera); (C) eruciform (Danaus p lexi pp u s , Lepidoptera); (D) eucephalous (B ibio s p., D i ptera); (E) h em i cep h a l ous (Tanyptera fronta l is , D i ptera); an d (F) acep h a l ous (Musca d omestica , D i ptera). [A–E, from A. Peterson, 19 5 1 , L arvae o f Insects . B y permission of Mrs. Helen Peterson. F, from V. B. Wigglesworth, 1959, Metamorphosis, polymorphism, differentiation , Scienti fi c American, February 1959. By p erm i ss i on o f Mr. Er i c Mose, Jr. ] 63 1 P OS TEMBRY O NI C DEVEL O PMEN T of the Scarabaeidae and other beetle families; campodeiform larvae occur in Neuroptera , Coleoptera-Adepha g a, and Trichoptera . Pol y pod (eruciform) larvae (Fi g ure 21.5C) have, in addition to thoracic le g s, a varie d num b er o f a bd om i na l pro l egs. T h e l arvae are genera ll yp h ytop h agous an d re l at i ve l y i nact i ve , rema i n i ng c l ose to or on t h e i r f oo d source. T h et h orax an d a bd omen are wea kl ysc l erot i ze din compar i son w i t h t h e h ea d ,w hi c hh as we ll - d eve l ope d c h ew i n g mout h parts. Eruc if orm l arvae are t y pical of Lepidoptera, Mecoptera, and some H y menoptera [sawflies (Tenthredinidae)]. Apodous larvae, which lack all trunk appenda g es, occur in various forms in man y en d opterygote or d ers b ut i n common are a d apte df or m i n i ng i nso il ,mu d ,oran i ma l o r p l ant t i ssues. T h evar i a bili ty o ff orm concerns t h e extent to w hi c h a di st i nct h ea d capsu le is developed. In eucephalous larvae (Figure 21. 5 D), characteristic of some Coleoptera (Buprestidae and Ceramb y cidae), Strepsiptera, Siphonaptera, aculeate H y menoptera, and more p rimitive Di p tera (suborder Nematocera), the head is well sclerotized and bears normal appendages. The head and its appendages of hemicephalous larvae (Figure 21.5E) are re d uce d an d part i a ll y retracte di nto t h et h orax. T hi s con di t i on i s seen i n crane fl y l arva e (T i pu lid ae: Nematocera) an di nt h e l arvae o f ort h orrap h ous D i ptera. Larvae o f D i ptera- M uscomorpha are acephalous (Fi g ure 21. 5 F); no si g n of the head and its appenda g es ca n b e seen a p art from a p air of minute p a p illae (remnants of the antennae) and a p air of sclerotized hooks belie v ed to be much modified maxillae. Frequent l ya l arva i nt h e fi na li nstar ceases to f ee d an db ecomes i nact i ve a f ew d ay s b e f ore t h e l arva l -pupa l mo l t. Suc h a stage i s k nown as a prepupa. In some spec i es, t h e entire instar is a non-feedin g sta g e in which important chan g es related to pupation occur. Fo re x ample, in the prepupal instar of sawflies, the salivar yg lands become modified fo r secretin g the silk used in cocoon formation . 3.3.2. Heteromorphos i s I n most endopter yg otes the larval instars are more or less alike. However, in some species of Neuroptera, Coleoptera, Diptera, Hymenoptera, and in all Strepsiptera, a larv a u n d ergoes c h aracter i st i cc h anges i n h a bi tan d morp h o l ogy as i t grows, a p h enomeno n k nown as h eteromorp h os i s( h ypermetamorp h os i s). In suc h spec i es severa l o f t h e l arva l ty pes described above ma y develop successivel y (Fi g ure 21. 6 ). For example, blister beetles (Meloidae) hatch as free-livin g campodeiform larvae (planidia, triun g ulins) that activel y search for food (grasshopper eggs and immature stages, or food reserves of bees or ants). At t hi s stage t h e l arvae can surv i ve f or per i o d so f severa l wee k sw i t h out f oo d . Larvae t h at l ocate f oo d soon mo l ttot h e secon d stage, a caterp ill ar lik e (eruc if orm) l arva. T h e i nsect th en passes t h rou gh two or more a ddi t i ona ll arva li nstars, w hi c h ma y rema i n eruc if orm or b ecome scarabaeiform. Some s p ecies overwinter in a modified larval form known as the p seudo p u p a or coarctate larva, so-called because the larva remains within the cuticle of th e prev i ous i nstar. T h e pseu d opupa l stage i s f o ll owe d t h enextspr i ng b ya f urt h er l arva l f ee di ng stage, w hi c h t h en mo l ts i nto a pupa. 3.3.3. The Pupal Sta ge T h e pupa i s a non- f ee di ng, genera ll yqu i escent i nstar t h at serves as a mo ld i nw hi c h a d u l t f eatures can b e f orme d . For many spec i es i t i sa l so t h e stage i nw hi c h an i nsect surv i ves a d verse con di t i ons by means o fdi apause (C h apter 22, Sect i on 3.2.3). T h e terms “pupa” and “pupal sta g e” are commonl y used to describe the entire preima g inal instar. This is, 632 CHAPTER 21 F I GU RE 21.6. Heteromorphosis i n E pi caut a ( Coleoptera). (A) Triungulin; (B) caraboid second instar; (C) final f orm of second instar; (D) coarctate larva; (E) pupa; and (F) adult. [From J. W. Folsom, 190 6, E ntomo l ogy: Wit h S pecia l Reference to Its Bio l ogica l an d Economic Aspect s ,B l a ki ston.] str i ct l y spea ki ng, i ncorrect b ecause f oravar i e d per i o d pr i or to ec l os i on, t h e i nsect i s a “ p h arate a d u l t,” t h at i s, an a d u l t enc l ose d w i t hi nt h e pupa l cut i c l e. T h e i nsect t h us b ecome s an a d u l t i mme di ate ly a f ter apo ly s i so f t h e pupa l cut i c l ean df ormat i on o f t h ea d u l tep i cut i c l e ( Chapter 11, Section 3.1). The distinction between the true pupal sta g e and the pharate adult c ondition becomes important in consideration of so-called “pupal movements,” includin g l ocomot i on an d man dib u l ar c h ew i ng movements (use di n escap i ng f rom t h e protect i ve c ocoon or ce ll i nw hi c h metamorp h os i s too k p l ace). In most i nstances t h ese movements resu l t f rom t h e act i v i t y o f musc l es attac h e d to t h ea d u l t apo d emes t h at fi t snu gly aroun d t h e remains of the pupal apodemes (Fi g ure 21.7). FI GU RE 21.7. S ection throu g h mandible of a decticous p upa to show adult apodemes around remains of pupa l ap odemes. [After H. E. Hinton, 194 6 , A new classificatio n of insect p u p ae, Proc. Zool. S oc. Lond. 1 1 6 : 282–328. B y p ermission of the Zoological Society of London.] [...]... POSTEMBRYONIC DEVELOPMENT 648 CHAPTER 21 FIGURE 21. 16 Course of development in Zootermopsis angusticollis (Isoptera) Broken arrow indicates the potential for several molts Abbreviations: L1–L5, first to fifth instar larvae; 1N, 2N, first- and second-stage nymphs; PR, primary reproductive; SR, supplementary reproductive; PSol, presoldier; Sol, soldier [After C.-M Yin and C Gillott, 1975, Endocrine activity during... the endocrine system Variations in the relative levels of different hormones in an insect’s body determine the nature and extent of tissue differentiation that is expressed at the next molt In other words, it is the hormone balance that determines, in a holometabolous insect, for example, whether the next molt is larval-larval, 639 POSTEMBRYONIC DEVELOPMENT 640 CHAPTER 21 larval-pupal, or pupal-adult... above; that is, the wing rudiments form in a peripodial cavity and become everted at the larval-pupal molt (Figure 21. 11) The forming wing bud in the peripodial cavity is initially a hollow, 636 CHAPTER 21 FIGURE 21. 10 Sections through leg of Pieris (Lepidoptera) to show development of adult appendage (A) Leg of last-instar larva 3 hours after ecdysis; (B) same as (A) but 1 day after ecdysis; (C) same as... FIGURE 21. 13 Schematic comparison of endocrine control of development in hemimetabolous and holometabolous insects Pulses of prothoracicotropic hormone (PTTH) trigger synthesis and release of MH Levels of JH determine the nature of the molt: when JH is present during a critical period, a larval-larval molt occurs; if no JH is present, the next molt will be larval-adult (Hemimetabola), or larval-pupal... so-called “early” genes that encode regulatory proteins The latter, in turn, modulate the activity of “middle,” and eventually “late,” genes whose transcriptional products carry out the appropriate tissue-specific process (Riddiford, 1985; Doctor and Fristrom, 1985) The pathways of gene activation that are followed depend on the 20-HE concentration and receptor isoform; as well, they will be species-... the old exuvium are coordinated by the interplay of several hormones, including 20-HE, eclosion hormone (EH), pre-ecdysis-triggering hormone (PETH), ecdysis-triggering hormone (ETH), crustacean cardioactive peptide (CCAP), and bursicon (Truman, 1985, 1990, 1992; Reynolds, 1986; Horodyski, 1996; Myers, 2003) Only when the 20-HE level falls below a threshold value can molting occur, for two reasons First,... model for the control of molting, Zitˇ which includes three phases: pre-ecdysis I, pre-ecdysis II, and ecdysis (Figure 21. 14) The behavioral sequence is initiated when low levels of EH are released The EH induces PETH and ETH secretion from the Inka cells (Chapter 13, Section 3.4) PETH acts on the abdominal ganglia to trigger pre-ecdysis I; that is, it causes motor neurons to fire, bringing about strong... may act directly to affect development; most factors, however, exert their effect indirectly via the endocrine system 6.1 Endocrine Regulation of Development Postembryonic development is controlled by three endocrine centers: the brain-corpora cardiaca complex, corpora allata, and molt glands (see Chapter 13, Section 3, for a description of their structure and products) A molt cycle is initiated when,... threshold value, a larval-larval molt follows; at below-threshold concentrations the molt is larval-pupal or pupal-adult Polymorphism is the existence of several distinct forms of the same stage of a species It may have a genetic basis (as in transient and balanced polymorphism) or be induced by changing external conditions (polyphenism), whose effects are manifest via the endocrine system, specifically... host’s body in the pupal stage 634 CHAPTER 21 4 Histological Changes During Metamorphosis Though we have distinguished, in the preceding account, between hemimetabolous development (where partial metamorphosis occurs in the molt from larva to adult) and holometabolous development (in which metamorphosis is striking and requires two molts, larval-pupal and pupal-imaginal, for completion), the distinction . t h e i nsect en l arges (Carter an d Loc k e, 1993). 2.2. B i ochem i cal C han g es dur i n gG rowt h L ike the physical changes noted above, biochemical changes that occur during postem - b ryon i c d eve l opment. protect i on a g a i nst suc h a d vers i t i es t h e pupa t y p i ca lly h asat hi c k , tanne d cut i c l e . Also, in man y species it is enclosed within a cocoon or subterranean cell constructed. (cocoon cutters) on the pupal cuticle. In hi g her L epidoptera, adults may shed the pupal cuticle while still in the cocoon. In such specie s th e cocoon may possess an “escape h atc h ” or