Entomology 3rd edition - C.Gillott - Chapter 20 ppt

20 E mbr y onic Development 1 . Intr oduc t ion E mbr y onic development be g ins with the first mitotic division of the z yg ote nucleus an d t erminates at hatchin g . Not surprisin g l y , in view of their diversit y of form, function, and life h istor y , insects exhibit a variet y of embr y onic developmental patterns, thou g h certain evolu - ti onary tren d s are apparent. Eggs o f most spec i es conta i n a cons id era bl e amount o f yo lk .In exopterygote eggs t h ere i s suc h a prepon d erance o f yo lk t h at t h e egg cytop l asm i s rea dil yo b - v i ous on ly w h en i t f orms a sma ll i s l an d surroun di n g t h e nuc l eus. In e gg so f en d opter yg otes, t he y olk:c y toplasm ratio is much lower than that of exopter yg otes and the c y toplasm can be seen as a conspicuous network connectin g the central island with a la y er of periplasm l y in g b eneat h t h ev i te lli ne mem b rane. T hi s tren d towar d re d uct i on i nt h ere l at i ve amount o f yo lk i nt h e egg, carr i e d to an extreme i n certa i n paras i t i c Hymenoptera an d v i v i parous D i ptera (Cec id om yiid ae), w h ose e gg s are y o lkl ess an d rece i ve nutr i ents f rom t h e i r surroun di n g s, h a s some important consequences. Broadl y speakin g , the e gg s of endopter yg otes are smaller (size measured in relation to the bod y size of the la y in g insect) and develop more rapidl y th an t h ose o f exopterygotes. T h e i ncrease d quant i ty o f cytop l asm l ea d stot h e more rap id f ormat i on o f more an dl arger ce ll satt h eyo lk sur f ace t h at f ac ili tates t h e f ormat i on o f a l arger em b r y on i c area f rom w hi c hd eve l opment can ta k ep l ace. Compare d w i t h t h at o f exopter y - g otes, development of endopter yg otes is streamlined and simplified. There has been, as Anderson (1972b, p . 229) p ut it, “reduction or elimination of ancestral irrelevancies,” which wh en ta k en to an extreme, seen i nt h e apocr i tan Hymenoptera an d cyc l orr h ap h D i ptera, re- su l ts i nt h e f ormat i on o f a structura ll ys i mp l e l arva t h at h atc h es w i t hi nas h ort t i me o f egg l a yi n g . However, super i mpose d on t hi s process o f s h ort-c i rcu i t i n g ma yb e d eve l opmenta l specializations associated with an increasin g dissimilarit y of j uvenile and adult habits. 2. C leavage and Blastoderm Format i o n As it moves toward the center of an e gg after fusion, the z yg ote nucleus be - g i ns to di v id em i tot i ca ll y. T h e fi rst di v i s i on occurs at a pre d eterm i ne d s i te, t h ec l eav - age center (F i gure 20.1), l ocate di nt h e f uture h ea d reg i on, w hi c h cannot b e recogn i ze d morpholo g icall y but which appears to become activated either when sperm enter an e gg o r w hen an e gg is laid. Earl y divisions are s y nchronous, and as nuclei are formed and mi g rate 597 5 98 CHAPTER 20 FI GU RE 20.1. Positions of cleava g e center, activation center, and differen - t i at i on center i n eggs o f Pl atycnemis (Odonata). [After D. Bodenstein, 1953 , Em b r y on i c d eve l opment, i n : Insect P h ysio l og y ( K. D. Roe d er, e d .). Cop y- r ight @ 1953, John Wiley and Sons, Inc. Reprinted by permission of John Wil ey an d Sons, Inc.] throu g h the y olk toward the periplasm, each becomes surrounded b y an island of c y toplas m ( Fi g ure 20.2A). Each nucleus and its surroundin g c y toplasm are known as a cleava g e en- e rgid. In eggs of endopterygotes and possibly exopterygotes, but not those of apterygotes, t h e energ id s rema i n i nterconnecte db y means o ffi ne cytop l asm i c b r id ges. Th e rate at w hi c h nuc l e i m i grate to t h eyo lk sur f ace an d t h e met h o d o f co l on i zat i on ar e v aried. In e gg s of some species nuclei appear in the periplasm as earl y as the 6 4-ener g i d state (after six divisions); in others, nuclei are not seen in the p eri p lasm until the 1024 - e ner g id sta g e. In e gg s of most endopter yg otes and in those of paleopteran and hemipteroi d e xopterygotes, t h e per i p l asm i s i nva d e d un if orm l y b yt h e energ id s. However, i n eggs o f o rt h optero id i nsects t h e per i p l asm at t h e poster i or po l eo f t h e egg rece i ves energ id s fi rst , after which there is pro g ressive colonization of the more anterior re g ions. In e gg s of most insects not all cleava g e ener g ids mi g rate to the peripher y but continue to divide within the y olk to form primar y vitellopha g es, so-called because in most species t h ey b ecome p h agocyt i cce ll sw h ose f unct i on i sto di gest t h eyo lk (F i gure 20.2B). In eggs o f Lep id optera, D i ptera, an d some ort h optero id i nsects, h owever, a ll o f t h e energ id sm i grate t o t h e per i p l asm an d on ly l ater d o some o f t h e i r pro g en y move b ac ki nto t h e y o lk as secon d ar y v itellopha g es (Fi g ure 20.2F). Secondar y vitellopha g es are also produced in e gg s of other i nsects to supplement the number of primar y vitellopha g es. So-called tertiar y vitellopha g es are pro d uce di n eggs o f some cyc l orr h ap h D i ptera an d apocr i tan Hymenoptera f rom t h e anter i or an d poster i or m id gut ru di ments. A f ter t h e i r arr i va l at t h e per i p l asm, t h e ener gid s cont i nue to di v id e, o f ten s y nc h ronous ly, until the nuclei become closel y packed (the s y nc y tial blastoderm sta g e), after which cell m embranes form b y radial infoldin g , then tan g ential expansion of the ori g inal e gg plas- m a l emma (t h eun if orm bl asto d erm stage) (F i gure 20.2C–F). From t h e resu l t i ng mono l aye r of ce ll s d eve l op a ll o f t h ece ll so f t h e l arva lb o d y, except i na f ew spec i es w h ere v i te ll op h ages o ryo lk ce ll s contr ib ute to t h e f ormat i on o f t h em id gut (Sect i on 7.4). 3. Format i on and G rowth o fG erm Band T he next sta g e is blastoderm differentiation, g ivin g rise to the embr y onic primordium ( an area of closel y packed columnar cells from which the future embr y o forms) and th e 599 E MBRY O NI C DEVELOPMEN T F I GU RE 20.2. S tages in cleavage and blastoderm formation in egg of D acus tr y on i (Diptera). (A) Frontal s ection through anterior end during 6 th division; (B) transverse section after 8th division; (C) transverse section a f ter 12t hdi v i s i on; (D) transverse sect i on d ur i n g 13t hdi v i s i on; (E) transverse sect i on at s y nc y t i a lbl asto d erm s tage; and (F) frontal section through posterior end after formation of uniform cellular blastoderm. [After D. T . A n d erson, 1972 b ,T h e d eve l opment o fh o l ometa b o l ous i nsects, i n: Deve l opmenta l Systems: Insects ,V ol. I (S. J. VV Counce and C. H. Waddin g ton, eds.). B y permission of Academic Press Ltd., and the author.] extra-embryonic ectoderm from which the extra-embryonic membranes later differentiat e (F i gure 20.3). For more t h an a century, attempts h ave b een ma d etoexp l a i n h ow t h e b o d y pattern o f an i nsect i s d eterm i ne d .Fo ll ow i ng t h ec l ass i c exper i ments o f t h e German em b ry- olo g ist Seidel in the late 1920s, it was widel y believed that differentiation was controlled by two centers (Counce, 1973; Hemin g , 2003). As ener g ids move toward the posterior end of the e gg , the y interact with a so-called “activation center” (Fi g ure 20.1), and differen- ti at i on su b sequent l y occurs. Se id e l ’s exper i ments s h owe d t h at ne i t h er an energ id nor t he act i vat i on center a l one cou ld st i mu l ate diff erent i at i on. It was presume d t h at t h e center i s cause d to re l ease an un id ent ifi e d c h em i ca l t h at diff uses anter i or ly .T hi s diff us i on i s seen morpholo g icall y as a clearin g and sli g ht contraction of the y olk. As the chemical reaches 600 CHAPTER 20 F IGURE 20.3 . D i a g rammat i c transverse (A) an d sa gi tta l (B) sect i ons o f e gg o f P ontani a ( H y menoptera ) t o s how differentiation of blastoderm into embr y onic primordium and extra-embr y onic ectoderm. Note also th e g erm (pole) cells at the posterior end. [After D. T. Anderson, 1972b, The development of holometabolous insects , i n : D eve l opmenta l Systems: Insects ,V ol. I (S. J. Counce and C. H. Waddington, eds.). By permission of Academic VV P ress Ltd., and the author. ] the future prothoracic re g ion of the embr y o (the “differentiation center”) (Fi g ure 20.1), the blastoderm in this re g ion g ives a sharp twitch and becomes sli g htl y inva g inated. Blastoderm c ells aggregate within this invagination and differentiate into the embryonic primordium . ( Later i nem b ryogenes i s, ot h er processes, f or examp l e, meso d erm f ormat i on an d segmen - tat i on, b eg i natt h e diff erent i at i on center an d sprea d anter i or l yan d poster i or l y f rom i t. ) An alternate view for the cause of embr y onic differentiation is the “ g radient h y pothesis, ” w hich had its ori g ins at the end of the 19th centur y but then fell out of favor after Seidel’s pioneerin g work (Sander, 1984, 1997; Lawrence, 1992). Essentiall y , the h y pothesis propose s t h atac h em i ca l pro d uce d at eac h en d o f an egg diff uses t h roug h out t h e egg, pro d uc i ng tw o gra di ents o f concentrat i on (F i gure 20.4). Ce ll sw i t hi nt h e egg t h en “recogn i ze” t h e i r pos i t i o n wi t hi nt h ee gg by t h ere l at i ve concentrat i ons o f t h ec h em i ca l an d diff erent i ate accor di n gly. Initial support for the existence of chemical g radients in e gg s came from experiments in w hich e gg s either were li g atured at various distances alon g their len g th and at varied time s a f ter em b ryon i c d eve l opment b egan or were centr if uge d ,t h ere b y di srupt i ng t h e propose d gra di ent. Recent l y, t h e app li cat i on o f genet i can d mo l ecu l ar tec h n i ques to t h e stu d yo f pattern d eve l opment in D roso ph i la h as gi ven f urt h er support to t h e id ea o fg ra di ents. T h us , a modern interpretation of Seidel’s differentiation center is that it is a “commitment center”; that is, it is the point at which blastoderm cells are committed to followin g a particular path o f differentiation by virtue of their position within the gradients (Heming, 2003). F I G URE 20.4 . D i a g rammat i c representat i on o f t h e g ra di ent hy pot h es i s.Ac h em i ca l pro d uce d at eac h en d o f an e gg diffuses len g thwise, formin g two g radients of concentration. At an y point alon g the len g th of the e gg , the relative concentration of the two chemicals provides positional information to cells. 60 1 E MBRY O NI C DEVEL O PMEN T F IGURE 20.5 . F orm an d pos i t i on o f em b ryon i cpr i mor di um i n exopterygotes. (A) P erip l anet a ;( B ) P l atycnemi s ; ( C ) Z ootermops is ; an d( D ) N otonecta.[A f ter D. T. An d erson, 1972a, T h e d eve l o p ment o fh em i meta b o l ous i nsects, in : Developmental S y stems: Insect s , V ol. I (S. J. Counce and C. H. Waddington, eds.). By permission of Academic VV P ress Lt d ., an d t h e aut h or. ] As a result of the differin g amounts of y olk that exopter yg ote and endopter yg ote e ggs contain, important differences occur in the formation of the embr y onic primordium. I n exopterygote eggs where there is initially little cytoplasm, the embryonic primordium i s norma ll yre l at i ve l y sma ll ,an di ts f ormat i on d epen d sont h e aggregat i on an d , to some extent , pro lif erat i on o f ce ll s. In t h ese eggs i t usua ll y occup i es a poster i or m id ventra l pos i t i on (F i gure 2 0. 5 A–D). In contrast, in endopter yg ote e gg s with their g reater quantit y of c y toplasm, the primordium forms as a broad monola y er of columnar cells that occupies much of the ventral surface of the yolk (Figure 20.6A,B). In other words, the primordium in endopterygote egg s d oes not requ i re to un d ergo muc hi ncrease i ns i ze, as i s necessary i n eggs o f exopterygotes, so t h at t i ssue diff erent i at i on can occur di rect l yan d em b ryon i c growt h more rap idl y. At i t s extreme, seen in e gg s of some Diptera and H y menoptera, the primordium occupies bot h ventral and lateral areas of the e gg , with the extra-embr y onic ectoderm coverin g onl y th e d orsal surface (Fi g ure 20.6C) . T h es h ape o f t h epr i mor di um i svar i e d ,t h oug hi n most i nsects t h e anter i or reg i on i s expan d e dl atera ll yasapa i ro fh ea dl o b es ( = p rotocep h a l on), b e hi n d w hi c hi sareg i on o f varied len g th, the protocorm (postantennal re g ion) (Fi g ure 20. 5 ). In e gg s of Paleoptera , h emipteroid insects, and some orthopteroid species, the protocorm is semilon g and at its formation includes the mouthpart-bearin g se g ments, the thoracic se g ments, and a posterio r growt h reg i on f rom w hi c h t h ea bd om i na l segments ar i se. In eggs o f ot h er ort h optero id i nsects t h e postantenna l reg i on cons i sts i n i t i a ll yo f on l yt h e growt h zone. T h oug h t h e proto - corm i n most en d opter yg ote em b r y os i s l on g , i ta l so i nc l u d es a poster i or g rowt h zone f ro m w hich rudimentar y abdominal se g ments proliferate. As the embr y onic primordium elon- g ates and be g ins to differentiate, it becomes known as the g erm band. Durin g elon g atio n an d diff erent i at i on, t h ea bd omen grows aroun d t h e poster i or en d an df orwar d over t h e d orsa l sur f ace o f t h e egg (F i gure 20.7). In eggs o f some hi g h er en d opterygotes (Hymenoptera- Apocr i ta an d D i ptera-Muscomorp h a), t h ere i s no poster i or growt h zone an d t h ea bd om i na l se g ments arise directl y from the primordium . 602 CHAPTER 20 F IGURE 20.6 . F orm and position of embryonic primordium in endopterygotes. (A) T e n eb r io ;( B ) S iali s ; and (C) P imp l a.[A f ter D. T. An d erson, 1972a, b ,T h e d eve l o p ment o fh em i meta b o l ous i nsects, an d T h e d eve l o p ment o f holometabolous insects , in : Developmental S y stems: Insects ,V ol. I (S. J. Counce and C. H. Waddington, eds.). V V B y permission of Academic Press Ltd., and the author. ] It i s d ur i n g t h e diff erent i at i on an d e l on g at i on o f t h e g erm b an d t h at t h epr i mor di a l g erm cells first become noticeable in most endopter yg ote e gg s, thou g h in those of some Coleoptera the y are distin g uishable even as the s y nc y tial blastoderm is formin g . The y ar e l arg i s h , roun d e d ce ll s i na di st i nct group at t h e poster i or po l eo f t h eyo lk ,an d accor di ng l y are re f erre d to as po l ece ll s(F i gure 20.3). In eggs o f Dermaptera, Psocoptera, T h ysanoptera, an d h omopterans a l so, t h e g erm ce ll s diff erent i ate ear ly at t h e poster i or en d o f t h epr i mor di um . In those of most exopter yg otes, however, the y are not apparent until g astrulation or somit e f ormation has occurred . As the germ band elongates and becomes broader, segmentation and limb-bud formatio n appear externa ll yan d are accompan i e di nterna ll y b y meso d erm an d som i te f ormat i on. Growt h o f t h e germ b an d may occur e i t h er on t h e sur f ace o f t h eyo lk (super fi c i a l growt h )a s seen in e gg s of Dict y optera, Dermaptera, Isoptera, some other orthopteroid insects, and al l e ndopter yg otes (Fi g ure 20.7), or b y immersion into the y olk (immersed g rowth) as occurs i n eggs of Paleoptera, most Orthoptera, and hemipteroid insects (Figure 20.8). Immersion o f t h e germ b an d (anatreps i s) f orms t h e fi rst o f a ser i es o f em b ryon i c movements, co ll ect i ve ly k nown as bl asto ki nes i s. T h e reverse movement ( k atatreps i s), w hi c hb r i ngs t h eem b ryo back to the surface of the y olk, occurs later (see Section 6 ). Anatrepsis has developed secondaril y (i.e., superficial g rowth is the more primitive method) and conver g entl y amon g those exopterygotes in which it occurs. Its functional significance is, however, not clea r ( An d erson, 1972a; Hem i ng, 2003) . 4 . Gastrulation, Somite Formation, and Segmentatio n As the embr y onic primordium be g ins to increase in len g th, its midventral cells sink i nward to form a transient, lon g itudinal g astral g roove (Fi g ure 20.9A). The inva g inate d 603 E MBRY O NI C DEVEL O PMEN T F I GU RE 20.7 . S ta g es in elon g ation and se g mentation of g erm band i n Zootermo p s is (Iso p tera) (A–C) an d Bruchi d iu s (Coleoptera) (D–F). [After D. T. Anderson, 1972a,b, The development of hemimetabolous insects , an d t h e d eve l o p ment o fh o l ometa b o l ous i nsects, i n : Deve l opmenta l Systems: Insect s ,V ol. I (S. J. Counce and VV C. H. Waddin g ton, eds.). B y permission of Academic Press Ltd., and the author.] cells soon separate from the outer la y er, which closes to obliterate the g roove. It is fro m t he anterior and posterior points of closure of the g astral g roove that the stomodeum an d proctodeum, respectivel y , develop. The outer la y er can now be distin g uished as the em- b ryon i c ecto d erm. T h e i nvag i nate d ce ll s, w hi c h pro lif erate an d sprea dl atera ll y, f orm t h e meso d erm (F i gure 20.9B,C) except a dj acent to t h e d eve l op i ng stomo d eum an d procto d eum wh ere t h e yb ecome t h e anter i or an d poster i or m idg ut ru di ments, respect i ve ly .T h e meso- d ermal cells become concentrated into paired lon g itudinal tracts which soon separate int o 60 4 CHAPTER 20 F IGURE 20.8. Early embryonic development in Calopter y x to show anatrepsis andkatatrepsis. [A–E, after O. A. Jo h annsen an d F. H. Butt , 1941 , Em b ryo l ogy of Insects an d Myriapo d s . By perm i ss i on o f McGraw-H ill Boo k Co., Inc. F, After R. F. Cha p man, 1971 , T he Insects: S tructure and Function. By permission of Elsevier/North-Holland, Inc., and the author. ] se g mental blocks, leavin g onl y a thin lon g itudinal strip, the median mesoderm, from which hemoc y tes later differentiate. From these se g mental blocks, paired hollow somites usuall y ar i se (F i gure 20.9E). Som i te f ormat i on i s i n i t i ate d an d occurs more or l ess s i mu l taneous l y i nt h e gnat h a l an d t h orac i c segments, sprea di ng anter i or l yan d poster i or l ya f ter gastru l at i o n ta k es p l ace. Format i on o f t h e coe l om (t h ecav i t y w i t hi n a som i te) ma y occur i n one o f two w a y s, b y internal splittin g of a somite or b y median foldin g of the lateral part of each somite. 605 E MBRY O NI C DEVEL O PMEN T F I GU RE 20.9. F o rmation of gastral groove, somites, and embryonic membranes. [After D. T. Anderson, 1972a , Th e d eve l opment o fh em i meta b o l ous i nsects, i n D eve l opmenta l Systems: Insect s , V ol. I (S. J. Counce and C. H. VV W a ddi n g ton, e d s.). B y perm i ss i on o f Aca d em i c Press Lt d ., an d t h e aut h or.] I nem b ryos o f ag i ven spec i es, one or b ot h met h o d s may b e seen i n diff erent segments. For example, internal splittin g of the somites occurs in all se g ments of embr y os of Phasmida, most hemipteroid insects, and most endopter yg otes, and in the abdominal se g ments of Locust a embryos. Median folding is the method used in all segments in embryos of Odonata, D i ctyoptera, an d Ma ll op h aga, an di nt h e gnat h a l an d t h orac i c segments o f t h ose o f Locu s t a , an d some Co l eoptera, Lep id optera, an d Mega l optera. In exopterygote em b ryos, a ll som i te s u suall y develop a central cavit y , thou g h this ma y be onl y temporar y . Amon g endopter yg otes, members of more primitive orders retain a full complement of somites in their embr y os and t he latter usually develop a coelom. In embryos of some species, however, cavities may no t f orm, an d som i te f ormat i on may b e suppresse di nt h e h ea d segments. In em b ryos o f D i pter a an d Hymenoptera, no di st i nct h ea d som i tes appear, an di nt h ose o f some Muscomorp h a and Apocrita, somite formation is entirel y suppressed, so that mesodermal derivatives ar e produced directl y from a sin g le midventral mass. 5 . Format i on o f Extra-Embr y on i c Membrane s S imultaneousl y with g astrulation and somite formation, two extra-embr y onic mem - b ranes, t h e amn i on an d serosa, d eve l op f rom t h e extra-em b ryon i c ecto d erm (F i gure 20.9). Ce ll satt h ee d ge o f t h e germ b an d pro lif erate an d t h et i ssue f orme d on eac h s id e f o lds ventra lly to gi ve r i se to t h e amn i ot i c f o ld s. T h ese meet an df use i nt h e ventra l m idli ne to form inner and outer membranes, the amnion and serosa, respectivel y , the former enclosin g a central fluid-filled amniotic cavit y . Man y authors have su gg ested that such a cavit y woul d 606 CHAPTER 20 provide space in which an embr y o could g row and also prevent ph y sical dama g e. Ander - son (1972a) considered, however, that these functions are redundant and that the cavit y m ust have an as y et unidentified function. Another possibilit y is that the amnion and it s c av i ty are use d to store wastes, w hi c h are t h us k ept separate f rom t h eyo lk .T h e genera l m et h o d o f amn i on an d serosa f ormat i on out li ne d a b ove i s f oun di na ll i nsect em b ryos (w i t h some modification where immersion of the g erm band into the y olk occurs) except those o f Muscomorpha and Apocrita, in which, it will be recalled, the embr y onic primordiu m c overs most of the y olk surface. In these, embr y onic membranes are g reatl y reduced o r l ost. In em b ryos o f Apocr i ta t h e extra-em b ryon i c ecto d erm separates f rom t h ee d ge o f t h e pr i mor di um an d grows ventra ll yto f orm t h e serosa; t h at i s, amn i ot i c f o ld s are not f orme d .In e m b r y os o f Muscomorp h ane i t h er an amn i on nor a serosa f orms, an d t h e extra-em b r y on ic e ctoderm covers the y olk until definitive dorsal closure occurs (see below) . After the embr y onic membranes form, the serosa in most insect e gg s secretes a cuticle t h at i so f ten as t hi c k as t h ec h or i on. For severa l s p ec i es, p ro d uct i on o f t h e serosa l cut i c l e i s cl ose l y sync h ron i ze d w i t h a pea k o f mo l t i ng h ormone i nt h e egg (see Sect i on 9) . 6. Dorsal Closure and Katatrepsis W h en germ b an d e l ongat i on an d segmentat i on are comp l ete, li m bb u d s d eve l op, t he e m b r y on i c ecto d erm g rows d orso l atera lly over t h e y o lk mass, an di nterna lly or g ano g enes is be g ins. This phase of g rowth is ended abruptl y as the extra-embr y onic membranes fuse and rupture and the g erm band reverts to its ori g inal (pre-anatreptic) position (in most e xopterygotes) or s h ortens (en d opterygotes). In em b ryos o f most i nsects, t h e amn i on an d serosa f use i nt h ev i c i n i ty o f t h e h ea d ,an d t h e com bi ne d t i ssue t h en sp li ts to expose t h e h ea d an d ro ll s b ac kd orsa ll y over t h eyo lk ( Fi g ure 20.10A). As a result, the serosa is reduced to a small mass of cells, the secondar y dorsal or g an, and the amnion becomes stretched over the y olk, formin g the provisiona l dorsal closure (Figure 20.10B). In some endopterygote embryos, variations of this process c an b e seen. In t h ose o f Nematocera (D i ptera) an d Symp h yta (Hymenoptera), f or examp l e , i t i st h e amn i on t h at ruptures an di sre d uce d , l eav i ng t h e serosa i ntact. As note d a b ove, i n egg s of Apocrita onl y a serosa is formed, and this persists until definitive dorsal closure o ccurs, and in those of Muscomorpha no extra-embr y onic membranes develop, and the y olk remains covered by the extra-embryonic ectoderm until definitive dorsal closure. E xcept i n di ctyopteran em b ryos w h ere t h e germ b an d rema i ns super fi c i a l an d ventra l d ur i ng e l ongat i on, extens i ve movement o f t h e germ b an d now occurs i n exopterygote egg s w hich serves (1) to brin g an immersed g erm band back to the surface of the y olk and (2) to restore the g erm band to its pre-anatreptic orientation, that is, on the ventral surface of the yolk with the head end facing the anterior pole of the egg. This movement, the reverse of anatreps i s, i s k nown as k atatreps i s(F i gure 20.8). At t h e b eg i nn i ng o f prov i s i ona ld orsa l c l osure, t h e germ b an d o f most en d opterygotes i s quite lon g so that, althou g h its anterior end is ventral, its posterior component passes round the posterior tip of the y olk and forward alon g the dorsal side (Fi g ure 20.7F). Durin g c losure, the germ band shortens and broadens rapidly so that its posterior end now come s to li e near t h e poster i or en d o f t h e egg (F i gure 20.11A). D e fi n i t i ve d orsa l c l osure, t h at i s, t h e enc l os i ng o f t h eyo lk w i t hi nt h eem b ryo, t h e n o ccurs. It is achieved in all insect embr y os b y a lateral g rowth of the embr y onic ectoderm , w hich g raduall y replaces the amnion or, rarel y , the serosa (Fi g ures 20.10C and 20.11B). [...]... reduced to epidermal thickenings In embryos of Muscomorpha, the thoracic appendages never develop beyond the epidermal thickening stage 608 CHAPTER 20 FIGURE 20. 11 (A) Five-day embryo of Bruchidius (Coleoptera) after shortening of germ band Compare this figure with Figure 20. 7F; and (B) embryo of Bruchidius at hatching stage (9 days) [After D T Anderson, 1972b, The development of holometabolous insects in:... from which both neurons and glial cells differentiate (Figure 20. 12) Remarkably, the number and arrangement of neuroblasts is highly conserved across the Insecta: each half-ganglion has 30 or 31 neuroblasts from which all neurons are produced (Thomas et al., 1984) However, the number of neurons in 609 EMBRYONIC DEVELOPMENT 610 CHAPTER 20 FIGURE 20. 12 Transverse sections to show development of nervous tissue... Hessian fly larvae, for example, at the four-cell stage, the cells may separate into two groups so that twin embryos are formed In contrast, in the chalcidid Litomastix truncatellus, which parasitizes larvae of the moth genus Plusia, formation of embryos does not begin until the 22 0- to 225-blastomere stage At this stage, certain of the blastomeres become spindle-shaped and fuse to form a syncytial sheath... the follicle cells degenerate because, it is assumed, they are supplying nutrients to the developing embryo (Figure 20. 16) FIGURE 20. 14 Caterpillars parasitized by Litomastix [From R R Askew, r 1971, Parasitic Insects By permission of Heinemann Educational Books Ltd.] 616 CHAPTER 20 FIGURE 20. 15 Female reproductive system of the tachinid Panzeria (Diptera) (A) Newly emerged fly; and (B) mature female,... components of the light-sensitive structures Imaginal discs and histoblasts, from which many adult tissues are derived at metamorphosis in higher Diptera and Hymenoptera (Chapter 21, Section 4.2), can be recognized soon after germ-band formation They are groups of cells that separate from the ectoderm in characteristic numbers, sizes and shapes, at specific sites in the body (Heming, 200 3) Concurrently... Drosophila: A common plan for neuronal development, Nature 310 :203 207 Truman, J W., and Ball, E E., 1998, Patterns of embryonic neurogenesis in a primitive wingless insect, the silverfish, Ctenolepisma longicaudata: Comparison with those seen in flying insects, Develop Genes Evol 208 :357–368 White, M J D., 1973, Animal Cytology and Evolution, 3rd ed., Cambridge University Press, London ... exists between temperature and time required to complete development; that is, the total heat requirement (temperature 618 CHAPTER 20 above minimum required X duration of exposure to this temperature) is constant for a given species This heat requirement is typically measured in degree-days Outside these developmental limits, yet within the limits of viability, an egg may survive but does not develop Under... of the beetle Sitona, when kept at 20 C and 100% relative humidity, hatch in 10.5 days; at the same temperature but only 62% relative humidity, development takes twice as long In other species contact of the egg with liquid water is necessary for continued development Such is the case in the damselfly eggs mentioned above which pass the winter in snow-covered, dried-out Scirpus stems and do not continue... migrate through the yolk to the periplasm and form the blastoderm; ∗ This fluid is no longer in the amniotic cavity whose membranes were destroyed during dorsal closure 619 EMBRYONIC DEVELOPMENT 620 CHAPTER 20 some remain in the yolk as vitellophages that supply nutrients to the embryo Posteriorly moving energids receive a signal at the activation center, which stimulates differentiation of part of... toward a specific epidermal insertion site (Chapter 14, Section 2.1) The breaking up of the somite walls into discrete tissues means that in insects as in other arthropods there is no true coelom Rather, the latter merges with the epineural sinus (the space between the dorsal surface of the embryo and the yolk) and is correctly 611 EMBRYONIC DEVELOPMENT 612 CHAPTER 20 called a mixocoel (hemocoel) From mesodermal . ru di ments, respect i ve ly .T h e meso- d ermal cells become concentrated into paired lon g itudinal tracts which soon separate int o 60 4 CHAPTER 20 F IGURE 20. 8. Early embryonic development in Calopter y x. respectivel y , the former enclosin g a central fluid-filled amniotic cavit y . Man y authors have su gg ested that such a cavit y woul d 606 CHAPTER 20 provide space in which an embr y o could g row. dorsal surface of the embr y o and the y olk) and is correctl y 6 1 2 CHAPTER 20 c alled a mixocoel (hemocoel). From mesodermal cells at the dorsal j unction of the somatic and s p lanchnic walls