I E volution and Diversit y 1 A rthropod Evolution 1 . Intr oduc t ion Despite their remarkable diversit y of form and habits, insects possess several commo n features b y which the g roup as a whole can be distin g uished. The y are g enerall y small arthropods whose bodies are divisible into cephalic, thoracic, and abdominal re g ions. The h ea d carr i es one p a i ro f antennae, one p a i ro f man dibl es, an d two p a i rs o f max ill ae (t h e hi n d pa i r f use d to f orm t h e l a bi um). Eac h o f t h ree t h orac i c segments b ears a pa i ro fl egs an d , i nt h ea d u l t, t h e meso- an d /or metat h orac i cse g ments usua lly h aveapa i ro f w i n g s. Abdominal appenda g es, when present, g enerall y do not have a locomotor y function. The g enital aperture is located posteriorl y on the abdomen. With few exceptions e gg s are laid , an d t h e young f orm may b equ i te diff erent f rom t h ea d u l t; most i nsects un d ergo some d egre e o f metamorp h os i s . A l t h ou gh t h ese ma y seem i n i t i a lly to b ean i nausp i c i ous set o f c h aracters, w h en t h e y are examined in relation to the environment it can be seen quite readil y wh y the Insect a h ave become the most successful g roup of livin g or g anisms. This aspect will be discusse d i nC h a p ter 2 . I nt h e present c h apter we s h a ll exam i ne t h e poss ibl eor i g i ns o f t h e Insecta, t h at i s, t h e ev ol ut i onar y re l at i ons hi ps o f t hi s g roup w i t h ot h er art h ropo d s. In or d er to d ot hi s mean- in g full y it is useful first to review the features of the other g roups of arthropods. As wil l b ecome apparent below, the question of arthropod ph y lo g en y is controversial, and variou s th eor i es h ave b een p ro p ose d . 2. Arthropod Diversit y Art h ropo d ss h are certa i n f eatures w i t h w hi c h t h ey can b e d e fi ne d .T h ese f eatures are : se g mented bod y covered with a chitinous exoskeleton that ma y be locall y hardened and is periodicall y shed, ta g mosis (the g roupin g of se g ments into functional units, for exam - ple, head, thorax, and abdomen in insects), presence of preoral segments, paired jointed appen d ages on a var i e d num b er o f segments, h emocoe li c b o d ycav i ty conta i n i ng ost i ate h eart enc l ose d w i t hi n a per i car di um, nervous system compr i s i ng d orsa lb ra i nan d ventra l g an g lionated nerve cord, muscles almost alwa y s striated, and epithelial tissue almost alwa ys non-ciliated . 3 4 CHAPTER 1 T hou g h the “true” arthropods fit readil y within this definition, three small g roups, the On y chophora, Tardi g rada, and Pentastoma, whose members are soft-bodied, wormlike animals with un j ointed appenda g es, are less obviousl y arthropodan and each is usuall y g i ven separate p h y l um status . 2.1. O nychophora, Tard ig rada, and Pentastoma T he approximatel y 200 extant species of On y chophora (Fi g ure 1.lA) are terrestria l animals living on land masses derived from the Gondwanan supercontinent: Africa, Cen - tra l an d Sout h Amer i ca, an d Austra l as i a(Ta i t, 2001). T h ey are genera ll y con fi ne d to mo i st h a bi tats an d are f oun db eneat h stones i n rott i ng l ogs an dl ea f mo ld , etc. T h ey possess a com- bination of annelidan and arthropodan characters and, as a result, are alwa y s prominent in discussions of arthropod evolution. Althou g h covered b y a thin arthropodlike cuticle (com - prising procuticle and epicuticle, but no outer wax layer—see Chapter 11), the body wall is anne lid an, as are t h e met h o d o fl ocomot i on, un j o i nte dl egs, t h e excretory system, an d t h e n ervous system. T h e i r art h ropo d an f eatures i nc l u d ea h emocoe li c b o d ycav i ty, t h e d eve l op- m ent and structure of the j aws, the possession of salivar yg lands, an open circulator y s y stem, a tracheal respirator y s y stem, and claws at the tips of the le g s. Amon g livin g arthropods , m yriapods resemble the Onychophora most closely: their body form is similar, tagmosis i s restr i cte d to t h et h ree-segmente dh ea d , exsert il eves i c l es are present i nD i p l opo d aan d Sym- p h y l aaswe ll as i n some onyc h op h orans, a di gest i ve g l an di sa b sent, t h em id gut i ss i m il ar , the g enital tracts of On y chophora resemble those of m y riapods, the g onopore is subtermi - n al, and certain features of embr y onic development are common to both g roups (Tie g s and Manton, 1958). However, this resemblance is superficial. Recent on y chophorans are but th e remnants o f a more w id esprea df auna ( f oss il s f rom t h e Car b on if erous are very s i m il ar t o m o d ern f orms) t h at may h ave evo l ve df rom mar i ne l o b opo d s i nt h e Cam b r i an per i o d. Tardigrades are mostly very small ( < ( ( 0 . 5 mm lon g ) animals, commonl y known as water bears (Fi g ure 1.lB). The ma j orit y of the 800 extant species are found in the temporar y water films that coat mosses and lichens. A few live in p ermanent a q uatic habitats, either marine o r f res h water, or i n water fil ms i nso il an df orest li tter (K i nc hi n, 1994; Ne l son, 2001) . T h e i r b o d y i s covere d w i t h ac hi t i n i ze d cut i c l ean db ears f our pa i rs o f un j o i nte dl egs, eac h F I GU RE 1.1 . ( A ) P er ip ato p s i s s p. (On y chophora); (B ) P s eu d echini s cu ss uillu s ( Tardi g rada); and (C) C e p halobaena tetra p od a (Pentastomida). [A, from A. Sedgewick, 1909, A Student’s Textbook o f Zoolog y ,V ol. VV I II , Swan , Sonnenhein and Co. , Ltd. B , C , from P P. Grass´e , 19 6 8 , Tr ait rr ede ´ Z oo l ogi e ,V ol. 6. By permission of V V Masson et Cie. ] 5 A RTHR O P O D E V O LUTI O N pair bein g innervated from a se g mental g an g lion in the ventral nerve cord. The fluid-filled , h emocoelic bod y cavit y serves as a h y drostatic skeleton. The affinities of the tardi g rades remain unclear. The y were traditionall y ali g ned with pseudocoelomates. However, the y h ave a num b er o f onyc h op h oran an d art h ropo d an structura lf eatures, an d t h emo d ern v i e w i st h at t h ey are c l ose l yre l ate d to t h ese groups. Recent mo l ecu l ar ev id ence supports t hi s proposa l . Pentastomids (ton g ue worms) (Fi g ure l.lC), of which about 100 species are known, are parasitic in the nasal and pulmonar y cavities of vertebrates, principall y reptiles bu t i nc l u di ng bi r d san d mamma l s. T h e b o d yo f t h ese worms, w hi c h range f rom2to13cm l ong, i s covere d w i t h a cut i c l ean dh as two pa i rs o f anter i or un j o i nte dl egs. Interna ll y, t h ere i sa fl u id - fill e d cav i ty ( d e b ata bl ye i t h er a h emocoe l or a pseu d ocoe l om) conta i n i ngapa i re d v e ntral nerve cord with se g mental g an g lia innervatin g each le g . Larval development occur s in the tissues of an intermediate host, which ma y be an omnivorous or herbivorous insect, fi sh, or mammal. Though pentastomids are highly modified as a result of their parasitic life, th ey are un d ou b te dl y art h ropo d s. T h e i r exact pos i t i on rema i ns controvers i a l ,re l at i ons hi ps wi t h Acar i na, myr i apo d s, an db ranc hi uran crustaceans h av i ng b een suggeste d . 2 .2. Tr i l obi t a The trilobites (Fi g ure 1.2), of which almost 4000 species have been described, ar e marine fossils that reached their peak diversit y in the Cambrian and Ordovician peri- ods ( 5 00–600 million years ago) (Whittington, 1992). Despite their antiquity they were, h owever, not pr i m i t i ve b ut hi g hl y spec i a li ze d art h ropo d s. In contrast to mo d ern art h ropo d s th etr il o bi tes as a w h o l es h ow a remar k a bl eun if orm i t y o fb o dy structure. T h e b o dy , usua lly F I GU RE 1.2. T riarthru s eaton i ( Trilobita). (A) Dorsal view; and (B) ventral view. [From R. D. Barnes, 1968, Inverte b rate Zoo l ogy,2n d e d . By perm i ss i on o f t h e W. B. Saun d ers Co., P hil a d e l p hi a.] 6 CHAPTER 1 o val and dorsoventrall y flattened, is divided transversel y into three ta g mata (head, thorax, and p yg idium) and lon g itudinall y into three lobes (two lateral pleura and a median axis). The head, which bears a pair of antennae, compound e y es, and four pairs of biramous ap- pen d ages, i s covere db y a carapace. A pa i ro fid ent i ca lbi ramous appen d ages i s f oun d on e ac h t h orac i c segment. T h e b asa l segment o f eac hli m bb ears a sma ll , i nwar dl y pro j ect i n g e n di te t h at i s use d to di rect f oo d towar d t h e mout h. M uch about the habits of trilobites can be surmised from examination of their remains and the de p osits in which these are found. Most trilobites lived near or on the sea floor. W hil e some spec i es preye d upon sma ll ,so f t- b o di e d an i ma l s, t h ema j or i ty were scavengers. H owever, lik e eart h worms, a f ew sma ll er tr il o bi tes too ki nmu d an ddi geste d t h e organ ic m atter f rom i t. On t h e b as i so f X-ray stu di es o f pyr i t i ze d tr il o bi te spec i mens, w hi c h s h ow that trilobites possess a combination of chelicerate and crustacean characteristics, Cisne ( 1974) concluded that the Trilobita, Chelicerata, and Crustacea form a natural g roup with a c ommon ancestry. Their ancestor would have a body form similar to that of trilobites. Most aut h ors di spute t h e propose d tr il o bi te-crustacean li n k ,an d some even re j ect t h e assoc i at i on b etween tr il o bi tes an d c h e li cerates. In d ee d ,t h ere are t h ose w h o suggest t h at t h etr il o bi tes themselves are pol y ph y letic (Willmer, 1990) . Althou g h the decline of trilobites (and their replacement b y the crustaceans as the dominant aquatic arthropods) is a matter solely for speculation, Tiegs and Manton (1958) suggeste d t h at t h e i r b as i c, rat h er cum b ersome b o d yp l an may h ave pro hibi te d t h eevo l ut i o n of f ast movement at a t i me w h en hi g hl y mot il e pre d ators suc h as fi s h an d cep h a l opo ds w ere becomin g common. In addition, the man y identical limbs presumabl y moved in a m etachronal manner, which is a rather inefficient method in lar g eor g anisms . 2.3. The Chelicerate Arthro p od s Th enext f our groups are o f ten p l ace d toget h er un d er t h e genera lh ea di ng o f C h e li cerata because their members possess a bod y that is divisible into cephalothorax and abdomen , the former usuall y bearin g a pair of chelicerae (but lackin g antennae), a pair of pedipalps, and four pairs of walkin g le g s. Althou g h there is little doubt of the close relationshi p b etween t h eX i p h osura, Eurypter id a, an d Arac h n id a, t h e pos i t i on o f t h e Pycnogon id a i s uncerta i n. T h oug h t h ey are usua ll y i nc l u d e d asac l ass o f c h e li cerates, t h e i ra ffi n i t i es w i t h o t h er mem b ers o f t hi s g roup rema i n unc l ear, an d t h ere are some aut h ors w h o cons id er t h e y deserve more separated status (see Kin g , 1973; Manton, 1978; Ed g ecombe, 1998; Forte y and Thomas, 1998 ). Xi phosura. L imu l us po ly p h emus , t h e ki ng or h orses h oe cra b (F i gure 1.3), i son e of f our surv i v i ng spec i es o f ac l ass o f art h ropo d st h at fl our i s h e di nt h eOr d ov i c i an-Upper D evonian periods. Kin g crabs occur in shallow water alon g the eastern coasts of North and Central America. Three s p ecies o f Tac h ypleu s a n d C arcinoscor p ius o ccur alon g the coast s o f China, Japan, and the East Indies. Like trilobites the y are bottom feeders, stirrin g up the su b strate an d extract i ng t h e organ i c mater i a lf rom i t. I n Limu l u s t h ece ph a l ot h orax i s covere d wi t h a h orses h oe-s h ape d carapace. T h ea bd omen art i cu l ates f ree l yw i t h t h e cep h a l ot h orax an d at i ts poster i or en d carr i es a l on g te l son. On t h e ventra l s id eo f t h e cep h a l ot h orax are six pairs of limbs. The most anterior pair are the chelicerae, and these are followed b y fiv e pairs of le g s. Each le g has a lar g e g nathobase, which serves to break up food and pass it f orwar d to t h e mout h .S i xpa i rs o f appen d ages are f oun d on t h ea bd omen. T h e fi rst pa ir f use me di a ll yto f orm t h e opercu l um. T hi s protects t h e rema i n i ng pa i rs, w hi c hb ear g ill so n t h e i r poster i or sur f ace. 7 A RTHR O P O D E V O LUTI O N F IGURE 1.3 . T he horseshoe crab , L imulus pol y phemus . ( A) Dorsal view and (B) ventral view. [From R . D. Barnes , 19 6 8 , I nverte b rate Zoo l ogy , 2n d e d .B y perm i ss i on o f t h e W. B. Saun d ers Co., P hil a d e l p hi a.] Eurypter i da . T h e Eurypter id a(g i ant water scorp i ons) (F i gure 1.4A) were f ormer ly i nc l u d e d w i t h t h eX i p h osura i nt h ec l ass Merostomata. However, most recent stu di es h av e conc l u d e d t h at t h e two are not s i ster g roups an d t h at t h e Merostomata i s a parap hyl et ic assembla g e (authors in Ed g ecombe, 1998). More than 300 species of this entirel y fossil g roup of predator y arthropods, which existed from the Ordovician to the Permian periods, are known. Because of their sometimes large size (up to 2.5 m) they are also known a s Gi gantostraca. T h ey are b e li eve d to h ave b een i mportant pre d ators o f ear l y fi s h , prov idi n g se l ect i on pressure f or t h eevo l ut i on o fd erma lb one i nt h e Agnat h a. In b o d yp l an t h ey wer e rather similar to the xiphosurids. Six pairs of limbs occur on the cephalothorax, but, in contrast to those of kin g crabs, the second pair is often g reatl y enlar g ed and chelate formin g pedipalps, which presumably served in defense and to capture and tear up prey. The trunk o f eurypter id s can b e di v id e di nto an anter i or prea bd omen on w hi c h appen d ages (concea l e d gill s) are reta i ne d an d a narrow ta illik e posta bd omen f rom w hi c h appen d a g es h ave b een l ost . T hou g h the earl y eur y pterids were marine, adaptive radiation into freshwater and perhap s ev en terrestrial habitats occurred. Indeed, it was from freshwater forms that arachnids are b elie v ed to ha v ee v ol v ed . A r ac hni da. Scorp i ons, sp id ers, t i c k s, an d m i tes b e l ong to t h ec l ass Arac h n id aw h os e approximatel y6 2,000 species are more easil y reco g nized than defined. Livin g members of t he g roup are terrestrial (althou g h a few mites are secondaril y aquatic) and have respirator y or g ans in the form of lun g books or tracheae. In contrast to the two aquatic chelicerate g roup s d escr ib e d ear li er, most arac h n id sta k eon l y li qu id f oo d , extracte df rom t h e i r prey b y means o f ap h aryngea l suc ki ng pump, o f ten a f ter extraora ldi gest i on. Scorp i ons, o f w hi c h t h er e are about 1 5 00 livin g species, are the oldest arachnids with fossils known from the Silurian. Some of these fossils were aquatic (Polis, 1990). With about 35,000 species, spiders form 8 CHAPTER 1 F IGURE 1.4. (A) Eurypterid and (B ) Ny mphon rubru m ( Pycnogonida). [A, from D. T. Anderson (ed.), 2001 , I nverte b rate Zoo l ogy , 2nd ed. B y permission of Oxford Universit y Press. B, from R. D. Barnes. 19 6 8, I nverte b rate Z oolo gy , 2nd ed. B y permission of the W. B. Saunders Co., Philadelphia. ] an extremely diverse group. The earliest spider fossils are from the Devonian, and by th e Tert i ary t h esp id er f auna was very s i m il ar to t h at seen to d ay (Foe li x, 1997) . P ycno g onida . T h e approx i mate ly 1000 spec i es o fli v i n g P y cno g on id a (Pantopo d a ) are the remnants of a g roup that ori g inated in the Devonian. The y are commonl y know n as sea spiders because of their superficial similarit y to these arachnids (Fi g ure 1.4B). The y are f oun d at vary i ng d ept h s i na ll oceans o f t h ewor ld , b ut are part i cu l ar l y common i nt h e s h a ll ower waters o f t h e Arct i can d Antarct i c Oceans. T h ey li ve on t h e sea fl oor an df ee d on c oe l enterates, b r y ozoans, an d spon g es. On t h e cep h a l ot h orax i sa l ar g e pro b osc i s,ara i se d tubercle bearin g four simple e y es, a pair of chelicerae and an associated pair of palps, an d five pairs of le g s. The le g s of the first pair differ from the rest in that the y are small and pos i t i one d ventra ll y. T h ese ov i gerous l egs are use di nt h ema l e f or carry i ng t h e eggs. T h e a bd omen i s very sma ll an dl ac k s appen d ages. As note d a b ove, t h e prec i se re l at i ons hi ps o f t h e pycnogon id stoot h er art h ropo d s rema i n c ontroversial. Althou g h the presence of chelicerae, the structure of the brain, and the natur e o f the sense or g ans are chelicerate characters, the structure and innervation of the proboscis , the similarity between the intestinal diverticula and those of annelids, the multiple paire d gonopores, an d t h e suggest i on t h at t h e pycnogon id s h ave a true coe l om s h ow t h at t h ey must have left the main line of arthropod evolution at a very early date (Sharov, 19 66 ). Other n on-chelicerate features that the y possess are (1) the partial se g mentation of the le g -bearin g part of the bod y , (2) the reduction of the opisthosoma to a small abdominal component, and ( 3) the presence, in the male, of ovigerous legs . 2.4. The Mandibulate Arthro p ods Th e rema i n i ng groups o f art h ropo d s (crustaceans, myr i apo d san dh exapo d s) were or i g- i nall yg rouped to g ether as the Mandibulata b y Snod g rass (1938) because their members 9 A RTHR O P O D E VOLUTION possess a pair of mandibles as the primar y masticator y or g ans. Thou g h this view becam e w idel y accepted, some later authors, notabl y Manton, ar g ued forcefull y that the mandible of t he crustaceans is not homolo g ous with that of the m y riapods and insects. That is, the term M an dib u l ata s h ou ld not b e use d to i mp l yap h y l ogenet i cre l at i ons hi p b ut on l y a commo n level of advancement reached by several groups independently (Tiegs and Manton, 19 5 8; M anton, 1977). T h e d e b ate over w h et h er t h e Man dib u l ata const i tute a monop hyl et i c g rou p continues to be vi g orous (see chapters in Ed g ecombe, 1998; Forte y and Thomas, 1998; also Section 3.3.1), and the conclusion reached t y picall y hin g es on the t y pe of evidenc e presente d .Ev id ence f rom comparat i ve morp h o l ogy, bi oc h em i stry, an d mo l ecu l ar bi o l ogy o fli v i ng spec i es ten d s to support monop h y l y, w h ereas d ata f rom f oss il s genera ll ya li g n th e Crustacea w i t h t h eC h e li cerata. W i t h t h e i rtwopa i rs o f antennae, t h e Crustacea wou ld appear ver y distinct from the other two g roups. M y riapods and hexapods have a sin g le pair of antennae, a feature that led Sharov (1966) to unite these g roups in the Atelocerata. Tie gs and Manton (1958) and Manton (1977) went a step further, placing the two groups wit h th eOnyc h op h ora i nt h eUn i ram i a. However, l oo k s can b e d ece i v i ng, an d many mo d ern p h y l ogenet i c i sts cannot accept t h e Ate l ocerata (see Sect i on 3.3.1) an d t h eUn i ram i a (se e Sections 3.2.2 and 3.3) as monoph y letic taxa . C rustacea . T o t he Crustacea belong the crabs, lobsters, shrimps, prawns, barnacles, an d woo dli ce. T h e Crustacea are a success f u l group o f art h ropo d s: some 40,000 li v i ng spec i es h ave b een d escr ib e d an d t h ere i sana b un d ant f oss il recor d .T h e y are pr i mar ily aquatic, and few have mana g ed to successfull y conquer terrestrial habitats. The y exhibit a remarkable diversit y of form; indeed, man y of the parasitic forms are unreco g nizable i n t he adult stage. Typical Crustacea, however, usually possess the following features: bod y di v id e di nto cep h a l ot h orax an d a bd omen; cep h a l ot h orax w i t h two pa i rs o f antennae, t h re e pa i rs o f mout h parts (man dibl es an dfi rst an d secon d max ill ae), an d at l east fi ve pa i rs o fl egs; b iramous appenda g es . The reason for the success of Crustacea (and perhaps the reason wh y the y replace d t rilobites as the dominant aquatic arthropods) is their adaptability. Like their terrestrial counterparts, t h e i nsects, crustaceans h ave exp l o i te d to t h e f u ll t h ea d vantages con f erre d b y possess i on o f a segmente db o d yan dj o i nte dli m b s. Pr i m i t i ve crustaceans, f or examp l e , t he fair y shrimp (Fi g ure 1. 5 ), have a bod y that shows little si g nofta g mosis and limb specialization. In contrast, in a hi g hl y or g anized crustacean such as the cra y fish (Fi g ure 1. 6) t he appenda g es have become specialized so that each performs onl y one or two functions, an d t h e b o d y i sc l ear l y di v id e di nto tagmata. In t h e l arger ( b ottom- d we lli ng) Crustacea spec i a li ze dd e f ens i ve weapons h ave evo l ve d (e.g., c h e l ae, t h ea bili ty to c h ange co l or i n re l at i on to t h eenv i ronment, an d t h ea bili t y to move at high spee d over s h ort di stances by snappin g the flexible abdomen under the thorax). B y contrast, smaller, planktonic Crustacea are often transparent and have evolved hi g h reproductive capacities and short life c y cles to f ac ili tate sur viv a l . My ria p oda . T h e mem b ers o ff our groups o f man dib u l ate art h ropo d s(C hil opo d a, Diplopoda, Pauropoda, and S y mph y la) share the followin g features: five- or six-se g mente d h ead, unique mandibular bitin g mechanism, sin g le pair of antennae, absence of compound F I GU RE 1.5 . B r a n c h i n ecta s p., a fairy shrimp . [ From R. D. Barnes, 19 6 8, Inverte b rate Zoo l ogy , 2n d e d .B y perm i ss i on o f t h e W. B. Saun d ers Co., P hiladelphia.] 10 CHAPTER 1 FI GU RE 1.6. Cra yfi s h . Ventra l v i ew o f one s id etos h ow diff erent i at i on o f appen d a g es. e y es, e l ongate trun k t h at b ears many pa i rs o fl egs, art i cu l at i on o f t h e coxa w i t h t h e sternu m ( rat h er t h an t h ep l euron as i n h exapo d s), trac h ea l resp i ratory system, Ma l p i g hi an tu b u l es f or excretion, absence of mesenteric ceca, and distinctive mechanism b y which the anima l e xits the old cuticle durin g ecd y sis. Further, the y are found in similar habitats (e. g ., leaf m old, loose soil, rottin g lo g s). Fo rt h ese reasons, t h ey were tra di t i ona ll yp l ace di nas i ng l e l arge taxon, t h e Myr i apo d a . T h e monop h y l et i c nature o f t h e myr i apo d s h as b een supporte db y some, b ut not a ll ,c l a di st ic ana ly ses o fl ar g e d ata sets w i t h a com bi nat i on o f morp h o l o gi ca l an d mo l ecu l ar c h aracters o f livin g species (Wheeler et al ., 1993; authors in Forte y and Thomas, 1998). Yet other m orpholo g ical and molecular studies indicate that the m y riapods constitute a paraph y leti c o revenpo l yp h y l et i c group. Determ i nat i on o f t h ere l at i ons hi ps w i t hi nt h e Myr i apo d a h a s prove d diffi cu l t b ecause potent i a ll y h omo l ogous c h aracters are s h are db y diff erent pa i rs o f g roups. For examp l e, D i p l opo d aan d Pauropo d a h ave t h e same num b er o fh ea d se g ments and one pair of maxillae; Diplopoda and Chilopoda have se g mental tracheae; and S y mph y l a 11 A RTHR O P O D E V O LUTI O N F IGURE 1.7. M yriapoda. (A) L ithobiu s sp. (centipede), (B) Julus terrestri s ( millipede), (C ) Pauro p us silvaticus (p auro p o d ), an d (D ) S cutigere ll a immacu l at a ( s y mph y lan). [From R. D. Barnes, 19 6 8 , Inverte b rate Zoo l og y , 2n d ed. B y permission of the W. B. Saunders Co., Philadelphia. ] an d Pauropo d a d eve l op em b ryon i c ventra l organs t h at b ecome evers ibl eves i c l es, as we ll as contributin g to the ventral g an g lia. Boudreaux’s (1979) overall conclusion was that th e P a uropoda-Diplopoda and Chilopoda-S y mph y la are the two sister g roups within the taxo n (Figure 1.8). An alternative view, based on cladistic analysis of morphological characters of li v i ng f orms (Kraus, i n Fortey an d T h omas, 1998), i st h at t h e myr i apo d s are parap h y l et i c : th eC hil opo d a i st h es i ster group to t h eot h er t h ree. Un f ortunate l y, t h oug h t h ere i sar i c h fossil record of m y riapods extendin g back to the Upper Silurian, insufficient stud y has been d one to clarif y the monoph y letic nature or otherwise of this g roup (see Shear, in Forte y an d T homas, 1998) . S ome 3000 spec i es o f c hil opo d s (cent i pe d es) (F i gure 1.7A) h ave b een d escr ib e d (Lew i s , 1981). T h ey are typ i ca ll y act i ve, nocturna l pre d ators w h ose b o di es are fl attene dd orsoven - t ra lly .T h e fi rst pa i ro f trun k appen d a g es (max illi pe d s) are mo difi e di nto po i son c l aws t h at are used to catch pre y . In most centipedes the le g s increase in len g th from the anterior to th e p osterior of the animal to facilitate ra p id movement. The earliest known fossil centi p edes , f rom t h e Upper S il ur i an, are remar k a bl ys i m il ar to some extant spec i es, suggest i ng t h at t h e group may b e cons id era bl y more anc i ent. I n contrast to t h e cent i pe d es, t h e di p l opo d s(m illi pe d es) (F ig ure 1.7B) are s l ow-mov i n g h erbivorous animals. The distin g uishin g feature of the almost 10,000 species in the class is the presence of diplose g ments, each bearin g two pairs of le g s, formed b y fusion of two or i g i na ll y separate som i tes. It i s b e li eve d t h at t h e di p l osegmenta l con di t i on ena bl es t he an i ma l to exert a strong pus hi ng f orce w i t hi ts l egs w hil e reta i n i ng r i g idi ty o f t h e trun k re gi on. As t h e y cannot escape f rom wou ld - b e pre d ators by spee d , man y m illi pe d es h av e e v o lved such protective mechanisms as the abilit y to roll into a ball and the secretion o f [...]... Manton (19 58) give a detailed historical account of schemes for the evolutionary relationships of arthropods Other major contributors to this fascinating debate include Manton (19 73, 19 77), Sharov (19 66), Anderson 21 ARTHROPOD EVOLUTION 22 CHAPTER 1 (19 73), Boudreaux (19 79), Willmer (19 90), Wheeler et al (19 93), Bitsch (2001a,b), and authors in Gupta (19 79), Edgecombe (19 98), and Fortey and Thomas (19 98)... Collembola, and Diplura have been included with the thysanurans in Chapter 5 where details of their biology are presented 13 ARTHROPOD EVOLUTION 14 CHAPTER 1 FIGURE 1. 9 Schemes for the possible relationships of the hexapod groups as envisaged by Boudreaux (19 79), a Kristensen (19 91) , and Kukalov´ -Peck (19 91) 3 Evolutionary Relationships of Arthropods 3 .1 The Problem In determining the evolutionary relationships... and morphology, Nature 413 :15 7 16 1 Gupta, A P (ed.), 19 79, Arthropod Phylogeny, V Nostrand-Reinhold, New York Van Hopkins, S P., and Read, H J., 19 92, The Biology of Millipedes, Oxford University Press, London Hwang, U W., Friedrich, M., Tautz, D., Park, C J., and Kim, W., 20 01, Mitochondrial protein phylogeny joins myriapods with chelicerates, Nature 413 :15 4 15 7 Kinchin, I M., 19 94, The Biology of the... Victoria a Kukalov´ -Peck, J., 19 92, The “Uniramia” do not exist: The ground plan of the Pterygota as revealed by Permian Diaphanopterodea from Russia (Insecta: Paleodictyopteroidea), Can J Zool 70:236–255 Lewis, J G E., 19 81, The Biology of Centipedes, Cambridge University Press, Cambridge Manton, S M., 19 73, Arthropod phylogeny—A modern synthesis, J Zool (London) 17 1: 11 1 13 0 Manton, S M., 19 77, The Arthropoda:... proteins, J Comp Physiol B 17 2:95 10 7 Cisne, J L., 19 74, Trilobites and the origin of arthropods, Science 18 6 :13 18 Cook, C E., Smith, M L., Telford, M J., Bastianello, A., and Akam, L., 20 01, Hox genes and the phylogeny of arthropods, Curr Biol 11 :759–763 Edgecombe, G D (ed.), 19 98, Arthropod Fossils and Phylogeny, Columbia University Press, New York Emerson, M J., and Schram, F R., 19 90, The origin of crustacean... University Press, Stanford Ross, H H., 19 65, A Textbook of Entomology, 3rd ed., Wiley, New York e Sharov, A G., 19 66, Basic Arthropodan Stock, Pergamon Press, Elmsford, NY Shultz, J W., and Regier, J C., 2000, Phylogenetic analysis of arthropods using two nuclear protein-encoding genes supports a crustacean + hexapod clade, Proc R Soc Lond Ser B 267 :10 11 10 19 Snodgrass, R E., 19 38, Evolution of the annelida,... in Gupta, 19 79; Edgecombe, 19 98; Fortey and Thomas, 19 98; also Emerson and Schram, 19 90; Kukalov´ -Peck, 19 92) Only rarely have authors attempted to marshall all of the a evidence in order to arrive at an overall conclusion Even then, there may be no agreement! For example, the analyses of Boudreaux (19 79) and Wheeler et al (19 93) led them to favor a monophyletic origin whereas Willmer (19 90) concluded... natural groups, each with the rank of phylum (Figure 1. 11) The phyla are the Chelicerata, the Crustacea, and the Uniramia FIGURE 1. 11 A scheme showing a possible polyphyletic origin for the major arthropod groups and related phyla Hatched lines ending in a question mark indicate arthropod fossils not easily assigned to existing taxa (Onychophora-Myriapoda-Hexapoda) In some schemes the Trilobita are included.. .12 CHAPTER 1 FIGURE 1. 8 Schemes for the possible monophyletic origin of the arthropods as proposed by Snodgrass (19 38), Sharov (19 66), and Boudreaux (19 79) Note also the differing relationships of the Annelida, Onychophora, and Arthropoda defensive chemicals (Hopkins and Read, 19 92) Fossil millipedes are known from the Lower Devonian Pauropoda... included 11 segments (Kukalov´ -Peck, 19 92, and in Edgecombe, 19 98) a Over the 75 years since it was proposed, the merits or otherwise of Snodgrass’ scheme have been debated vigorously, and there is still no consensus Broadly speaking, evidence from morphological, biochemical, and molecular biological studies tend to support the scheme (e.g., Boudreaux, 19 79; W¨ gele, 19 93; Wheeler et al., 19 93; Wheeler, . t h e b as i so f X-ray stu di es o f pyr i t i ze d tr il o bi te spec i mens, w hi c h s h ow that trilobites possess a combination of chelicerate and crustacean characteristics, Cisne ( 19 74) concluded. cuticle (com - prising procuticle and epicuticle, but no outer wax layer—see Chapter 11 ), the body wall is anne lid an, as are t h e met h o d o fl ocomot i on, un j o i nte dl egs, t h e excretory. Man dib u l ata const i tute a monop hyl et i c g rou p continues to be vi g orous (see chapters in Ed g ecombe, 19 98; Forte y and Thomas, 19 98; also Section 3.3 .1) , and the conclusion reached t y picall y hin g es