The sejugal furrow in camel spiders and acariform mites

2012·6.9 MB·English

by  Jason A Dunlop

2
Arachnologische Mitteilungen 43:29-36 Nuremberg,July2012
The sejugal furrow in camel spiders and acariform mites
Jason A.Dunlop,Jessica Krüger&Gerd Alberti
doi:10.5431/aramit4303
AbstractiCamelspiders(ArachnidaiSolifugae)areoneofthearachnidgroupscharacterisedbyaprosomaldorsal
shieldcomposedofthreedistinctelements:thepro-,meso-andmetapeltidium.Theseareassociatedrespectively
with prosomal appendages one to four,five,and six.What is less well known,although noted in the historical
literature,isthatthecoxaeofthe4^*^ and 5^^ prosomal segments (i.e.walking legs 2and 3) ofcamel spidersare
alsoseparatedventrallybya distinctmembranous region,which isabsent betweenthecoxaeoftheotherlegs.
We suggestthatthis essentiallyventral division ofthe prosoma specifically between coxae 2 and 3 is homolo-
gouswith the so-called sejugal furrow (the sejugal interval sensu van der Hammen).This division constitutes a
fundamentalpartofthebodyplaninacariformmites(Arachnida:Acariformes).lfhomologous,thissejugalfurrow
could represent a further potential synapomorphyfor (Solifugae -h Acariformes);a relationshipwith increasing
morphological and molecularsupport.Alternatively,outgroup comparison with sea spiders (Pycnogonida) and
certain earlyPalaeozoicfossilscould implythatthesejugalfurrowdefinesan oldertagma,derivedfrom a more
basalgradeoforganisation.Inthisscenariothe(still)dividedprosomaofacariformmitesandcamelspiderswould
be plesiomorphic.This interpretation challengesthe textbookarachnid character ofa peltidium (or'carapace')
covering an undivided prosoma.
Keywords:Acariformes,morphology,outgroups,phylogeny,Solifugae,tagmosis
Camelspiders(Arachnida,Solifugae)areafascinating In camel spiders, schizomids (Schizomida) and pal-
groupofarachnidswhich,astheirnameimplies,pre- pigrades (Palpigradi) the peltidium is not a single
dominantlyoccurinaridhabitats.Thesefast-moving plate, but is divided into a series ofdiscrete dorsal
and voracious predators are also sometimes referred sclerites.These are conventionallyreferred to as the
to aswind scorpions orsunspiders. Overathousand pro-, meso- and metapeltidium. In fact the camel
living species are known (HARVEY 2003) and they spiderpropeltidium seems to be even more complex
occur in suitable environments in all subtropical to andcomposedofmultipleelements(KÄSTNER1932,
tropicalzones,withthecuriousexceptionofAustralia. ROEWER 1932).
For a summary oftheir biology see PUNZO (1998). Authors such as BERNARD (1896, 1897) and
Camel spiders are morphologically and phyloge- Kästner(1932)interpretedthisbasictagmosispat-
neticallyofinterest in that theydiffer in certain key tern in camel spiders as plesiomorphic, presumably
aspects from the typical arachnid groundplan. The reflectingagrade oforganisationwhichpredates the
best example ofthis is that the prosoma is not cov- traditionalarachnidprosoma.Otherworkersexplicitly
eredbyasingledorsalshield.Thisstructureiswidely treateda‘dividedcarapace’asaderivedcharacterstate
referredtointhearachnidtaxonomicliteratureasthe (WEYGOLDT&PAULUS 1979,SHULTZ1990,2007).
carapace. Strictly speaking - from the perspective Irrespectiveofpolarity,thecamelspiderconditionhas
of comparative arthropod morphology - the term interestingparallelswithcertainmites(Acari),which
‘carapace’shouldbe restrictedto crustaceans andthe alsoexpressadorsalscleriteagainassociatedwiththe
arachnid structure is betterreferred to as a prosomal chelicerae, pedipalps and the first two pairs ofwalk-
&
dorsal shield, or (sensu BÖRNER 1904) a peltidium. inglegs (COINEAU 1974, EVANS 1992,ALBERTI
Coons 1999,WeigMANN 2001).This whole body
JasonA.DUNLOP,MuseumfürNaturkunde,LeibnizInstitute regiondowntothesecondpairoflegshasbeentermed
forResearchonEvolutionandBiodiversityattheHumboldt the proterosoma and the dorsal sclerite covering it
UniversityBerlin,Invalidenstrasse43,D-10115Berlin,Germany, is usually called the prodorsum (e.g. WEIGMANN
E-Mail:[email protected]
JessicaKRÜGER,InstitutfürBiologie,HumboldtUniversityBerlin, 2001). The name ‘aspidosoma’ can also be found in
Philippstrasse 13,D-10115Berlin,Germany, the literature but, as discussed byWEIGMANN, this
E-Mail:[email protected] termshouldrefertotergitesexplicitlyassociatedwith
GerdALBERTI,ZoologischesInstitutundMuseum,University
ofGreifswald,J.-S.-Bach-Strasse 11/12,D-17489Greifswald, the gnathosoma, and there is no evidence that these
Germany,E-Mail:[email protected] structureshaveovergrowntherestoftheproterosoma
aspertheevolutionaryscenariosproposedbyauthors
submitted:29.11.2011;accepted:14.2.2012;onlineeady:25.4.201
30 JA.Dunlop,J.Krüger&G.Alberti
such as Grandjean (1969), Coineau (1974) and Historically,KiTTARY(1848)differentiatedthecamel
VANDERHammen (1989).Ingeneral,issuesremain spider prosoma into a ‘head’ (the propeltidium) and
among mites with respect to questions ofsegmental ‘thorax’ (meso- and metapeltidium) and observed
homologyand the use ofa standard terminology. paired spiracles opening ventrally on a membrane
Theseobservationsalsoreflecttworecurrentprob- betweenthem.ThecomprehensivestudyofBERNARD
lems in arachnid comparative morphology (see e.g. (1896,p. 308) statedthat“The Galeodidae showthe
Dunlop2000).The firstis the use ofdivergentter- primitive metamerism of the body more markedly
minologiesforessentiallythesamestructuresinmites than any other Arachnid“. He added (p. 308) “The
andnon-mitetaxa.Thesecondistheuseofthesame Galeodidae canbendthe bodynot onlybetween the
term, e.g. carapace’, for non-homologous structures 6thand7thsegments(atthewaist),butalsobetween
acrossdifferentarthropodgroups.Suchdiscrepancies the 4th and 5th”. While Bernard did not explicitly
innomenclaturecanmaskpotentialsynapomorphies. describe the ventral membrane between segments
Here, we draw attention to an older - albeit largely 4 and 5, its presence can be easily inferred from his
overlooked-observationthatcamelspiders notonly illustrations (pi. 27, fig. 15, pi. 29, fig. 6). RoeWER
have an obvious dorsaldivision ofthe prosoma, but
alsoexpressadistinctventraldivision(Fig. 1),specifi-
callybetween the coxae ofthe second and third pair
ofwalkinglegs(BERNARD 1896,ROEWER1932,VAN
DERHammen 1989).Webelievethischaractertobe
ofsomesignificanceandpotentiallyhomologouswith
theso-calledsejugalfurrow,whichalsorunsbetween
legs two and three in certain lineages ofmites (Figs
2-4).
Severalstudies eitherproposedthatmites should
besplitintotwodistinctclades (e.g.VANDERHAM-
MEN 1989, Alberti 2006) or did not recovered
these two lineages as sister taxa in their cladograms
(DABERTetal.2010,PEPATO etal.2010,REGIERet
al.2010).Thesegroups are here termedAcariformes
andParasitiformes (=ActinotrichidaandAnactinot-
richida)andthesepublicationsimplythatAcari,inits
traditionalsense,maynotbemonophyletic.Theseju-
galfurrowiswidelycitedasafundamentalpartofthe
bodyplan in numerous acariform lineages only (e.g.
Coineau1974,Alberti&Coons 1999,Alberti
2006, Dunlop & Alberti 2008). We argue here
that it is present in camel spiders too, and should be
scoredassuchinfuturecladisticanalyses.Thesejugal
furrowmaythereforecontributetowardsalargersetof
&
morphological and moleculardata (AlBERTI PE-
RETTI2002,DABERTetal.2010,PepATOetal.2010,
and references therein) explicitly supporting a novel
(Solifugae + Acariformes) clade. However, as noted
byBernard and Kästner above (see also Discussion),
an alternative interpretation would be that the body
region defined by the propeltidium/sejugal furrow Fig.1:Camel spider{SoWfugae:Galeodessp.) inventralview.
is part ofan older arthropod groundplan. Ifso, this Prosomal regionartificiallybentslightlybackwardsto
teaseouta natural,membranousdivision (arrowed)
would raise questions about the original pattern of
betweenthesecondandthird leg coxae.Abbreviations:
anterior tagmosis among arachnids: namely did the ch =chelicerae,mb= membrane(interpreted hereas
first arachnids have a prosoma or a proterosoma? homologouswiththesejugalfurrow),op=opisthosoma,
legcoxae numberedfrom 1-4.
Sejugalfurrowincornelspiders 31
(1932: 43, fig. 33) explicitlystated for the coxae that specimens, six ofwhich are illustrated here (Figs.
“Only these ofthe 2nd and 3rdwalking legs are di- 5—10). SpecimenswerephotographedusingaCanon
vided by a wide, soft membrane.” [our translation]. Eosdigitalcamerawitheitheraxl orax3 macrolens.
KÄSTNER(1932)didnotexplicitlymentionaventral The resultingimageswere cleaned and assembled in
division, but seems to have been more concerned Adobe Photoshop. Comparative scanning electron
with the compositionofthe dorsalprosoma. He did, micrographs (Figs. 2-4) ofrepresentative acariform
however,mentionstructures(alsonotedbyBERNARD mites were producedbyGA.
1896) which partly divide the body internally and
further help to define and offset this anterior body Results
region. KÄSTNER (1952: fig. 9) seemed to indicate Inventralview,the prosomaofcamel spiders from a
thisventralmembraneinalateralviewofalate-stage rangeofdifferentfamilies(Figs5-10)presentsafairly
camel spider embryo. He labelled
the region between coxae two and
three ‘G’, but did not define this in
the figure legend. It may refer to
“Gelenkhaut” [= membrane]. Most
recently,VANDERHAMMEN (1986,
1989: 249), formally stated that
for camel spiders “The coxisternal
regions oflegs II andIII (epimera2
and 3) are transverselyseparated by
thesejugalinterval(anintersegmen-
tal area of soft skin, which allows
ofprosomatic articulation).” Here,
we confirm these observations and
further discuss their potential phy-
logenetic significance.
MaterialandMethods
Camelspidergrossmorphologywas
examinedunderadissectingmicro-
scope. Specimens were carefully
bentbackwardsand/ormanipulated
with tweezers to investigate where
the basic division(s) in the ventral
body surface lay. To determine
whether the resulting observations
were typical for the whole order,
representativesofnineofthetwelve
currently recognised families (cf.
Harvey2003)were examinedbased
on alcohol-preserved specimens in
the Museum für Naturkunde, Ber-
lin. Specimens ofMelanoblossidae,
Mummuciidae and Eremobatidae
were not available, but all other Figs.2-4:Comparativescanning electron micrographsofselected acariform
families revealed a consistent mor- mites.Noteagainthe principal division betweenthesecondandthird pair
phology which we thus presume ofleg coxae(arrowed);specificallyformed here bytheso-called sejugalfur-
row.2-NeonanorchestesammolitoreusMcDaniel &Bolen,1981 (Endeostigmata:
to be the groundplan character for Nanorchestidae).3-MicropsammuslittoralisTheron&Coineau,1983 (Endeosti-
Solifugae. The ventral prosomal gata:Micropsammidae).4-Epilohmanniacylindrica(Berlese,1905) (Oribatida:
anatomyiseasiertoresolveinlarger Epilohmannidae).Nottoscale.
32 JA.Dunlop,J.Krüger&G.Alberti
compact series ofpedipalp and limb coxae.There is dorsosejugal) furrow:
no plate-like sternum between the leg coxae, as in 1)is“Pertainingtothefurroworintervalseparating,in
spiders (Araneae) forexample,noris there aseries of Actinotrichida,propodosomaandmetapodosoma.”
ventral sclerites between the coxae as per Palpigradi. (VAN DER Hammen 1980: 140),
Furthermore, there is no superficial evidence of a 2)is“atransversefurrowrunningbetweenlegsII and
‘break’between the successive coxalpairs. Infactthe III and separating them.This furrow [...] extends
dividinglineelaboratedhereisbestrevealedbysimply dorsallyand thus divides thebodyinto an anterior
taking a specimen and gently bending the prosoma part,theproterosomaandaposteriorpartthe hys-
backwards orsideways.Theventral surface naturally terosoma.” (AlberTI 2006: 327),
opens up between the second and third pair ofleg 3) is a “circumferential zone ofbody flexibility that
coxae (Fig. 1);preciselybecausetheyareseparatedby passesbetweenthe coxaeoflegs2 and3”(SHULTZ
apale,flexible membrane; superficiallysimilarto the 2007: character 7).
pedicel(orpetiolus) ofaspider.Ingross morphology
this membrane is similar in form to an arthrodial We argue here thaton allthese criteria a sejugalfur-
membrane between adjacent limb articles and does rowcanreasonablybe scoredaspresentforSolifugae
notrevealanyembedded sclerites.Itforms adistinct too.VanderHammen(1989:249) cameclosestto
narrowing,withamaximumwidthaboutathirdofthe this by recognising (and naming) a ‘sejugal interval’
widthoftheadjacentcoxalpairs,andcanbefollowed in camel spiders,but idiosyncrasies in his workhave
as a dividingline up the lateral sides ofthe animal- limited the impact ofhis views. First, he frequently
where it merges smoothlyinto the dorsal membrane referred to the coxae as ‘epimera’, as part ofa novel
dividing the propeltidium from the mesopeltidium. hypothesisaboutcoxaloriginsandevolution.Theuse
Significantly, physical manipulation ofthe prosoma ofthetermepimera-andhisgeneralhabitofdescrib-
revealsthatnoneoftheothercoxalpairscanbeteased ing all arachnids using mite terminology - tended
apart in thiswayto the same extent. In otherwords, to marginalise his work. Second, van der Hammen
thecoxaeofthepedipalps,pluslegs 1 and2,essentially rejected cladistics, and his (sometimes detailed and
form an anterior functional unit. The coxae oflegs accurate) observations have been largely overlooked
3 and 4 form a corresponding posterior functional by later authors scoring characters for phylogenetic
We
unit. interpret this as clear ventral evidence of analyses.
tagmosis;wherebythesoft,membranoussuture (Fig.
1: mb) defines an anterior body region bearing the Poecilophysidea
chelicerae, pedipalps and first two pairs ofwalking Thepresenceofwhatweinterpretasasejugalfurrow
legs: the samebodyregion thatis dorsallyassociated in camel spiders further emphasises their morpho-
with the propeltidium. logicalsimilaritytocertainmites(Figs.2-5) (seealso
Dunlop1999,2000).Specifically,thesejugalfurrow
Discussion is anotherpotential synapomorphyforarelationship
Hereweconfirmandillustratepreviousobservations ofthe form (Solifugae +Acariformes).Mostauthors
about the flexibility of the camel spider body be- have recovered camel spiders as the sister group of
tweenthe second and thirdpairofwalkinglegs.The pseudoscorpiones (WEYGOLDT &c PAULUS 1979,
body region defined dorsallyby the propeltidium in VANDERHammen1989,Shultz1990,2007).Basal
Solifugaeisalsodelimitedventrallybyamembranous (i.e. chthoniid) pseudoscorpions do indeed resemble
region (Fig. 1), which essentially continues laterally camel spiders quite closelyand this traditional Hap-
and forms a flexible ring around the animal more or locnemata clade (BÖRNER 1904) is supported by a
less in the middle ofits prosoma.This membrane is, rangeofcharacterssuchaslegswithaveryshortfemur
incidentally, also the place where a pair ofspiracles and a correspondingly long patella, two-segmented
opensonthelateralsidesofthebody.Insearchingfor andchelatechelicerae,andtrachealspiraclesopening
comparabletagmosisfeaturesamongotherarachnids on the and 4^^ opisthosomal segments.
the most obvious candidate is the sejugal furrow of Nevertheless, there is also evidence linking
acariform mites; a characterwhichwe reiterate does mites and camel spiders; a hypothesis with histori-
not occur in the parasitiform lineage. Precise defini- cal precedent (BANKS 1915). Mites, solifuges (and
tions ofthis character in the literature vary slightly, also pseudoscorpions) have a mouth on a projecting
but to quote some recent authors the sejugal (or ‘beak’, or rostrum in some terminologies, and also
Sejugalfurrowincamelspiders 33
Galeodidae Solipugidae Daesidae
Rhagodidae Hexisopodidae Ammotrechidae
Figs.5-10:Ventral prosomal region in sixofthetwelvecurrentlyrecognised camel spiderfamilies.Noteagain in all
casesthe principal division betweenthesecondandthird pairofcoxae(arrowed);in largerspecimensa pedicel-like
membranous region hereisclearlyevident.5-GaleodesarmeniacusBirula,1929(Galeodidae:ZMB ]7972).6-Zeriakeyser-
lingi(Pocock,1895) (Solpugidae:ZMB 15646).7-Biton{Biton)kolbei(Purcell,1899) (Daesiidae:ZMB 15517).8-Rhagodoca
termes(Karsch,1885) (Rhagodidae:ZMB 15642).9-ChelypusbarberiPurcell,1902 (Hexisopodidae:ZMB48436).10-Pseudo-
deobisandinus(Pocock,1899) (Ammotrechidae:ZMB 15634).Nottoscale.
havecheliceraeinwhichthemovabledigitarticulates namelysimple,aflagellatespermandalargeglandular
ventrallyrelative to the fixed digit (BERNARD 1896, area ofthe testis producing secretions. The present
Dunlop 2000).Two characters ofthe reproductive tagmosis character of a propeltidium/proterosoma/
system have been elucidated exclusively for Solifu- propodosoma/aspidosoma/sejugal furrow can now
gae and Acariformes (cf AlberTI 1980a, b, 2000, potentiallybe added to this list; althoughwe should
&
Alberti Peretti 2002, Klann et ah 2009): caution againstthe riskofcharacterduplication. For
34 JA.Dunlop,J.Krüger&G.Alberti
exampleSHULTZ(2007)scoredthe‘dividedcarapace’ sejugalfurrow(in acariform mites) as derivedcondi-
and the sejugal furrow as two separate characters. tions; justifying polarity by using Limulus (Xipho-
However, but it maybe better to treat them as parts sura) -with its large, unitaryprosomal dorsal shield
ofa single charactercomplex relatingto tagmosis. andlackofventralsegmentaldifferentiation-as the
In addition to this morphological data, recent outgroup.
molecular (DABERTet al. 2010) and combined (Pe- Furtherdownthe euarthropodtreeweencounter
PATOetal.2010) studieshavealsopickedupastrong alternative outgroups such as sea spiders (Pycnogo-
molecularsignalfor(Solifugae+Acariformes).Itwill nida) inwhich the fundamentaltagmosis isbetween
beinterestingtoseewhetherfurtherinvestigationsof a so-called cephalosoma, bearing four pairs of ap-
this nature continue to support these results. Pepato pendages (ViLPOUX &WALOSZEK 2003: Fig. 13),
et al. (2010) evenwent so far as to recognise a clade and the successive separate segments of the trunk.
Poecilophysideaforcamelspidersandacariformmites This cephalosomais segmentallyhomologous to the
-andaclade Cephalosomataforpoecilophysidsplus anteriortagmaofcamelspiders,acariform mites and
palpigrades. The latter group potentially share the palpigrades (DUNLOP & ArANGO 2005: Fig. 5).
characterofa‘cephalosoma; adiscrete anteriorbody Adopt sea spiders as the outgroup and the ‘divided
region (see above) covered by the propeltidium and carapace’ / sejugal furrow could be interpreted as a
bearing the firstfourpairs ofappendages. plesiomorphic state; retained from an earlier grade
Inthiscontext,weshouldbrieflyconsiderwhether oforganisation.This is essentiallythe argument put
asejugalfurrow/intervaloccursintheotherarachnids forwards byBERNARD (1886) and KÄSTNER (1932,
with a divided peltidium. VAN DER HAMMEN’s 1952) who thought that the divided camel spider
(1989) account ofpalpigrade morphology does not prosoma revealed the original arachnid morphology.
explicitlymentionsuchafurrowbetweenlegcoxae2 Authors such as RemANE (1962: 214) have argued
and3,andthischaracterisprobablyhardtotesthere thatthearachnidprosomafundamentallyconsistsofa
sincethehighlyflexiblebodyofthese animalsisonly four-segmentedheadregion-bearingthechelicerae,
weakly sclerotised. In palpigrades the coxae of the palps and legs 1 and 2 - plus two additional seg-
pedipalpsandfirstwalkinglimbsareassociatedwitha ments bearinglegpairs 3 and 4 respectively. Further
sclerite,andeachofthesuccessivepairsoflimbcoxae discussioncanbefoundin KRAUS (1976),whoagain
areassociatedwithacorrespondingseparateplate(see favoured the idea that separate prosomal elements
e.g.Börner1904:%.4).OrtoquoteROWLAND&c reflect a ‘4+2’ arachnid groundplan, orWEYGOLDT
SiSSOM(1980:76),“Followingthedeutotritosternum 8cPaulus (1979)whopreferredinsteadtointerpret
and lyingbetween the second, third, and fourth pair these divisions as derived and homoplastic features,
ofwalking legs are the tetrasternum, pentasternum, possiblyadapted forincreasingprosomal mobility.
and metasternum, respectively.”Thus in palpigrades Finally,weshouldmentionaseriesofearlyPalaeo-
leg coxae 2, 3 and 4 are all to a certain extent ‘free’. zoic arthropods expressing raptorial anterior limbs
Forschizomids,thereisagainnomentionofaspecific - the ‘great appendage’ arthropods, or Megacheira
furrowbetween legs 2 and 3 in VAN DER HAMMEN in some schemes -which some authors interpret as
(1989). The classic and detailed study of BÖRNER stem-group Chelicerata (CHEN et al. 2004). These
(1904: fig. 2) is likewise circumspect about a specific fossilsalsoappeartopreserveananteriorbodytagma
zone offlexibilityhere. bearingfourpairs ofappendageswhich authors such
as Waloszek and co-workers have termed the ‘euar-
A
cephalosomaoradividedcarapace? thropod head’ (see also ReMANE‘s 1962 hypothesis)
Butis‘Cephalosomata’acladeoragrade?Wesuggest andwhichtheyinterpretasafundamentalpartofthe
thatbothacariformmites andcamelspiders share an body plan in early arthropods (cf Chen 2009: Fig.
anteriortagmabearingfourpairsofappendageswhich 11).Usingmegacheirans as anoutgroupwouldagain
is essentially separated from the rest ofthe body by polarise the tagmosis pattern ofmites,camel spiders
a membranouszone forwhichthe mite term‘sejugal (and palpigrades?) as a plesiomorphic, groundplan,
furrow’ is available and appropriate.WeYGOLDT & character state for arachnids. In this scenario, a uni-
Paulus (1979) and Shultz (2007: characters 6-7) taryprosomaldorsalshield(orpeltidium) emergesas
interpretedbothadividedcarapace(incamelspiders, a derived character state; perhaps even homoplastic
palpigrades and schizomids) and the presence of a across Arachnida.
X
Sejugolfurrowincamelspiders 35
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