Divergent transformation of chelicerae and original arrangement of eyes in spiders (Arachnida, Araneae)

6 Pages·1993·1.3 MB·

by  O Kraus

DIVERGENTTRANSFORMATION OFCHELICERAEAND
ORIGINALARRANGEMENTOFEYES IN SPIDERS (ARACHNIDA, ARANEAE)
OTTOKRAUS ANDMARGARETEKRAUS
Kraus, O. and Kraus, M. 1993 11 II: Divergent transformation ofchelicerae and original
arrangementofeyesinspiders(Arachnida, Araneae).MemoirsoftheQueenslandMuseum
33(2): 579-584. Brisbane. ISSN0079-8835.
In various higher taxa of the Araneae (e.g., Mesothelae, Migidae, Hypochilidae), the
chelicerae and their fangs show an intermediate position between those commonly called
orthognathyandlabidognathy.Thisstageisconsideredtoformpartofthegroundpatternof
spiders; accordingly, itiscalledplagiognathy (newterm). Itisconcludedthatplagiognathy
gaverisetoorthognathyandlabidognathyasdivergentadaptational developments. Inmost
instances,plagiognathy iscorrelatedwiththemaintenanceoftheoriginal(plesiomorphous)
arrangementofthelateraleyes(=ALE+PLE+PME)intriadsorsemi-triads.Theprevious
assumption that orthognathy andthearrangementofeighteyesin two subparallel rows are
charactersthatwerealreadypresentinancestralspiders isrefuted.
Bei verschiedenen hoheren Taxa der Araneae (z.B. Mesothelae, Migidae, Hypochilidae)
weisendieChelicerensowiederenKlaueneineintermediarePositionzwischenOrthognathic
undLabidognathieimublichenSinneauf.DieseAnordnungwirdalsTeildesGrundmusters
derEchten Spinnenangesehen undhierfiirdieneueBezeichnungPlagiognathieeingefiihrt.
Vondiesem primarenplagiognathen Zustandwerden sowohldieOrthognathicals auchdie
Labidognathie als divergente Entwicklungen mit unterschiedlichem Anpassungswert ab-
geleitet.IndenmeistenFallenistPlagiognathiekorreliertmitdemErhaltderurspriinglichen
(plesiomorphen) AnordnungderSeitenaugen (VSA + HSA + HMA) in Form von Triaden
oder Semi-Triaden. Die bisherige Annahme ist nicht langer aufrecht zu erhalten, wonach
Orthognathic und die Anordnung von 8 Augen in zwei Querreihen als Komponenten des
Grundmusters der Spinnen angesehen worden waren. \Z\Araneae, plagiognathy, orthog-
nathy, labidognathy, lateraleyes, triads.
Otto Kraus, Margarete Kraus, Abteilungfiir Phylogenetische Systematik, Zoologisches
Institut und Zoologisches Museum, Universitat Hamburg, Martin-Luther-King-Platz 3,
D-2000Hamburg 13, Germany; 12November, 1992.
It is generally believed that the chelicerae in thognathyhadbeen maintainedin aconsiderable
spiders can bearranged in eitheroftwodifferent numberofhighertaxa.
ways, described by the terms orthognathy and Simon (1892: 64, 82) pointed out that the
labidognathy. Orthognathyiscommonlythought LiphistiidaeandMigidaehadarrangementsofthe
to represent the primitive (plesiomorphic) char- chelicerae that did not fit very well into the
acterstage(Foelix, 1982:3;PlatnickandGertsch, generally accepted orthognathy/labidognathy
1976: 13). At first glance, this view seems to be scheme. Laterauthors ignored such deviations',
supportedbythefactthatastrictlyorthognathous however, andcontinued tobasethedistinctionof
arrangement ofthese mouthparts is also present two major subtaxa of spiders-Mygalomorphae
in the outgroup of the Araneae, i.e., in the (=Orthognatha) and Araneomorphae (=Labido-
Amblypygi. Hence, the ideathatorthognathy isa gnatha) on different positions of the chelicerae.
plesiomorphic feature seems to be the most par- Kaestneralone remarked on the intermediate ar-
simonious explanation. Accordingly, labidog- rangementofthesemouthparts inActinopodidae
nathy is regarded as a derived (apomorphic) and in Hypochilus, but apparently he too con-
feature. Kaestner(e.g., 1952, 1953a,b)presented tinued to adhere to the typological ortho-
arguments supporting the assumption that gnathy/labidognathyconcept.Onemainaspectof
labidognathous, i.e., cooperating chelicerae had his study was therefore toclassify the chelicerae
various functional advantages. He produced a in Hypochilus as orthognathous or labido-
model (Fig. 1)illustratingthetransformationofa gnathous.
'primitive' orthognathous arrangement into the In this paper, we will present relevant facts,
labidognathous position. However, it is difficult most of them already known for decades, and
to imagine how this could have happened discuss conclusions allowed by alternative con-
gradually, and Kaestner did not explain why or- cepts.
MO MhMOlXS OFTHEQUEENSLANDMI
IS
FIG. 1. Transformation ofchelicerae as supposed by
Kaesrner.A,orthognathy,lefteheliceraomitted,Front
of prosoma nearly vertical. B, labidognathy. dotted
linesandarrowsindicatehowfrontofprosoma(with
chelicerae)shiftedfromoriginalverticaltoahorizon-
tal position (rotation ofbasal segments ofchelicerae
not indicated). C, suggested economy of relatively
small cooperating labidognalhous chelicerae com-
pared with a single orthognathous chclicera
(hatched); both seize objects of same size.-iProm
Kaestner, 1953b).
This approach leads directly into a critical
discussion ofanothergenerallyaccepted dogma
inarachnology—thattheeighteyespresentinthe
ground pattern in spiders were originally ar-
ranged in two more or less parallel rows. In the
nearest outgroups (Amblypygi, Uropygi). how-
ever, the arrangementoftheseeight eyes is quite
different:thelateraleyesformtriadsonbothsides
ofthe prosoma- Such triads alsooccur in certain FIG. 2. Position of chelicerae and fangs, lateral and
spiders. We therefore also plan to adopt a some- ventral views. A-B, Mesothelae: Liphisttus sp. C-D,
what unconventional approach, discussing the Atypidae:Atypusaffiriis. E, Migidae:Migasquintus.
questionas towhetherthepresenceofsuchtriads F, Aclinopodidae: Missulena accuiona. G,
Hypochilidac: Hvporhiiusgerlscht H, H. ihorellj-
in various subtaxa of the Araneae could be a (A-B from Millot, 1949; CD, F, H from Kaestner,
persisting plestomorphic characterexpression 1952; EfromWilton, 1968).
RESULTSANDINTERPRETATIONS
mentsintheMigidae,referringto 'chelicerestxes
courtes,convexesalabase,maisensuitebrusque-
Chelicerae ment inclinees, presque verticalement ...* (see
FACTS. Fig. 2e). Kaestner (1952: 118) studied Sason
Orthognathy is commonly regarded as robustum (O. P.-Cambridget 1883) as a repre-
plesiomorphic. However, precisely thosespiders sentative ofthe Barychelidae and characterized
that have the greatest number of plcsiomorphies thecheliceraeasshortandsubverticallyinclined
in common (Platnick and Gcrtsch, 1976) do not In the same paper, Kaestner demonstrated that
show an orthognathous position of their obliquely arranged chelicerae were also present
chelicerae: in the Liphistiidae (Figs 2a-b) the in the Actinopodidae (Fig. 2f); he described the
basal segment (paturon) of these mouthparts is situation in Missulena occatoria (Walckenaer,
relatively short, inflated and obliquely posi- 1805)andconcluded: 'Icannotseeanybiological
tioned. Further, the longer axis ofthis basal seg- reasonforsuchconditions. Butas torsions ofthis
mentisorientated obliquelydownwards, and not kind play an important role in the origin of
horizontally and paraxially as in 'true* Orthog- labidognathy, it is interestingtosec thatthey [the
natha (Figs 2c-d). The corresponding position of torsions] may also occur in the Orthognathia'
the fangs is also oblique, and anything but (transl. from German).
paraxial, in contrasttothe positionofthefangsin It is worth mentioning that thecheliceraeeven
Atypus, forexample (seeSimon, 1892: 64). oftheoldestknownspider,Aiterocopusfimbriim-
ThesamesituationasinLiphistius'isalsofound guts (Shear. Selden and Rolfe. 1987) (Middle-
jn various subtaxa of the Mygalomorphae. In Devonian), had short basal segments and also
1892, Simon (. 82) described similar arrange- fangs (Selden eta/., 1991, e.g., plate 1. figs6-8).
TRANSFORMATION OFSPIDERCHELICERAE 581
of the chelicerae. Accordingly, the plagio-
gnathouspositionpresentinthegroundpattern
- t
the Araneaehasbeen secondarily transformed in
two different directions, both apomorphic char-
acterstates: orthognathy and labidognathy (Fig.
Wc
3), see various arguments in support of
hypothesis:
a) It explains why orthognathy is not en-
countered in the Mesothelae [Liphisiiu^ Hep-
mtht>Li\.
bt The absence of orthognathy in repre-
sentatives of several mygalomorph families is
explained.
FIG. 3. Orthognathy (left) and labidognathy (right)ets Ib-j fact that the Hypochilidac arc not
BpOsnorphk ctiarft s derived from plagiog- labidognathous is explained by the simple as-
nadiy (bottom). sumptionthattheoriginal plagiognathy has been
maintained in this group of the Araneomorphae.
Unfortunately, their original position is un- Nonetheless, in all other Araneomorphae (this
known.
means in the Neocribellatae, the sister taxon to
Chelicerae with an oblique position are also theHypochilidae) labidognathyhasbeenachicv
found in a taxon that unquestionably belongs to edandisregardedasanapomorphyofthistaxon.
the Araneomorphae (= Labidognatha auct.y. the Thisconclusion is notinvalidatedby thefactthat
Hypochilidac(Figs2g-h>. Again,itwas Kaestner superficially orthognathous arrangements orig-
(1952: 132) who studied details. He concluded inated secondarily in a few sexually dimorphic
dial the mouthparts in Hypochilus were of the araneomorph ta.xa (e.g., in males of the salticid
orthognathoustypeinconstructionandexpressed genusMyrmarachne).
the view (Kaestner, 1952: 114)that 'themajority d)Kaestnerstypologicalandentirelytheoreti-
ofimportant characters present in Hypochilus is cal model suggesting how a supposed transition
111 accordance with the Orthognathia, whereasthe from orthognathy to labidognathy could come
insuvmebreyrloofwf.eaFtourretshipsrerseeanstoni,n LIambuisdtogrneamtohvaeontlhye a3)b.ouTthi(sFipgo.st1u)liastresepdliavceerdgebnytaannedwgrcaodnucaelptev(oFliug-.
genus fromthe suborderLabidognathaandeither tionary change of the ground pattern, that is to
place it in the Orthognathy or set it up in a say, ofplagiognathy.
suborderofits own1 (transl. from German). e) Kaestner's complicated assumption that
INTERPRETATION obliquelyarranged cheliceraeoriginated in pQfg
Kaestnermaintained that labidognathy was an lie! bothin theMygalomorphac and the Araneo-
.ukanced character state, which had developed morphaeis replaced by asimple, comprehensive
fromanorthognathousgroundpatternby gradual hypothesis
transformation (Kaestner, 1953a: 60; Fig. 1). He The only remaining conflict seems to be that
felt that Hypochilus (and the Hypochilidae) reflected inthe strictly orthognathouspositionof
should be regarded as transitory stages and ex- the chelicerae in the most closely related out-
plained the oblique position also present in the groups ofthe Araneae (Amblypygi, Uropygi). If
Baryehelidae and Actinopodidae as a parallel our 'plagiognathy hypothesis" is correct, it must
development. Furthermore, he regarded the be assumed that orthognathy in the Araneae is a
*semi-orthognathous' chelicerae in Dysdera different and thus independent secondary
(Dysdendae) as intermediate. Kaestner thought, development within the mygalomorph spiders
then, that various transitory stages still existed There is no question but that this contradiction
forming a "phylogenctic link' between the two needs some furtherexamination.
extreme character states. Preliminary investigations have already sug-
We reject thisjudgement based on typology. gested that orthognathy in Amblypygi may be
and postulate that an oblique position of the different from orthognathy in spiders: the basal
chelicerae, including the fangs, really represents segment in amblypygid chelicerae has a long.
the plcsiomorphic situation (Fig. 3). As a new stoutapoderneatitsproximaldorsolateralborder,
term is needed, we would like to suggest which reaches deeply into the broad. Hal
'plagiognathy' todesignatethisoriginal position prosoma. This peculiarity is lacking in plagio-
5S2 MEMOIRSOFTHEQUEENSLANDMUSEUM
gnaihousand alsoin orthognathouscheliceraeof
spiders. Weexpectthat more detailed studies on -
the functional morphology, including the mus- "
culature, will demonstrate that orthognathy in
uropygids and amblypygids differs from ortho-
i
gnathy in spiders. This would support our view n '
andcouldperhapsconstitutepointf) inthelist of
positiveargumentsabove. 1
• i
O
Eves I C
Surprisingly, plagiognathous spiders (for ex-
ampleMesotheiae,Migidae,Hypochilidac)share
a special arrangement oftheeyes (Fig> 4g, e,c):
anterior lateral, posterior lateral and posterior '
median eyes are grouped closely together. This E
promptsthefollowingremarksonthequestionas
to how the eyes were grouped in the ground FIG. 4. Position of median (black) and lateral eyes in
patternofthe Araneae. Trigonotarbida. Amblypygi and Araneae. A,
As designations widely used in taxonomie
descriptions (AME, ALE, PME, PLE) disregard nf prosoma (from Shear et at., 1987). - B,
the origin of these 'ocelli', some notes on the Amblypygi:Damonsp.C, HypochiJidae:Hyptw:/ii/H.y
homolgy of the eyes ot spiders may be ap- gerttchi. D, Atypidac: Atypus affinis. E. Migidae.
propriatetoensurethatweunderstandeachothei PMcoMeicIihleolmagca:ssLipp.hiFs,fPihuoslcspi.daHe,;PDhyosldecruisdcaier;cutDayrsiisf.aCnJi,
the anterior median eyes (AME) will be called Sp I. Aeelenidae:Agelena sp. (Notto scale).
'medianeyes' byus,astheyarehomologouswith
themedianeyesofotherarthropods(forexample Araneae*Amblypygi+Uropygi+Schiz-omida).
those in Xiphosura, 4ocelU' in insects, and the Devonian trigonotarbids had the usual two
three components of crustacean naupHus eyes). median eyes, and the lateral eyes were repre-
AH other eyes, three on each side, will becalled sentedby upto9 (12 ?)lenses(Fig. 4a).Threeof
'lateral eyes1 (ALE + PLE + PME), as they are these were major lenses, while the others were
homologous with the paired original compound minorlenses arranged in the interspace between
eyes in arthropods, forexample, inxiphosurar.v the major ones (Shear et fll, 1987). This kind of
PACTS transformation of the original compound eyes
It is commonly believed that an arrangement clearlyindicates that atriad ofmajorlateral eyes
in twotransverserowsofeighteyesisplesmmor- is a feature of the ground pattern of the pul-
phir Only two weak aspects support this s monaies as a whole; accordingly, the loss ofthe
however: (a)thereis noreasonatalltodoubtthat minor lateral eyes could be regarded as an aut-
the presence of eight eyes forms part of the apomorphy of all other pulmonates, including
araneidgroundpattern,and(b)theirarrangement spiders. This secondary reduction of the minor
in two rows is widely observed both in the lateral eyes may explain why most triads arenot
Mygalomorphae (for example the Actinopod- perfectly closed, not even in amblypygids (Fig.
idae; see Simon. 1892: 79, figs 81-83) and in the 4b).
Araneomorpha (forexample Araneidae. Euspar- The peculiarity 'lateral eyes in triads' is com-
assidac,Thomisidac). monlyused asacharacterin spideridentification
On the other hand, lateral eyes more or less keys,butasfaraswecan tell,itspotentialbearing
distinctly grouped in triads occur in various on phylogeny has neverbeendiscussed. Couldit
groups of spiders. Mesotheiae, Vligidae and bethat triadsoflateraleyesarepartoftheground
Hypochilidac have already been mentioned. Al- pattern in the Araneae'7
most perfect triads occur in Pholcidae (Fig 4T). A surveyofhowthe lateral eyes arepositioned
The same is true of Amblypygi (Fig. 4b) and in representativesofhighertaxaofspidersshows
Uropygi, thedirect outgroups to spiders! that almost perfectly 'closed' triads (as in phol-
The arrangement of the eyes in the extinct cids) are rare. In most instances, the three lateral
Trigonotarbida deserves special attention. Ac- eyes on each side are somewhat dissociated. In
cordingtoSeldenetal. (1991:254),theyformthe addition to the Mesotheiae and Migidae already
sister group of all other pulmonale taxa (= mentioned, we should also like to draw attention
TRANSFORMATION OFSPIDER CHELICERAE 583
to the Atypidae (Fig. 4d) and to the illustrations (e.g., 1894: 517), thatis tosay, theoccurrenceof
in Raven's comprehensive study ofthe Mygalo- diadscanbeexplainedaspartoftheoriginaltriad.
morphae (1985). In many cases the posteriorlat- To some extent, thequestion remains open, as
eral and the posterior median eyes are closely to how it is possible to distinguish between eye
connected,withsomedistancebetweenthemand positions that can be regarded as more or less
the anterior laterals. Hypochilus shows slightly modified triads and otherpositions, with secon-
dissociatedtriads(Fig.4c).TheDysderidae(Fig. darily approximated ALEandPLE.
4e) and Oonopidae are six-eyed spiders, having
the median eyes completely reduced. In dys- PERSPECTIVES
derids, the lateral eyes are closely grouped
together, resembling the arrangement of the Apparently, plagiognathy ispartoftheground
lateralsintheMesothelae. Inmanygroupswithin pattern of the Araneae. Developments in the
the Araneomorphae, diads are present instead of directions of orthognathy and labidognathy can
triads. They are formedby the ALE + PLE, with easily be explained as divergent evolutionary
PME
the separated. This arrangement can be changes (Fig. 3).Thequestion thereforearisesof
found in Austrochilidae and especially in most how these might be correlated with functional
Theridiidaeand Linyphiidae,forexample. Diads aspects. As an impetus forfurtherdiscussion, we
alsooccuringroupscharacterizedbyasecondary propose the following working hypotheses:
loss ofthe PME, such as Scytodidae. a)IntheMygalomorphae,orthognathy maybe
INTERPRETATION correlatedwiththecaptureofpreyontheground.
Theassumptionthateighteyesarranged intwo Under such conditions, the two parallel fangs of
transverse rows were already present in the the chelicerae can easily penetrate the victim on
groundpatternoftheAraneaeisnotsupportedby a substrate like two stabs of a dagger. It seems
anyconcretefact;norwouldthisatallcorrespond remarkablethatasemi-orthognathouspositionof
with the situation in the nearest outgroups. It the chelicerae has originated secondarily in the
would mean that triads and triad-like arrange- Dysderidae: theykill woodlice on the substrate.1
ments ofthe lateral eyes in spiders were classifi- b)In theAraneomorphae, theoriginoflabido-
able as parallel developments (homoplasies). gnathy may be correlated with the evolution of
This is unlikely. In accordance with the position capturewebs(sheet,frame, orbwebsetc.).These
of the eyes in the Amblypygi and Uropygi, we could make itmoreefficienttobite theprey with
expectthatthelateralswereprimarilygroupedas two opposing chelicerae or fangs, whereas
triads (ALE + PLE + PME). This hypothesis is plagiognathous and, even more, orthognathous
supportedby five arguments; chelicerae might not penetrate but rather push
a)Triads ofmajorlateral eyes (lenses) already away the victim: there is no longer any substrate
existedinDevonianTrigonotarbida;hence,triads 'supporting' prey animals.
apparently form part ofthe ground pattern ofall c)Plagiognathyand the maintenanceoflateral
pulmonates among arachnids. triadsorsemi-triadsofeyes apparently formpart
b) The postulated configuration is in good ofthegroundpatternofspiders;thesefeaturesare
agreementwiththearrangementoftheeyesinthe confined to more 'primitive' groups. The
directoutgroups. presence and the various types oftransformation
c) Triads and semi-triads present in various ofthese twocharacters should be integrated into
groups of the Mygalomorphae and also of the currentconceptsonthephylogenyoftheAraneae
Araneomorphae must no longerbe explained by (see, for example, Raven, 1985; Coddington,
assumed parallel origin. 1990). At present, our view of features of an
d) Various types ofsomewhat dissociated lat- araneidgroundpatternandsucceedingevolution-
eraleyescanbeexplainedbyasecondarysepara- ary changes seems to be somewhat at odds with
tion of the ALE or ofthe PME from the others, various published cladograms; they hence could
which frequently remain in contact with each bepartially wrong. Wefeel thatthisconflict may
other. be due to the possibility that characters assumed
e) Simon's 'oculi laterales utrinque contigui* tobesynapomorphiesinvariouscladograms(see,
1 But see Kaestner (1953a: 62). He believed that theposition ofthechelicerae in Dysderawas a 'phylogenetic
link'betweenorthognathyandlabidognathy.Unfortunately,hewasnotawarethatthefirstpostembryonicstages
were nearly labidognathous, with relatively shorter basal segments and only slightly oblique fangs (pers.
observ.). In Dysdera,the final semi-orthognathousposition wasgraduallyacquired in laterinstars.
S84 MEMOIRSOFTHEQUEENSLANDMUSEUM
e.g., Platnick and Shadab, 1976, fig. 1; Raven, der Lahidognathie. Zoologische Jahrbiicher
1985, fig. 1) may well turn out to be symples- (Anatomie)730): 47-68.
iomorphies; e.g., Raven's characters 35 (eyes 1953b, Die Mundwerkzeuge der Spinnen, ihr Bau,
spread widely across the prosoma; same as Plat- ihreFunktionundihreBedeutungftirdasSystem.
nick and Shadab*s character 1) and 36 (male 3p.neTuemilo:neDsi)e.ChMeiltitceeirleunngedneraLuasbiddeomgnaZtoohlaog(iDsi--
pedipalps: conductorofbulbpresent; see Kraus, chen Museum inBerlin 29(1): 3-54.
1978,figs 12, 14-16). KRAUS,0. 1978.Liphistiusandtheevolutionofspider
genitalia. Symposiaofthe Zoological Society of
ACKNOWLEDGEMENTS London42: 235-254.
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589-743.InGrasse,P.P.(ed.),TraitedeZoologie.
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Tubingen, and Prof. Dr. H.W. Levi, Cambridge, Paris).
Massachussets, for critical advice. We express PLATNICK,N.I.&GERTSCH,W.J. 1976.Thesubor-
ourthankstoDr, W.A. Shear, Hampden-Sydney, ders of spiders: A cladislic analysis. American
Twrhiogondortaawrboiudra aatntdenitnioonthetor ftohsesisliatruaacthionnidsi.n Dtrh.e PLATNMIuCsKe,umN.NIo.v&itaStHesAD26A0B7:,1M-.15U.. 1976. Arevision
of the mygalomorph spider genus Neocteniza
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