Criteria for identifying thermal behaviour in spiders: a low technology approach

8 Pages·1993·1.4 MB·

by  W F Humphreys

CRITERIA FOR IDENTIFYINGTHERMALBEHAVIOUR IN SHDliKS
ALOWTECHNOLOGY APPROACH
W.F HUMPHREYS
Humphreys, W.F 1993 1 ! IJ: Criteriaforidentifying thermal behaviourin spiders: a low
technologyapproach.MemoirsoftheQueenslandMuseum33(2): 543-550. Brisbane- ISSN
0079-8835.
The widespread occurrence ofthermal behaviourin diumally active web spiders is either
largely ignored or not recognised. Thus it obfuscates someexplanationsofthe function of
the stabilunentum on spiders webs.Thermal behaviours offourspiders (Nephilaedulis,N.
macutata, Gasteraeantha minaxandNeogea sp.) are examined using technology which is
often inappropriate for field studies. Many thermal behaviours are recognised as well as
behaviours which facilitate thermal behaviour. The thermalcorrelatesofthese behaviours
arcestablished. Some observational criteria are derived, which require only simple equip-
ment, by which thermal behaviour* may be recognised in the field anddistinguished from
other behavioural patttms.[jAmncae, orb-weavers, thermal behaviour, thermoregulation,
stahilimentum.
W.F. Humphreys, Western Australian Museum, Francis Street, Perth, Western Australia
60(H), Australia. 26 October, 1992,
Many orb-webspidersremainactiveatthe web gressively the. heat loading, behaviours thai are
hmb during the day. where they can continue to themselves mostly graded (Humphreys, 1992).
feed, mate, and defend their web site from the These include stilting, drooping, orientation,
same and other species, as well as produce or front leg raising, abdomen pointing, posturing,
respond to attractant signals (acoustic, tactile, front leg rotation and web abandonment. As-
visual or chemical). The seasonal and diurnal sociatedbehavioursincludesilklayingandagita-
durationofthisactivitycanbeextendedbyadopt- tion (Humphreys. 1992 and unpublished), In
ing behaviours that warm the spider (Robinson addition, the useofadisx:stabilimentum as a sun
andRobinson, 1974, 197K; BiereandUelz, 1981) shade, suggested by Robinson and Robinson
when it would otherwise be too cold or else that (1973: 283), is an effectivethermal behaviour in
preventit from overhauling (l.tibinand Henschel, Neogeasp. (Humphreys, 1992), Suchbehaviours
1990; Humphreys. 1978, 1987a, 1991, 1992). not only maintain the animal within its heat
I consider below mainly behaviours that tolerance range, but also serve to maintain Ihe
prevent overheating. In essence, in hot weather body temperature (T*>) within a narrow range
the spiderpostures so as to align the long axis of (presumablysomeoptimumtemperature) forex-
its body with thesun's rays and in this position it tended periodsoftime (Humphreys, 1974, 1978,
tracks the sun's apparent movement during the 1991). As body temperature has wide implica-
day- Suchbehaviourisnotexclusive tospidersof tions in physiological, behavioural, ecological
opencountry norintropicalclimates butis found and evolutionary contexts (Willmer, 1991), it is
also in both temperate and tropical forest spiders important to recognise thermal behaviour in
(Biereand Uetz, 1981; Humphreys, 1991, 1992 spiders in order to allow different types of ex-
ami unpublished)- The standard interpretation of planations for theirbehaviours.
thisthermalbehaviourreliesonasimplephysical That the thermal behaviour of spiders is not
model;posturingminimisestheprojectedsurface beiftg rCCOgni$e<j or is not being reported in the
area (silhouette) exposed to the sun's radiation literaturecan bedrawn from workon stabilimcn-
and so reduces the heat load (Fig. 1), lowering ta on spiders' webs. While stabilimenta have
equilibrium body temperature (Willmer and many different forms, they aremostly thought lo
Unwin, 1981: butseeHumphreys, 1986)or, inan provide mechanical support (Robinson and
anti-predator hypothesis, minimising the sil- Robinson, 1973), to function as anti-predator
houetteagainstthehrightestbackground,elsethe devices (Eberhard, 1973, Edmunds and Ed-
body area most brightly illuminated. munds, 1986;Lubin, 1986), toattractprey(Ewer,
Recent observations have shed some light on 1972; Craig and Bernard, 1990) or to collect
this seemingly simple process and revealed a water (Olive, 1980; sec alsoEwer, 1972; Robin-
sequence of behaviours serving to reduce pro- son and Robinson, 1973: 283). Neogea sp. in
MEMOIRSOFTHEQUEENSLAND MUSEUM
Papua New Guinea uses a discstabilimcntumas sociobiological problems There needs to bo
a pArteo), which, together with a sequence of some observational criteria using only low tech-
other behaviours, each themselves graded, nology by which thermal behaviour may be
reducesitsheal loading(Humphreys, 1992).The recognised and distinguished from other be-
demonstrationthatstabilimentamay heused in a havioural patterns.
thermoregulatory rule raises questions concern- The examination of thermoregulation in orb-
ing many observations which have been inter- weaving spiders is problematical as most
preted as supporting the anti-predator role of methods used on vagrant spiders (thermal
stabihmenta. preferendum apparatus, thermocouple implanta-
Some observations and deductions have tion, lemperature transmitters) arc not ap-
resulted in an anii-prcdator function being propriate.Themostpromisingapparatusforsuch
ascribed to the stabilimenuim. However, in the studies is the use of remote infrared telemetry
absence ofother, information, these observations (Suier, 1981; Humphreys, 1991, 1992), although
are equally open to interpretation in terms of model spiders may be effectively used to deter-
thermoregulatory hypotheses (see Humphreys, mine Te (Riceherl and Tracy, 1975; Hensehel tt
1992). Forexample -1. Only spiders that remain oi. 1992).
at the hub of the web during the day produce Cancriteriabeestablished whichwouldenable
stiibilimenla (Eberhard, 1973). 2. Spiders with workers usingcheapandreadilyavailableequip-
stabilimentamayshuttlefromonesideoftheweb ment, to.establish that spiders ure behaving if] a
totheother(ibid.). 3.Thelegsassumean 'aligned manner consistent withthermoregulation and to
posture' by day but not by night (ibid.). 4. The identify the lhermal conditions underwhich they
amountofsilkusedisdirectlyrelatedtoopenness initiatesuchbehaviour?
ofthe habitat (Marson. 1947),
Eberhard (1973) found that Ulohorusdivtrsus METHODS
Marx usedmore silk initsstabilimentumon light
than ondarknights (see also4. above).Thetrend
for larger stabihrnentum at brighter Mies ObservationsonNephikiedulis(Labillardiere)
camouflaged those spiders most susceptible to were made both on Rottncst Island and in Perth.
attack. However, without pertinent behavioural on Gasteracuntlni minax Thorell in the south-
data,itisnotpossibletorefutethehypothesisthat west, of Western Australia, and on Nephita
theopensitesarc moreexposedtodirectsunlight nweuliitij(Fabrictus) inmangrove atPort Benoa,
and thus that the stabilimenta are used to protect Bali,Indonesia
the spider from ultra violet light (but sec Craig Spidertemperatures were recordedfrom 1100
andBernard, 1990)ortoreduce its heatload. -1500 hours using an infra-red thermometer
Posturing by spiders that remain by day at the described elsewhere (Humphreys, 1991, 1992).
web hub is rarely mentioned in the literatureon The temperature was recorded of undisturbed
stabilimenta and defense mecham r ex- spiders resting above and below the hub of the
ample, although Edmunds and Edmunds (1986: web. bo!h in the shade and in the sun. It was
83) found that species of Argute, Septula, recordedatintervalsandassoonaspossible(<3s)
U'ucau^e, Cyrtophora and the Gastcracanthinae after a change in behaviour. Spider behaviour
remain atthe hubofthe webduringthe day, they changed according to the incident light; this
made no mention of posturing. In the varied becausethesite wassometimes shaded by
Australasian region species in all these genera or by clouds. To induce more behavioural
readily posture and track the sun (Humphreys, sequences the spider was sometimes shaded ar-
1991, 1992; W.R Humphreys,unpublished). tificially and the direction and strength of the
Clearly, thermoregulation hypotheses need incidentsunlightadjustedusingamirror Aplane
more consideration in field studies uf diurnally mirror was used to alter the apparent position of
active spiders. More recently this has occurred the incidentradiation at about thesame intensity
(Henscbel elal., 1992; Humphreys. 1991. 1992; as the natural sunlight and aconcave mirror was
Lubin and Hensehel, 1990; Ward and Hcnschcl, used toalterthe apparentposition ofthe incident
1992) but such studies often require expensive radiation and at an intensity continuously vari-
equipment to examine the thermal behaviour. able from greater to less than the intensity ofthe
Such equipment may he inappropriate to a field direct sunlight. In the field, control of intensity
biologist primarily interested in observation and AftS cl tide in the wind owingtomovementofthe
manipulation to examine behavioural or web,andhencethespider;controloftheintensity
THERMALBEHAVIOUR IN SPIDERS 545
Behaviour Mean s.d. n 1Range f wnt tegs raised: legs 1 and II are raised offthe
Refuseinshade 32.6a 2.10 J2 129.0-35J webandaligned parallel to the incident radiaiwn;
Reposeinpartialsun 32.4ah !J0 19 [295-343 ihisOCCURSasagraded sequencewiththefirstpair
Reposeinsun ';! m;- 1.66 47 |3l6-3R;i beingrai*edbefore the second pair.
Orientation: the angle ofthe saggital plane ofihc
TRAeBpLoEse1.pMoesiatniotnemopenraRtoutrten{eTsbtDCIs)laonfdN,. Werefsaltiesrinn wsphiidleertihseroJtoantgedaxtioslioefptahraelbleoldytostthaeyssoilnarthaezipmuth
Australia. Tb ofspiders in Repose differs according ofthe web.
loenergy intensities oitheir location(shaded,partly Parrorientation: the spider is not in the Repose
shaded, sunlight ANOVA - Fs %j$ = 19.895, positionandhaspartlyorientateditssaggitalplane
I'<0.001). Common letters include means not differ- between the Repose position and the orientated
ingsignificantly (Fisher'sPLSDat«=0.05). position
ofradiantenergy isthereforerelativeand greater Posturing: change in the angle between the web
planeand theanterior-posterioraxisofthe spider.
or less than the natural incident radiation.
The following temperatures arc mentioned: T Repose'.position: spidersoccupythelowerorupper
=ofambientshadedair;Tb=ofthespiderTsbody;, surface of the hub with the prosoma pointing
which by defaultis the abdomen (Taj), other downwards; the anterior-posterior axis of the
the thorax (Tifa). The environmental temperature spideris parallel totheplaneofthe web.
(TV)whichis usedasashorthandfortheeffective Rotatefronrlegy. following front legs raising die
heat load on the spider taking account of all 1 are rotated ftirwimlv such that Ihey ar:-
energy gains and losses. Means are followed in Stretched out in from of the spider f\nd lie in rhr
parentheses by the standard deviation of the shadeofitsbodywhenitisfullypostured,thismay
meat), and sample si7.e occura>agradedsequencewith thefirstpairbeing
rotated before the second pan.
Definitions required for this discussion arc
p. en below (sec also Humphreys, 1991 1992). Silk laying:adjusts the webstuctrurc nearthe hub
Abandonweb:thespiderleavestheweb,of1tenafter apparently to aid leg placement the better lo pos
asequenceofveryagitatedmovements,andmoves tureand orientate toachieve the full Fabian posi-
to the shadeprovided by the objects to which the tion. This facilitates subsequent thermoregulatory
main anchor lines ol the web arc attached. behaviourbut is not itselfthcrrnorcgulatoiy
Abdomen pointing: ihe abdomen alone is orien- Stantoposture: whenthespiderchangestheangle
tated to the sun as a prelude to full posturing.This betweenitsanterior poslcnot axisandthe planeoJ
behaviour is strongly tcprcscnted in some spei the web such thai its long axi-- is parallel to (be
.'Humphreys 1992). incident radiation
Agttailonandbody Uft: the spiderappearsagitated SMiing:, describes the 'standing on tiptoe* be-
and circles its body around the web's hub and in haviour ofscorpions used toprevent overheating
tThheeprlaotcteerssisthseombeotdiymiessrasieseendoanwaitys forwonmatnhedwtehbe.y (siAmlielxaranbdeehravainodurEmwesrp,ide1r9s58()H;umhperhereiyts,des1c9r92).
are included here under the same behaviour. This
;dbnce
hodyliftisnotcomparabletostiltine(Humphreys,
1992).
Drooping: the spider hangs limply from the back Statementsotherwiseunsupportedarebasedon
legswithapparent lossofhydrostaticpressure;the my unpublished observations.
appearance is like that adopted by B spider imme- Repose Position and Thermoregulation
diately after moulting while the new cuticle is Spidersadopt therepose position ifTeis below
hardening. some critical level and they do so whether they
Fabian position (Humphreys. 1991): the spider arc in shade or sun, hence, direct sunlight alone
aligns its long axis parallel to the direction of doesnotcausespiderstoposture, However,IV in
incident sunlight with ihe prosoma facing away sun is higher than in shade (Table 1). For ex-
from the sun. This position may be achieved by ample, during cool weather (low T«) in the
orientation and/or posturing. When the incident and during hot weather in the shade all in-
sunlightisparalleltothewebplanethentheFabian dividualsarein theReposepavilionontheir v
position may be the same as the Repose position if not otherwise engaged in activities such as
(Humphreys, 1991). Continued adoption of the maring, web building etc
Fabianpositionresultsinthelongaxisofthespicier When Te is not sufficient to cause posturing in G.
kfng the sun during theday. rmnuA the proportion of spiders in the Repose
MEMOIRSOFTHEQUEENSLANDMUSEUM
position during daylight docs not differ between Suchactivitypattenshavebeenreportedformany
sunlit (36/38) and*shaded 132/3?) sites (\ 2i = heliothcrmal spiders (Humphreys, 1978, 1987a,
0.016.P^0.90). 1987b) and other taxa (e.g. reptiles: Heatwole,
The mean Tt> temperature of ft. edutis in the 1970).
Repose position wasdirectly related to the inten- S- edutis abandons the web at 44.8°C (±0-50, 3)
sity ofthe incident radiation such that spiders in and moves into shade. In Perth, Western Aust-
the sun were hotter than partly or fully shaded ralia,whentheshadedairtemperaturewasextreme
spiders (Tabic I). (46.2°C) many N. edidis failed to seek cooler
Spiderson non-horizontal websalmostinvariably places and fell dead from their shaded webs
reston the undersideofthewebwiththeprosnma through heat stress (G.A. Harold, pers. coonm,
pointing down. However, spiders resting in posi- 1991).
tionsotherthan the Reposepositionshouldnotbe LargespidersassumetheFabianpositionearlierin
taken as proofofthermoregulatory behaviourbe- the day Lhan do small spiders (e.g.,A?. vxacuiataY-
cause some species, such as Verrucosa and this is consistent with the thermoregulation
Cyclasereputedly adoptaheadupstance iFoelix, hypothesis because larger bodies have a higher
1982: 139), as does G. minax at night and Ar- tempeiature excess (Willmer and Unwin, 1981).
gyrodes amipodianusOP.Cambridge,generally. However the threshold temperatures for given
behaviourscouldbesizerelatedandlowerinsmall
Orientation and Posturing: Evidencefor lhan in large spiders. This is consistent with the
Thermoregulatory FUNCTION seeming generality that the tolerance zones of
Inhotweather, spidersorientorpostureon the animals are related to the temperatures ex-
web to attain the Fabian position and then track perienced. Forexample, very smallN~ edulis start
the sun's apparent movement. They do this ir- to posture at 36.0°C (±2.39. 4>. significantly
respective of web orientation. Heat and sunlight cooler, by an average of 4.4°C (Fsijrj =13.279.
are needed to obtain Lhese behaviours. On very p=0.002), than adult spiders undergoingthe same
hot days, spiders may leave the web altogether behaviour(40.4°C ±2.16.27).
and seek shade. Large spiders assume Fabian
postureearlier than small ones possibly because
Orientation andPosturing inResponseto
under given environmental conditions large
Manipulation
spiders reach higher body temperatures. How-
ever, small spiders may have lower threshold Experiments with redirected and intensified
sunlight can be conducted to influence the be-
Tb's.
In hotweather an individualspiderin the sun will haviour of spiders to assist in determining
use reorientation aiwVor posturing to align the whether the behaviour is thermoregulatory
anterior-posterior axis of its body parallel to the withouthavingtomeasurebodytemperature.The
direction of incident sunlight with the prasuma results arc consistent for several species includ-
facing away from the sun and thus achieve the ing Arachnum higghsi, N. edulis, N. tnacuiata.
Fabian position (Robinson anil Robinson, 1974, G. minaxand Neogeasp-
1978; Humphreys, 1991, 1992). The spider will in cool sunny weatherwhenthe spiders are in the
adjust this position during the day and track the Repose position, additional healing (by con-
apparcntmovementofthesun(Humphceys, 1991 ). centratingredirectedsunlightusingaconcavemir-
In hot weathei all individuals in the sun orientate ror), results in the spider orientating-and/or
in lhesame-directionirrespectiveuftheorientation posturingifnecessary-toassumetheFabianposi-
oftheir webs, namely they all assume the Fabian tion. The redirected sunlight does not alone alter
position by posturing andVor Teonentatton thebehaviourofthe spider (sunlight redirected at
Humphreys, 1991). natural intensity using a plane mirror). Hence,
Hl eat atone does notcause the thermal behaviour posturing is dependent on the intensity ofthe heat
because in hot weather tinder heavily overcast applied.
conditions spidersdonotassumethe Fabian posi- In hot weather a spider which has assumed the
tion. However, if intermittent direct sunlight Fabianpositionwill resumethe Reposepositionif
strikes the spider it assumes the Fabian position clouds obscure the sun or it is artificially shaded
intermittently eveniflowerthanthenaturalinsolationisreflected
On very hot days spiders may assume the Fabian ontoithy means ofa concave m»ror.
positioninthemorningandafternoonbutleavethe Spiders in the Fabian position in the sun will, if
web to seek shade during the middle of the day. artificially shaded, assume a new Fabian position
THF.RMALBEHAVIOUR IN SPlDbkS 547
HG. 1. Thermoregulatory postures adopted by golden web spider, M clavipes. Lines projecting from force
successivepositionsofsun(S1,S2,S3)indicatecorrespondingorientationsoflongaxisofspider'sbody.Lateral
views(a,c)andplanview(b)of webshowpostureassumedinresponseto ventral(throughtheweb)insolation;
h lateralinsolation; cdorsal insolation (redrawn fromRobinson and Robinson. 1974).
ifthedirectionofthesunisartificiallychangedby increasing temperature ofthe abdomen (Humph-
means ofa mirror. reys, 1992). When aspiderinthe Repose position
In hot weatherwhen thespidersarc in the Repose on the disc stabilimentum is heated by the sun it
position in the shade, redirection of unconcen- initially 'stilts*, as has been described for scor-
trated sunlight, by means of a plane mirror or a pions. The spider gradually raises its body away
concave mirror, results in the spider orientating from the disc surface until it has full downward
and/orposturingifnecessarytoassumetheFabian extension ofall legs and seems to be standing on
position. 'tiptoe', thus removing the body of the spider as
In hot weather, when the spiders are in a Fabian faraspossible from the disc'ssurface.
position, adding reflected sunlight from a plane On further heating, Neogea sp. progressively
mirrorresultsinthespiderorientatingwithrespect orientates its body and then gradually postures,
tothemeanangulardirectionofthetwosourcesof starting with the abdomen. It rotates the tipofthe
incident radiation (when angle <90°). A spider abdomen towards the incident radiation and this
posturing between two heat sources will turn to- minimises the projected surface area of the ab-
wardstheoneincreasedinintensityand vice versa. domen exposed to the sun. As the posturing
Hence, the Fabian position moves towards the develops the prosoma also is aligned with the
incidentradiationofthemirroriftheheatreflected abdomen so that the entire spider is orientated
from the mirroris increased and towards incident prosomafromthesunwithminimalsilhouettearea
radiation ofthe sun ifthe heat reflected from the exposed. Eventually the legs themselves are rot-
mirrorisdecreased. atedforwardsuntiltheyareparalleltothelongaxis
Ifredirectedlightcomesfromabovethehorizontal of the spider in which position they arc in the
plane of the web spiders rapidly change their shadow ofthe abdomen, as is the prosoma; thisis
Fabian position; if redirected light comes from the full Fabian Position from which the spider
below thehonzontal planeoftheweb, anaturally tracksthe sun (Humphreys, 1991, 1992).Bythese
impossible position, spiders appear confused and
meansthespiders potentially can obtain very fine
changepositionfrequenUyandsomespeciesnever
control oftheirsilhouette area and hence on their
achievethe Fabian position (e.g. N. maculoia).
temperature.
InN. edulismany behaviourswererecognisableas
Cascading Behaviours similar to those observed in other spiders (e.g.
Many thermally related behaviours that have orientation, posturing, agitation: Humphreys,
been categorised are themselves graded so that 1991, 1992), whereas others have not previously
they each develop progressively rather than beenreportedorrecognised(e.g.drooping). Some
switch from one state toanother behaviours recognised may be components ofthe
For example as Neogea sp. warms in the sun, il same behavioural sequence For example, Agita-
exhibitsaprogressionofdistinct behaviours,each tion, in which the spider circles around the bub.
ofwhich is graded and which are associated with involves the body being raised slightly from the
548 MEMOIRSOFTHEQUEENSLANDMUSEUM
web,abehavioursometimesseenonitsown. Both Behaviour —Mean —s.d. —n Range
behaviours are included here under the same Repose-seeTable 1
category.Thermallythereappearslittledifference Partorientation 33.9 0.57 6 33.1-34.7
between three categories recognised here as dif- Drooping 36.5 1.57 8 35.4-38.8
ferent behaviours (agitation and body lift, front Orientation 38.7a 2.30 9 34.2-40.3
legs raised, and startto posture). Work conducted Agitationandbodylift 38.7a 2.09 21 35.2-43.5
umnadyersempoarreatceotnthreorlmlaeldlycotnhdeisteiobneshaivnitohuerlsaobroraaltloorwy l:rontlepsraised 38.9a 2.84 M 35.0-43.3
Starttoposture 4D4b 2.16 27 34.8-43.6
theirpooling using more rigorouscriteria.
Silklaying 41 Ibc 1.34 6 39.3-42.6
ElevenbehaviouralcategoriesarerecognisedinN.
Rotatefrontlegs 42.4cd 0.99 10 41.2-44.3
edtdlis, ranging from Repose toweb abandonment Abandonweb 44.8d 0.50 3 44.3-45.3
whichoccurbetweenTbof33.9and44.8°C(Table
2). The spider temperature associated with many TABLE 2. Mean temperature (Tb °C) ofA/, edulison
ofthese behaviours is significantly different from Rottnest Island, Western Australia, associated with
others. Some of these behaviours reduce the different behaviours (ANOVA - Fs 8,93 = 15.233,
projectedbodysurfaceareaexposedtothesunand P<0.001). Common letters includemeans notdiffer-
thus, under the predictions ofthe physical model, ingsignificantly(Fisher's PLSD ata=0.05).
should result in lower equilibrium body tempera- orientation (Humphreys, 1992) makes no sense
tiunrge,, allleeglserabiesiingnge,qualleg(e.gr.otoraiteinotantioann,dposwtuerb- under alternative hypotheses but is entirely con-
sistent with, and predicted from, the ther-
abandonment). Otherbehavioursmaynotbether-
moregulationhypothesis.
moregulatory but are associated with the onsetof The body temperature ofa spideris acomplex
the next behaviourin thegraded series (e.g. agita- functionofmany intrinsicfactors(size,morphol-
tion and body lift) orfacilitate a subsequent stage
(e.g. silklayingtoenablecorrectlegplacementfor ogy, attitude, physiology, reflectance, etc. ) as
well as factors extrinsic to the individual (e.g.
full posturing).
wind speed, turbulence,airtemperature, incident
hAasviionurNseotgheeamsespl.ve(sHufmoprhmreaygsr,ad1e9d92s)e,riseosmwehibceh- radiation and its spectral characteristics; Mon-
should proffer gradually increased ther- teith and Unsworth, 1990). It is because Tb is a
moregulatory effects. Both front leg raising and complex function of intrinsic and extrinsic fac-
front leg rotation occur initially in the front legs tors that makes Ta a poor predictor of thermal
followed by the second pair of legs. In addition behaviour. Hence, the observation that a spider
contralateral legs are not necessarily lifted or may not always assume the Fabian position (or
rotated at the same time. other presumptive thermoregulatory behaviour)
at the same Ta does not imply that the behaviour
DISCUSSION has no thermoregulatory significance. For ex-
ample, Lycosa godeffroyi Koch in Canberra
Two classes of observation refute the began basking at much lowerTa in winter (4°C)
thaninsummer(17°C)and reached 35°Conclear
hroylpeotahsesstiastetdhaitnptohsetuinrtirnogduscetrivoens.an anti-predator winter days at Ta of 11°C (Humphreys, 1974,
1978); the latter shows the dominant role of
Firstly, if the Fabian position reduce the sil-
boundary layereffects for such surface dwelling
houette area against the brightest part ofthe sky
asananti-predatordefense(i.e.tomakethemless spiders. Although orb-weaving spiders are often
highabovetheground,suchboundarylayersmay
visible)then they shouldposturetothesun under
clear conditions irrespective of the intensity of assume more importance in those orb weaving
the sunlight; they do not. Furthermore, under spiders that incorporate a surface in their web
conditions ofpatchy heavy clouds (cumulus and (e.g. Neogea sp. and leaf curling species;
eumulo-status) against a clear sky, the spiders Humphreys, 1992).
should assume a Fabian position with respect to Thesemanyclassesofobservation supportthe
the brightest sectorofthe sky; they do not. hypothesisthattheposturingand/orreorientation
Secondly, many thermally related behaviours that spiders undergo in intense sunlight is of
are themselves graded so that they are exhibited thermoregulatory significance. Many are not
progressively asthe spiderwarms. This provides aloneadequatetosupportunequivocallythether-
the strongest evidence for the thermoregulation moregulation hypothesis (e.g. Table 3: 4, 6 , 9),
hypothesis because partial stilting, posturing or some, in combination with others, support the
THERMAL BEHAVIOURIN SPIDERS 549
Behaviouralresponseandthermal TABLE 3. Summary of
Conditionatspider Manipulation consequences characteristics of ther-
1 Coolweatherinshade Nil Repose;Th=Ta moregulatory behaviour in
2 Coolweatherinsun Nil Repose;Tb>Ta orb-weaving spiders.
Definitions: Spiders as-
3 Coolweatherinsun =S Repose;Ti,>Ta sume Fabian position in
4 Coolweatherinsun >S Orientateand/orposture;TS>TC hotbutnotincoldweather.
'" Hoi weatherinshade Nil Repose;TY^Tj S denotes sunlight
6 Hotweatherinshade =sor>s Fabian redirectedontothespiderat
7 Hotweatherinsun Nil Omriineinmtiasteea1Yn,d/orpostureXtrackssun; asubnoluitghtinutseinnsgitaypolfanneatmuirra-l
«5
8Hotweatherinsun Repose ror(=S)oratgreaterorless
9Hotweatherinsun =Sor>S Fabian thannaturalintensityusing
1 Veryhotdayinshade Nil wRietphoosuet;pionsetxutrrinegmismaysufferheatdeath Sa).conTchaevespmiidrerroris(>sSimorul<-
111321 PPVooeppruuyllaahttoiitoodnnaiiynnihhnoottsussnuunn LVNiaalrrigeed&wesmbaolrliesnptiadteirosn LmSAlaeilrengkisesmpiissdspheeiardds1eero1,rsipeonstattuereineasralimeerdiinrdeactyiotnh;ansmall tsbalelhoaroawnsndeeeedwo,diutb(sahsyln)yd*ocraonrnmaeuormtmctob(oiSennf)rsi.icslNdieeatuftlrmoelelr-yds-
14Hotweatherinsun =5perpendiculartosun Sinpciiddeerntpohsetaulressoumricdes-waybetweentwo are considered together, to
support strongly the ther-
15*Graden-1 behaviour >S GcurlamdienanlbienhgaivnioFuarb;inbiiehaviouralcascade moregulationhypothesis.
16Hotweatherinsun =Sbelowhorizontal Apparentconfusioninsomespecies;>Tb
17Coolweatherinshade =s Postureto>exposuretohealsource;Tb>Ta
hypothesis (e.g. 1-7), while others support no tangling thermoregulatory from other be-
otherhypothesis (e.g. 15). haviours.
While the emphasis here has been on behav- A thermoregulator with the battery of finely
ioursthatreducetheheatload,spidersshoulduse graded behaviours seen here should, under ideal
behavioursto warm them in ordertoenhance the conditions, be able to maintain a near constant
time they are atoptimal temperatures. This is the bodytemperature undera widerangeofenviron-
case in burrow dwelling lycosids (Humphreys, mental conditions. In practice theirtemperatures
1974, 1978, 1987a, 1987b) as well as in orb web fluctuate markedly with every air movement, at
spiders which may seasonally orientate their leastpartlyowingtotheirsmall thermalcapacity.
webs to maximise the projected surface area to If the spiders are innately incapable, owing to
warm more or faster (Carrell, 1978; Tolbert, their small mass, of precise thermoregulation,
why have they developed such a wide range of
1979).
sophisticated behaviours which should permit
While there is an indication of size related precise thermoregulation?
effects in thermoregulatory behaviourinNephila
spp., as may be expected theoretically using a ACKNOWLEDGEMENTS
simple physical model, no such size effect was
observed in Stegodyphus lineatus Latreille
(HenscheU/ a/., 1992). It is a pleasure to acknowledge the occasional
moWrheilseopthhiesttihceartmeadltbheahnavhiaosurbeofenspaicdceersptiesdm,utchhe YasosuisntganfcaemiolfyRfaoreaYlloouwnigngimnethteo wfioerldk oanndthtehier
presumed advantages ofsuch fine tuning are not property 'Mandalay'. I thank Mark Elgar and
Yael Lubin for their many constructive com-
understood.Nonetheless,therecognitionofsuch ments. Someofthis work wasundertaken during
behaviour is an important aspect offield studies the tenure of a Christensen Research Institute
andthemeanstodosoarerequired,especiallyfor Fellowship and with the approval ofthe Madang
smallerspiders which are intractable subjects for Provincial Council; the advice and assistance of
directrecordingoftemperatureinthefield. How- MatthewJebbandhisstaffisgreatlyappreciated.
ever, thesensibleuseofthisschemashouldallow TheworkwasfundedbytheAustralianResearch
easy appraisal of the overt body positions of Grants Scheme (#D181/15274) and the Austral-
spiders in the field as to their likely thermoreg- ian Research Council (#A18831977 and
ulatory significance and it should assist in disen- #A18932024).
» .
sv. MEMOIRSOFTHEQUEENSLAND MUSEUM
LITERATURECITED Western Australia. Behavioural Ecology and
Sociobiology28: 47-54.
ALEXANDER, A.J. & EWER, D.W 1958.Tempera- 1992. Stamlimenta as parasols: shade construction
lure adaptive behaviour in the scorpion. Opis- byNeo$easp.(Araneae: Araneidae, Argiopinae)
ihopkthaimus tatimanus Koch. Journal of anditsthermal behaviour. Bulletinofthe British
Experimental Biology35; 349-359 Arachnological Society9: 47-52
BIERE.J.M. & UETZ,G.W. 1981 Weborientation in LUBIN,Y.D. 1986. Webbuilding and preycapture in
the spicier Micrathena graciti.s (Araneae: Uloboridae. Pp. 132-171. In Shear, *W.A. (Ed).
Arane>dae). Ecology62: 336-344. Spiders:webs,behavior,andevolution. (Stanford
CARREL. IE 1978. Behavioural thermoregulation University Press: Stanford, California),
during winter in an orb-weaving spider LUBIN. Y.D. & HENSCHEL. J.R. 1990 Foraging at
possumofthe Zoological Society ofLondon 42: the thermal limit: burrowing spider {Seoihyru,
41-50. Eresidae) in the Namib Desert dunes. Oecologia
CRAIG.C.Lv&BERNARD, G, D. 1990. Insectattrac- 84:461-467,
tion toultraviolet-reflecungspiderwebsandweb MARSON,J. 1947 Some obscivationsontheecologi-
decorations. Ecology 71 616-623. calvariationanddevelopmentofIhecruciatezig-
EBERHARD, W.G. 1973. Sla:bilimcntaonthewebsof zag camouflage device of Ar$lop6 pufchem
Uloborus diversus (Araneae: Uloboridae) and (Thor.), Proceedingsofthe Zoological Societyof
other spiders. Journal of Zoology, London 171: London 117:219-227
367-384. MONTEJTH. J.L. & UNSWORTH, M.H. 1990. Prin-
EDMUNDS,J.&EDMUNDS,M.1986.Thedefensive Cipit-sofenvironmentalphysics.(EdwardArnold.
mechanismsoforbweavers(Araneae;Araneidae London).
in Ghana. West Africa. Pp. 73-89. In Eberharu. OLIVE. C.W. 1980. Foraging specializations in orb-
W.G., Lubin, Y.D. and Robinson, B.C. [Eds) weaving spiders. Ecology61: 1133-1144.
Proceedings ofthe Ninth International Congress RIECHERT. S.E. & TRACY, CR. 1975 TV
ofArachnology,Panama, 1983.(SmithsonianIn- balanceandprey availability Bases foxa model
stitution Press: Washington, D.C.K relating web-site character:istics to spider
EWER, R.F 1972.Thedevicesin thewebofthe-West reproductive success. Ecology56: 265-284.
African spider Argiope flavipulpis. Journal of ROBINSON, M.H. & ROBINSON, B. 1973. The
Natural History6: 159-167. stabilimentaofNephilaciaVlpesandtheoriginof
POEUX, R-F. 1982. Biology of Spiders. (Harvard stabiliinentum-building in Araneids. Psyche 80:
University Press:Cambridge. Massachusetts). 277-288.
HEATWOLE,H. 1970.ThermalecologyofIhe Desert
Dragon Amphibolurus inermis. Ecological 197p4o.stAudraepstiovfe tchoemplgeoxlidtey:n-twheebthseprimdoerregNuelpahtiolray
Monographs40: 425-457.
HENSCHEL,J.R..WARD,D.&LUB1N.Y. 1992.The ctavipt's at low altitudes. American Midland
Naturalist 92: 386-396,
importance ofthermal factors fornest-site selec-
tion, web construclion and behaviour of St4tgo* 197n8e.wTdheesrcmroirpetgiuonlsatoifonthienrmoorrbe-gwuelaatvoirnyg psopsitduerres:s
dyphuslineatus(Araneae: Ercsidac)intheNcgev
and experiments on ihe effects of coloration.
Desert.Journalofthermal Biologv 17:97-106.
HUMPHREYS,W.F 1974. Behaviouralihcrrnoirguiu Zoological Journal ofthe Linncan Society, Lon-
lion in a wolf spider. Nature. London 251 SUTEdRo.n6R4.:B.87-1190821.. Behavioural thermoregulation:
503.
197a8n.dTohtehetrhbeurrmraolwbiinohlaobgiytionfgGLeyocloysriodsaaeg(oAdrfafnfenaqe)p sioLilnayrphioirdiaeen)t,aati6o-nmginspiFdreorn.tiBneehlalvuioucroamlmEucnoils-
in Australia. Oecologia, Berlin31: 319-347. ogyand Sociobiology8:77-K1
1986. Heal shunting in spiders. Pp. 41-46. In Con- TOLBERT, W.W. 1979. Thermal stress of the orb-
oxesso Intemacionale Arachnologia. vol. I. (Ed. weaving spider Argiope irifasciuta (Araneae).
J.A. Bairiemos). Jaca. Spain. Oikos32: 386-392.
1987a, The thermal biology of the wolf spider WARD. D &. HENSCHEL, J.R. 1992. Experimental
Lycouttorentula (Araneae: Lycosidae) in north- evidence thai a desert parasitoid keeps its host
ern Greece. Bulletinofihe British Arachnologi- cool. Ethology92:135-142.
cal Society.73 117-122. WILLMER, P. 1991. Thermal biology and mate ac-
1987b. Behaviouraltemperatureregulation. Pp.56- quisition in eclotherms. Trends in Ecology and
W
65, In. Nentwig, fed). 'Ecopbysioloey of Evolution6: 396-399.
Spiders*.(Springer-\''erjyg: H-.ilm.) WILLMER. PC, ft UNWIN, DM. 19X1 Field
1991. Thermal behaviour of n small spider analyses ofinsect heat budgets: reflectance, size
(Araneae: Araneidae: Arancinuc)onspinifcx in and"healingrates. Oecologia50: 250-255.