*Author ([email protected]) for correspondence Little Rock, Little Rock, AR72204,USA. Louisville, KY40292, USA. © 2014.PublishedbyTheCompanyofBiologistsLtd|JournalExperimentalBiology(2014)217,2163-2170doi:10.1242/jeb.101600 Received 19December 2013;Accepted3March2014 1 relatively ineffective andclumsy onland.Thegaitsoftheseinsects locomotion beloworonthewatersurface(Nachtigall,1974),but are such asdivingbeetlesandwaterstridersfacilitateefficient fortuitous. lacking suchtraits,escapefromthewatersurfaceusually is Milne andMilne,1978;Vogel, 1994;Huetal.,2003).Forspecies morphologies ofneustonicinsectsandspiders(Andersen,1976; generalities includethespecializedwater-treading behaviorsand death viadrowningorpredation.Conspicuousexceptionsto these slender kickingappendagesoftenleadstoafutilestruggleending in Additionally, therelativelylowphysicalresistanceofwaterto size makesthemvulnerabletoentrapmentbysurfacetensionforces. helpless whentheyfallontowater, inpartbecausetheirsmallbody uncommon innature(Vogel, 1994).Mostterrestrialarthropodsare Efficient locomotionattheair–waterinterfaceisrelatively Skototaxis KEY WORDS:Aquatic,Behavior, Forest,Formicidae,Neustonic, swimming atspeeds>3 assess theirswimmingability. Ten speciesshoweddirectedsurface into naturalandartificialaquaticsettingsinPeruPanamato swimming) intropicalarborealants.We dropped35speciesofants document sustained,directional,neustoniclocomotion(i.e.surface escape bywalkingorswimmingatthewatersurface.Herewe Exceptions tothisoutcomemostlyoccuramongripariantaxathat helplessly struggleandarequicklyeatenbyaquaticpredators. Upon fallingontothewatersurface,mostterrestrialarthropods S. P. Yanoviak Water surfacelocomotionintropicalcanopyants RESEARCH ARTICLE to includevariousspeciesoftropicalarborealants. These resultsexpandthelistoffacultativelyneustonicterrestrialtaxa and midlegs,respectively, whilethehindlegsproviderollstability. synchronized treadingandrowingmotionsofthecontralateralfore when swimming;theygeneratethrustatthewatersurfacevia Pachycondyla skototaxis). Analysesofhigh-speedvideoimagesindicatethat water bydirectingtheirswimmingtowarddarkemergentobjects(i.e. workers ofOdontomachusbauri association withdirectedaerialdescentbehavior. Experimentswith genera indicatesparallelevolutionandatrendtowardnegative the surface.Thephylogeneticdistributionofswimmingamongant movement. Theremainingspeciesshowednolocomotorycontrolat swimming abilitycharacterizedbyrelativelyslowbutdirected absolute speeds>10 INTRODUCTION ABSTRACT Department of Biology, Louisville, 139Life SciencesBuilding, of University of Department The veryspecializedlegsofobligatepelagicandneustonictaxa spp. and 1, * andD.N.Frederick

2 cm Department of Biology, at Arkansas of University of Department O. bauri body s − 1 . Ten otherspeciesexhibitedpartial lengths use amodifiedalternatingtripodgait showed thattheyescapefromthe s − 1 , withsomeswimmingat 2 example, thehydrophobictarsiofriparian with highefficiency usingthesamegaitsandappendages.For contrast, riparianarthropodsfrequentlytreadonbothlandandwater by terrestrialspeciesinthesameclades(Andersen,1976).By often arekinematicallydistinctfromthealternatingtripodgaitused described indetailfor workers andbroodonthewatersurface.Thisbehaviorwas remarkably well.Inthesimplestcase,entirecoloniesformraftsof Nonetheless, afewantspeciesareabletolocomoteonorinwater but completeimmersiontypicallyisproblematicforants. foraging (Young andHermann,1980;DejeanSuzzoni,1997), some speciesusewatersurfacetensiontotheiradvantagewhen particularly unlikelytointeractfavorablywithwater. Workers of (Franklinetal.,1977;DuBoisandJander, 1985). a watersurfacewithmodifiedorunmodifiedgaits,dependingonthe taxa(e.g.mantids,cockroaches)arealsocapableoftraversing 2003; Suter, 2013).Althoughlesscommon,afewentirelyterrestrial water whileothersdonot(SuterandWildman, 1999;Suteretal., and land(Strattonetal.,2004);someuseamodifiedgaitwhenon Sutton, 2012).Manyspidersalsolocomoteequallywellonwater from bothlandandwater(BurrowsPicker, 2010;Burrowsand modified hindlegsoftridactylidgrasshoppersfacilitatelarge jumps them toalightonwaterorvegetation(Burrows,2013),andthe 2011), butalsooccursinsome falling fromthe canopytotheunderstory (D.N.F. andS.P.Y., of Panamashowthatantsconstitute ca.70%ofwinglessarthropods However, many speciesdonotglide,andsurveysinlowlandforest understory byglidingtonearby treetrunks(Yanoviak etal.,2005). to theircolonies,andsomespecies avoidlandinginthehazardous (Saint-Paul etal.,2000;Small al.,2013).Lostworkersarecostly by fishthatconsumeterrestrial insectstrappedinthewatersurface and riparianareasareespeciallydangerous;theyofteninhabited and complexterrain(Yanoviak etal.,2011). Tropical floodedforests Longino andColwell,1997),wheretheyfaceunfamiliarpredators fall fromthecanopytoforestunderstory(Haemig,1997; and Oliveira,2007;Klimesetal.,2012).Arborealantsfrequently high abovetheground(HölldoblerandWilson, 1990;Rico-Gray tropical forests,andmanyspeciesnestforageintreecrowns water surfacelocomotioninvariousNeotropicalants. we describetheoccurrence,basickinematicsanddirectionality of lundii carli (DuBois andJander, 1985),andcertainleaf cutterants( (Robson, 2010),Camponotus sokolova on watersurfacesisknowninonlyafewanttaxa,including Nepenthes for insectpreybycrawlingorswimmingintothewaterheld personal observation),andworkersofCamponotusschmitzi walk beneaththesurfaceofpuddlesonforestfloor(S.P.Y., such asEctatommaruidum (Robson, 2010;S.P.Y., personalobservation).Otherrainforesttaxa, As small,terrestrialandobligatelycursorialorganisms, antsseem areamongthemostabundantandconspicuousarthropods in ) inhabitingAmazonianfloodedforest(Adis,1982).Here pitchers inAsia(Bohnetal.,2012).Directedlocomotion Solenopsis workers ofAustralianmangroveforest in CentralAmerica,willvoluntarily spp. inMalaysiaandNorthAmerica Wasmannia (Adams etal.,2011; Mlotetal., Hydrophorus and Pheidole flies allow species forage 2163

The Journal of Experimental Biology Paraponerinae 2164 methods. *Epigeic species. intheMaterials and Each testedspecies wasassignedtooneofthreegroups basedonasemi-quantitativeassessment ofswimmingabilityasdescribed Taxon traversing seasonalfloodwaters(Irmler, 1973). ground-dwelling beetlesuseskototaxistolocatetreetrunkswhen darkest nearbylandmarks(Campan,1997).Indeed,Amazonian surface reflectsambientlight,andtreetrunksconsequentlyarethe effective inecotonal,riparianandfloodedforestsettings;thewater (Taylor, 1988;Campan,1997).Skototaxismaybeespecially skototaxis, althoughreasonsforthebehaviorarenotalwaysobvious skototaxis (DuBoisandJander, 1985)].Various exhibit suggest thattheyspecificallymovetowarddarkobjects[i.e. mantids (Miller, 1972).Otherstudiesofantstreadingonwater escape maneuverstowardnearbyemergent objects,asdoswimming Likewise, antslandingonwatershoulduselocalcuestodirecttheir locate treetrunksduringafall(Yanoviak andDudley, 2006). Nicholson etal.,1999),andglidingcanopyantsusevisualcuesto chemical senses(Hölldobler, 1980;HölldoblerandWilson, 1990; from awatersurface. it islikelythatsomespeciespossesstraitsfacilitatingtheirescape unpublished). Giventhatfallingantswilloccasionallylandinwater, RESEARCH ARTICLE Table Ecitoninae Ectatomminae clavata Hypoponera distinguenda Wasmannia rochai Solenopsis trigona P. foetida Pachycondyla apicalis O. bauri Solenopsis Procryptocerus belti Pheidole Pheidole Nesomyrmex anduzei