Abstract Earlier studies have shown that the sarcopha-gid fly Arachnidomyia lindae is the principal egg-sacpredator of the colonial orb-weaving spider Metepeiraincrassata, and that risk of egg-sac loss increases withgroup size. Observations of specialized behaviors for at-tack (flies) and defense (spiders) suggest that this preda-tor–prey relationship may incorporate elements of ployand counterploy behavior. Here we explore this relation-ship in detail and test hypotheses regarding efficacy ofattack and defense behaviors. Egg-sac guarding by thespider includes defensive behaviors specific to this flypredator, which were observed during experimental “attacks” with tethered A. lindae, but not with similarpresentations of non-predatory Musca domestica. Exper-imental studies also show that Metepeira incrassata rec-ognizes this predatory fly by airborne cues (i.e., the sig-nature frequency of wing-beats), and can distinguish be-tween this predator and other flies (potential prey) on thebasis of wing-beat frequency differences. Removal of fe-male spiders results in a significantly higher probabilityof unguarded egg-sacs being parasitized, demonstratingthe adaptive value of spider defensive behaviors. Wealso present evidence that A. lindae utilizes a behavioralploy for circumventing spider guarding behavior (ag-gressive mimicry – producing vibrations of capturedprey in the web), and that Metepeira incrassata, in turn,exhibits a counter-ploy behavior (signal thread cutting)to eliminate this potentially distracting vibratory infor-

mation. While previous studies have shown that this colonial web-building spider uses a number of generalattack-abatement mechanisms against a diversity of pre-dators and parasitoids, results of this study suggest thatselection pressures from a highly specialized predatormay also result in evolution of predator-specific prey re-sponses.

Keywords Aggressive mimicry · Egg-sac predator ·Ploy–counterploy behavior · Predator–prey interaction ·Spiders

Introduction

Increased rates of predation and parasitism are often cit-ed as a major cost of group-living, especially for speciesthat occupy fixed spatial locations, for example, socialinsects, colonial nesting birds (Vulinec 1990; Mooringand Hart 1992; Brown and Brown 1996; Hart et. al.1997). Numerous studies have demonstrated that thesecosts may be reduced by a number of group level attack-abatement strategies, for example, encounter-avoidanceand dilution effects, predator mobbing (Hamilton 1971;Turner and Pitcher 1986; Inman and Krebs 1987; Wronaand Dixon 1991; Brown and Brown 1996). Less isknown about the role of individual anti-predator behav-iors of group members, but selection pressures fromhighly specialized predators may sometimes result in theevolution of predator-specific prey responses (Endler1991; Rayor 1997).

Predator and prey are generally coupled in a long-term relationship that has varied evolutionary implica-tions depending on its intimacy. If the relationship is notclose, for example, groups of prey evolving in responseto groups of predators, their joint evolution can be de-fined as an “arms race” (Dawkins and Krebs 1979), “dif-fuse coevolution” (Gilbert and Raven 1975; Jantzen1980), or “escalation” (Vermeij 1987). For a preda-tor–prey relationship to be considered “coevolved”,however, it must be demonstrated that responses are spe-

Communicated by M. Elgar

C.S. HieberDepartment of Biology, Saint Anselm College, Manchester,NH 03102, USA

R.S. WilcoxDepartment of Biological Sciences, State University of New York,Binghamton, NY, USA

J. Boyle · G.W. Uetz (✉)Department of Biological Sciences, P.O. Box 210006, University of Cincinnati, Cincinnati, Ohio 45221–0006, USAe-mail: George.Uetz@uc.eduTel.: +1-513-5569752, Fax: +1-513-5565299

Behav Ecol Sociobiol (2002) 53:51–60DOI 10.1007/s00265-002-0547-2

O R I G I N A L A RT I C L E

Craig S. Hieber · R. Stimson Wilcox · Jay BoyleGeorge W. Uetz

The spider and fly revisited: ploy–counterploy behavior in a unique predator–prey system

Received: 16 July 2002 / Revised: 25 September 2002 / Accepted: 8 October 2002 / Published online: 12 November 2002© Springer-Verlag 2002

cialized, reciprocal and specific; any new defensemounted by the prey must be counteracted by the preda-tor and vice versa through evolutionary time; and thepredator and prey must be the primary evolutionaryagents of one another across a geographic mosaic ofpopulations (Janzen 1980; Endler 1991; Thompson1994, 1999; Soler and Soler 2000). Highly specialized orcoevolved predator-specific defense behaviors are infre-quently seen among species with multiple predators andespecially those whose primary anti-predator defensesderive from grouping (Vulinec 1990; Endler 1991; Rayor1997; Uetz et al. 2002). In this study, we examine aunique predator–prey system – a colonial web-buildingspider and a predatory fly – and document complexploy–counterploy interactions in predator defense by in-dividual spiders.

Colonies of web-building spiders are vulnerable toegg-sac predators and pay a cost, increasing predatorload with increasing colony size (Lubin 1974; Rypstra1979; Smith 1982; Hieber and Uetz 1990), because thehigh density and conspicuous silk of colonies provideeasily located patches for these specialized predators(Lubin 1974; Rypstra 1979; Buskirk 1981; Uetz and Hieber 1994, 1997). Within colonies, females may beunder pressure to secure high quality web-sites becauseof predation pressure (Rayor and Uetz 1990, 1993), andspiders holding these sites may have to actively defendthem against prey theft and attempted takeover by con-specifics (Lubin 1974; Buskirk 1975a, 1975b; Uetz1985). Because of these constraints, female spiders ofmany species are present in their respective territorieswithin a colony at all times (Lubin 1974; Uetz and Burgess 1979; Buskirk 1981; Burgess and Uetz 1982).Since colonial spiders may deposit their egg-sacs at theweb-site, many species have also evolved active mater-nal defense of the egg-sac against predators (Hieber andUetz 1990), rather than a passive defense utilizing cocoon architecture (Austin 1985; Hieber 1992).

Some spider species in the genus Metepeira (Araneae:Araneidae) exhibit a “communal-territorial” or “colo-nial” form of social organization in which individualslive communally, but occupy individually defended terri-tories (an orb-web connected by a signal thread to a re-treat) within the colony and do not share prey (Burgessand Witt 1976; Uetz and Burgess 1979; Burgess andUetz 1982; Uetz 1986). For these species, the individualterritory is multifunctional and may be used for foraging(the orb), as a habitation (the retreat), as a mating site(orb and/or retreat), and as a breeding/egg-laying site(the retreat).

Metepeira incrassata F.O. Pickard-Cambridge occursin the East-Central Sierra Madre mountains above theGulf of Mexico, in Fortin de las Flores, Veracruz, Mexico.The spider is colonial and lives in small groups of ten in-dividuals to immense groups of several thousand span-ning large spaces between trees and in gaps in moisttropical (second growth) forest, along forest edges, andfrequently in coffee and banana plantations (Uetz 1985;Uetz and Hodge 1990).

Previous research with M. incrassata has shown thategg-sac predators represent a significant source of mor-tality for this colonial spider (Hieber and Uetz 1990;Rayor and Uetz 1990). Chief among its egg-sac preda-tors is the fly Arachnidomyia lindae Souza-Lopez (Diptera: Sarcophagidae). Within a colony, the fly at-tacks female spiders guarding egg-sacs, successivelymoving from spider to spider in a “trapline” fashion. Theattack strategy of the fly may involve aggressive mimic-ry as well. Previous observations suggest that female spiders are able to detect an approaching fly prior to anattack, and that during and after an attack they performspecific behaviors related to guarding the egg-sac. Theseobservations have caused us to hypothesize that the interaction between M. incrassata and A. lindae may in-clude elements of “ploy and counterploy” behavior(Dawkins and Krebs 1979).

Here we use a number of observational and experi-mental approaches to examine interactions between M.incrassata and its principal egg-sac predator A. lindae.Our objectives were: (1) to examine in detail the com-plex set of behaviors exhibited during interactions be-tween this spider and fly; and (2) to test hypotheses con-cerning cues that M. incrassata may use in predator detection, and the effectiveness and adaptive value ofspider guarding behavior in preventing egg-sac predationby A. lindae.

Methods

To examine the interactive behaviors of Arachnidomyia lindae andMetepeira incrassata, we collected data in a number of ways during visits to our study site in Fortin de las Flores, Veracruz,Mexico during July and August 1990–1992.

Spider–fly interactions

In each year of the study, we observed naturally occurring attacksby A. lindae on M. incrassata, and conducted experimental stud-ies. Behavioral observations were recorded ad libitum on hand-held microcassette recorders whenever attacks occurred during ob-servations of spider colonies (for details, see Uetz and Hieber1994) and transcribed later. Whenever possible, we recorded thebehaviors displayed by interacting flies and spiders, and the posi-tion, behavior, and time spent in those behaviors by flies as theymoved through colonies.

After tape transcription, we pooled the behavioral data collect-ed from natural fly/spider interactions across attacks observed ineach year and analyzed data separately. In 1990, individual flies(n=32) were followed through colonies, and we pooled the datafor all flies and calculated the percentage of total time spent, andthe mean duration of behavioral activities involved. In 1992, wemade more detailed observations of fly and spider behavior (n=20)with an activity recorder (Psion Organizer, with The Observerevent recording software; Noldus et. al. 1989), and calculated themean number of times each fly behavior and each spider behaviorwas performed during an attack.

We also conducted experimental studies to test the hypothesesthat M. incrassata: (1) recognizes A. lindae as a predator; and (2)uses predator-specific defense behaviors. This was accomplishedby comparing the responses of Metepeira incrassata to staged en-counters with the predatory fly A. lindae and the prey fly Muscadomestica (Diptera: Muscidae) in 1990. As the stimulus for these

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experiments, we presented live A. lindae (n=13) and Muscadomestica (n=16) tethered to insect pins glued to their thoraces(and mounted on a wooden stick) to randomly chosen spidersguarding egg-sacs within several observation colonies. Flies teth-ered in this manner “fly” when their legs are not in contact with asubstratum. An experimental presentation consisted of three 1-min“attacks” with a tethered fly, separated by 30 s of fly removal.During each attack, we moved the fly around the egg-sac (as muchas possible) at a distance of approximately 2 cm. This presentationpattern is similar in both timing and spacing to observed naturalattacks. We recorded the behaviors performed by the spiders dur-ing the presentation attacks, and after the presentation, we contin-ued to record spider behavior until the spider moved into its retreator stopped its activity, and remained motionless for 1 min. The be-haviors observed during and after these experimental attacks wererecorded on tape as above, then later transcribed using The Observer software. Care was taken to ensure that spiders were notused more than once, and that spiders within the same colonywere not exposed to a fly stimulus presentation within the same 3-h activity period (see Rayor and Uetz 1990, 1993; Uetz and Hieber 1994, 1997).

We pooled the spider behavioral data collected during the ex-perimental presentations by fly stimulus type. From these data, wecalculated the mean number of times each behavior was per-formed during an attack, and the mean number, mean time, andcumulative time spent in each behavior after attacks. We com-pared these means using Mann-Whitney U-tests (variances withingroups could not be made equal) to determine if the spiders re-sponded differentially to A. lindae and Musca domestica.

Fly identification cues

As orb-weaving spiders respond to both airborne vibration andweb-borne vibrational stimuli from prey (Barth 1982), we testedthe hypothesis that guarding Metepeira incrassata detect the pres-ence of A. lindae through a vibrational cue(s) with a number of re-lated experiments in 1991. We first recorded the wing-beat signalsfrom four A. lindae using a Marantz field-portable tape recorder.The recordings were made using tethered flies positioned approxi-mately 2–3 cm distance from the microphone. Flies mounted inthis manner provided reasonably long sections of wing-beat vibra-tion. We also recorded the wing-beat of four Musca domestica as acontrol.

We analyzed the fly wing-beat recordings using the softwareprogram Canary in the following manner. We selected “clean” sec-tions of fly wing-beat containing no background noise. Fromthese, ten 100 ms samples were collected for each fly. We generat-ed a spectrogram for each sample using the default settings for theprogram, with the amplitude set to quadratic, and the frequencyset to 0.5 kHz/inch. From each spectrogram, we used the data-logfunction to measure the first six peaks of each sample for frequen-cy and energy amplitude.

We subjected these frequency and energy amplitude data to avariety of statistical analyses. To determine if there were differ-ences between the two fly species for wing-beat frequencies or energy, we ran two-way repeated measures ANOVA’s with fly species and peak as the main treatment effects and either frequen-cy or energy as the repeated measure. We then used post hocScheffe S tests to test for differences between the flies at each ofthe peaks. We ran two-way ANOVAs with individual flies and fre-quency or energy as the main treatment effects, and then ran posthoc Scheffe S tests comparing all individual flies to one anotherfor frequency or energy at each of the peaks to determine if therewere differences between individual flies.

To determine if predator recognition is based solely on vibra-tion cues, we conducted a series of experimental studies in 1991.We presented guarding spiders (n=27) with both a silent speakerand a speaker playing a recorded A. lindae wing-beat (from thesamples recorded above). This method removed any attribute ofthe fly other than its wing-beat vibration as a cue for the spider.Order of presentation was random (a coin toss) for each spider.

Each presentation lasted one min. and was recorded as positive ifthe spider showed characteristic behaviors (i.e., shuttle or groom)in response. As a control, we presented guarding spiders (n=21)with both a recorded A. lindae wingbeat and a live tethered A. lindae to determine if the spiders could distinguish between recordings and live flies. Presentation of the two stimuli for thisexperiment was as above.

We conducted another experiment to test the possibility thatairborne olfactory signals are used as cues for identifying preda-tors by presenting guarding spiders (n=10) with dead (freshly fro-zen and thawed) specimens of A. lindae and Musca domesticamounted on an insect pin attached to a wooden stick (as above).This experimental design removed any vibrational cue(s) fliesmight be providing. Both flies were presented in a random orderto each spider, with each presentation lasting 1 min. We scored re-sults as positive if the spiders responded with appropriate behav-iors. Care was taken during this experiment, and all other presen-tations using live flies, to touch neither the spider, the egg-sac, northe web with the flies.

Guarding behavior

To test the hypothesis that spider guarding behavior has an adap-tive value (i.e., contributes to fitness via improved egg survival),we collected egg-sacs from one of the large Metepeira incrassatacolonies where observations were conducted in 1990. This includ-ed 30 egg-sacs with a guarding female, and 26 egg-sacs that wereunguarded as a result of experimental manipulation (females wereremoved during a time when attacks were occurring in a colonyand left for several days). We compared the rates of parasitism be-tween these two experimental groups using a Log-likelihood test(with Williams correction).

Ploy–counterploy behaviors – signal line cutting

To determine whether the web-vibrating behavior shown by A. lin-dae and the signal line-cutting response of Metepeira incrassataconstitute ploy–counterploy behaviors, we tested two hypotheses:(1) signal line cutting has no effect on spider response to the fly,and (2) signal line cutting has no effect on response to web-bornesignals. We did this with an experiment during our field season in1991 that simulated a prolonged attack by A. lindae utilizing aggressive mimicry behavior (vibrating the signal thread). We pre-sented spiders with either a tethered A. lindae (n=6 trials) or aspeaker playing a recorded fly wing-beat (n=8 trials) for 60 s,waited 30 s, and then vibrated the web for 10 s as if prey werecaught in it. We vibrated the web by touching a small piece ofmagnetized material glued to a cat whisker to the hub of the orband exposing it to an electrical field pulsing at 90 Hz. After twocomplete sets of alternating presentations, we cut the signal line,presented the fly stimulus (live or taped) to the spider, and then vi-brated the web one last time. We scored all fly vibration presenta-tions as positive if the spider performed guarding or grooming be-haviors in response to the stimulus; all web vibrations were scoredpositive if the spider left the egg-sac and moved out onto the sig-nal line or orb-web. We used the responses shown by the spider tothe last two sets of attacks (before and after signal line cutting) foranalysis.

Results

Spider-fly interactions

From observations of naturally occurring attacks byArachnidomyia lindae on colonies of Metepeira incrass-ata, we developed a standardized ethogram of attack (fly)and defense (spider) behaviors, which we used wherever

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possible in all comparisons (Table 1). The majority of be-haviors performed by A. lindae moving within a colonyduring attack “runs” in 1990 (n=32) involved bouts of at-tacking spiders guarding egg-sacs, examining unguardedegg-sacs, and resting on debris within the silk frameworkbetween attacks (Fig. 1). Examining non-target individu-als (females without egg-sacs, males, immatures), wasperformed less often. While females with egg-sacs, un-guarded egg-sacs, and other non-target individuals werevisited throughout the colony, the flies spent significantlymore time (72.95% of total time) in the core of the colony(where >60% of egg-sacs are found; Rayor and Uetz1990) during attacks. Comparison of attack frequencieswith expected values based on the proportion of potentialtarget types in a colony (minus values for debris) suggest-ed that flies target adult females with egg-sacs far moreoften than expected, and most other categories less often(χ2=2493.8336; P<0.001). Flies frequently sat upon thesignal line connecting the egg-sac and the hub of the orbweb, vibrating the line in what appears to be mimicry ofcaptured prey in 28 of 32 attacks (87.5% of total). Al-though these bouts of aggressive mimicry were brief(3.85±0.56 s), they represented approximately 4% of theflies’ total time during attacks.

Rates of occurrence of fly and spider behaviors dur-ing natural attacks were examined in more detail in1992, with very similar results. Flies in observed colo-nies performed the behaviors move, vibrate, and attackegg-sac more often than sit and examine (Fig. 2a). Lar-vaposition, the deposition of a single fly maggot on thesurface of the egg-sac, occurred in 50% of the attacks.During these observed attacks, the most commonly per-formed behaviors by Metepeira incrassata were shut-tling around the egg-sac and attacking the fly, followedby grooming (Fig. 2). The behaviors sit, check web and,cut signal thread were observed less often.

In staged experimental encounters (presentations withtethered flies) spiders performed behaviors similar tothose seen in natural attacks with A. lindae more oftenthan in encounters with Musca domestica, supporting thehypotheses that spiders recognize the predatory fly and

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Table 1 Ethogram of recorded fly and spider behaviors

Behaviour Definition

The fly: Arachnidomyia lindaeAttack Actively trying to get past a guarding female spider to larvaposit on an egg-sac.Vibrate Landing on the hub or signal line of an orb–web and vibrating it.Examine Activity associated with “investigating” non-target material, e.g., spider retreats without egg-sacs,

abandoned egg-sacs, discarded prey, or debris within the colony.Sit Sitting on non-target material (see Examine above) within the colony.Move Moving through the colony.Larvaposit Gaining access to a guarded egg-sac and depositing a larva upon its surface.

The spider: Metepeira incrassata

Attack fly Swinging front legs at a fly, extending front legs toward a fly without swinging, or jumping off of the egg-sac at a fly while it is hovering near the egg-sac.

Shuttle Moving rapidly around the egg-sac during an attack to maintain a position between the egg-sac and a hovering fly.Groom Searching the cocoon surface with chelicerae and pedipalps both during and after an attack.Check web Tugging on the signal line to the web, moving on to the center of the orb-web and plucking

the various component radii, or moving off the egg-sac into the space web and flexing or bouncing it.Sit Maintaining a non-moving position in the retreat, on the egg-sac(s), or on the space web in which

the orb-web is hung.Cut Cutting the signal thread connecting the orb-web to the retreat during an attack.Interact Interactions with conspecifics within the territory (space web, orb-web, or retreat).

Fig. 1a, b Activity by the fly Arachnidomyia lindae during natu-rally occurring attack “runs” in colonies of the spider Metepeiraincrassata in 1990. a The proportion of total time spent in eachactivity. b The average amount of time spent in each activity. Er-ror bars represent 1 SD; n=32 attacks

respond with predator-specific defensive behavior(Fig. 3a). Spiders responded to tethered A. lindae withsignificantly more shuttle behavior (U=175, P<0.01),and significantly less attacks on the fly (U=482, P<0.05)than they did to tethered Musca domestica. Spiders per-formed, on average, three grooming bouts in response toA. lindae during the presentation of the fly stimulus ver-sus none to Musca domestica. There was no significantdifference in response to the two flies for the behaviorssit or check web during the presentation phase.

There was no significant difference in the number oftimes spiders groomed their egg-sacs after attack in re-sponse to the two flies (Fig. 3b). However, the averagelength of a grooming bout (Fig. 3; U=3509, P<0.001),and the cumulative time spent grooming (Fig. 3; U=38;P<0.05), were significantly longer in response to experi-mental attacks by A. lindae. There were no significantdifferences in the frequency of sitting or web-checkingbehaviors, the number of times these behaviors were per-formed per bout, or the cumulative times for these be-haviors after attacks in response to either of the flies.During this post-attack period, spiders responded to allconspecifics that moved into their territories, regardlessof fly type.

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Fig. 2a, b Observation of fly attack and spider defense behaviorsobserved simultaneously during naturally-occurring attacks in1992. a The mean number of times behaviors of the fly Arachnid-omyia lindae were observed during an attack run.) The mean num-ber of times behaviors of the spider Metepeira incrassata were ob-served during an attack by the fly. represent 1 SD; n=20 attacks

Fig. 3a–d Behaviors exhibited by the spider Metepeira incrassatain response to experimental presentation of tethered Arachnid-omyia lindae and Musca domestica flies. are 1 SD; n=13 for A.lindae, n=16 for M. domestica. a Mean number of times each be-havior was observed during presentation of a tethered fly. b Meannumber of times each behavior was observed after presentation ofa tethered fly. c Mean length of bouts of each behavior after thepresentation of a tethered fly. d Cumulative time spent in each be-havior after the presentation of a tethered fly

Fly identification cues

Analysis of the wing-beat signatures of A. lindae(Fig. 4a) and Musca domestica (Fig. 4b) reveals that thefirst six frequency peaks and average amplitude (ener-gies) are disimilar (Table 2). A repeated measures

ANOVA revealed significant differences between themain effects: fly species (F1,28=20.438; P=0.0001), andfrequency peaks (F5,28=158.00; P=0.0001). In addition,the repeated measures term (frequency measure) for theANOVA was significant (F9,252=3.229; P=0.001), aswere the interaction terms frequency measure × fly(F9,252=3.065; P=0.0017), and frequency measure × fly ×peak (F45,252=1.531; P=0.0227). Further post-hoc com-parisons between flies at each peak revealed significantdifferences in wing-beat frequency at all six peaks(Scheffe’s S tests; all P values=0.001) with A. lindae sig-nificantly higher in component frequency than Metepeiraincrassata.

A two-way ANOVA comparison of all individual fliesat each peak also showed significant differences betweenthe fly species for wing-beat frequencies (all Ps=0.001).Further post-hoc Scheffe S tests showed the differencesto be primarily caused by between-individual variationamong A. lindae, and between-species variation for Musca domestica and A. lindae. For all six peaks, the A. lindae exhibited more variation in wing-beat frequen-cy among individuals than did Musca domestica Thepredatory fly A. lindae had, for the most part, signifi-cantly higher wing-beat frequency, although there wassome signal overlap between the two fly species.

For measures of signal energy or amplitude, the re-peated measures ANOVA revealed no significant differ-ences for the main effect fly species, or the repeatedmeasure term (amplitude measure). There was a signifi-cant difference for the other main effect, peak(F5,28=4.653; P=0.0033), and for the interaction term,amplitude measure × fly (F9,252=7.202; P=0.0222). Fur-ther post-hoc tests revealed the source of these differ-ences. There were significant differences between A. lin-dae and Musca domestica for wing-beat amplitude atpeak no. 5 (Scheffe S test; P=0.001), and significant dif-ferences among and between individual flies of both spe-cies at peaks 1, 2, 3, 5, 6 (Scheffe S tests; all P’s<0.05),although there was no apparent pattern among these dif-ferences.

We tested whether spider recognition of A. lindae isbased on wing-beat vibrations with several audio play-back and fly presentation experiments in the field. Spiders responded significantly more often (McNemar’stest: χ2=20.04, P<0.001) to presentations of speakersplaying a recorded A. lindae wing-beat signal (22 of 27presentations) than they did to a silent speaker (none of

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Fig. 4a, b Spectrograms showing frequency (Hz) and power am-plitude (nJ/Hz), and oscillograms for representative: a Arachnid-omyia lindae, and b Musca domestica wing-beats. Numbers abovepeaks in the spectrogram indicate those selected for statisticalanalysis

Arachnidomyia lindae Musca domestica

Frequency Energy Amplitude Frequency Energy Amplitude

Peak 1 (Hz) 227.47±6.85 8.43±1.37 179.95±0.90 10.79±2.25Peak 2 (Hz) 477.14±10.68 5.74±1.09 374.96±3.93 4.54±1.01Peak 3 (Hz) 717.91±16.04 2.28±0.28 571.17±6.39 1.59±0.26Peak 4 (Hz) 935.74±22.18 1.19±2.92 802.65±12.93 0.681±0.12Peak 5 (Hz) 1.22±0.027 1.22±0.27 1.02±0.02 0.48±0.02Peak 6 (Hz) 1.38±0.33 0.553±0.73 1.20±0.02 0.56±0.09

Table 2 The frequency (±SE)in Hz (peaks 1–4) or kHz(peaks 5–6) and amplitude(±SE) in nJ/Hz for the first sixpeaks in the wing-beat frequen-cies (as in Fig. 4) of Arachnid-omyia lindae (n=4) and Muscadomestica (n=4), based uponten wing-beat sound samplesrecorded for each individual fly

those same 27 spiders responded). In an additional experiment, spiders did not respond differently to pre-sentation of a live tethered A. lindae (15/18) versusspeakers (15/18) playing a recorded A. lindae wing-beat(McNemar’s test: χ2=0.25, P>0.5). Additionally, spidersdid not respond at all (0/10) to the presence of eitherdead A. lindae or Musca domestica flies (McNemar’stest: χ2=0, P>0.90), eliminating recognition based onchemical cues as a possibility.

Guarding behavior

In our study, egg-sac guarding had a significant effect onthe survival of Metepeira incrassata eggs, a result simi-lar to previous studies (Rayor and Uetz 1990). Egg-sacswhich had females removed suffered significantly higherrates of predation (11 of 26; 42.3%) than those for whichguarding females were present (5 of 30; 16.6%) (Loglikelihood test, Williams correction; Gadj=4.39, P<0.05).

Ploy–counterploy behavior – signal line cutting

During repeated and prolonged attacks on individualsguarding egg-sacs, flies appeared to use an aggressivemimicry ploy by vibrating the hub of the web (to simu-late captured prey and distract the attention of the spider). This behavior was seen in 28 of 32 naturally-oc-curring attacks in 1990 (87.5%), and in 15 of 20 attacksobserved in 1992 (75%). There were no differences be-tween years in the frequency of these behaviors(χ2=0.61; P>0.4). In response to web vibration by A. lin-dae, spiders immediately rushed to the hub and pluckedthe web to locate prey (100% of observations). In re-sponse to repeated web vibration, some spiders exhibiteda counter-ploy, signal line cutting behavior, in 8 of 32(25.0%) natural fly attacks in 1990, and in 4 of 20 (20%)of attacks in 1992. There were no differences betweenyears in the frequency of these behaviors (χ2=0.01;P>0.9).

In experimental studies, signal line cutting behaviorwas observed in 5 of 13 (38.46%) presentations of teth-ered Arachnidomyia, and in 0 of 16 (0.0%) presentationsof tethered Musca. The frequency of these behaviors wasnot independent of fly stimulus type (reject H0), as atwo-by-two contingency test revealed a significant dif-ference (Log likelihood test, Williams correction;Gadj=7.363, P<0.01).

Our test of the effectiveness of signal line cutting be-havior in reducing distracting vibration (predator vibra-tion at the egg-sac → prey vibration at the hub of the orbweb → manual cutting of the signal line) supports thehypothesis that these are ploy and counterploy behaviors.All spiders responded appropriately to attack by tetheredA. lindae (grooming and shuttle behaviors) both before(14/14) and after (14/14) the signal line was cut (McNe-mar’s test: χ2=0.00, P>0.9). However, while experimen-tal cutting had no effect on spider response to attack by

tethered A. lindae it did have a significant effect on spi-der response to prey-like vibrations in the web (McNe-mar’s test: χ2=16.9, P<0.001). All spiders respondedpositively to to the presentation of vibrating artificial“prey” in the web before the signal line was cut (14/14),running out onto the web and and attempting to captureit. Once the signal line was cut, however, none of thespiders responded to further stimulation (0/14), suggest-ing that signal line cutting is an effective counter-ploybehavior.

Discussion

For the colonial orb-weaving spider Metepeira incrass-ata, the risk of egg-sac loss as a consequence of preda-tion by the sarcophagid fly Arachnidomyia lindae in-creases with group size (Hieber and Uetz 1990), but maybe offset by spider defense behaviors at the individuallevel. Our work suggests that interactions between thefly predator and spider prey include elements of “ployand counterploy” behavior (sensu Dawkins and Krebs1979) and reciprocal selection. While a convincing dem-onstration of coevolution in these species would requirea more thorough examination of interactions from aphylogeographic perspective (Thompson 1999), our ob-servations do provide at least some evidence of adaptivespecialization and the potential for coevolution as sug-gested by Janzen (1980), Endler (1991) and Soler andSoler (2000).

The specialized fly predator A. lindae uses a specificset of behaviors to attack spiders guarding egg-sacs,traplining through colonies and attacking guarding spi-ders repeatedly. In addition, it forages preferentially inthe central core of colonies, where the greatest concen-tration of females and egg-sacs are found (Rayor andUetz 1990, 1993). In response, M. incrassata hasevolved the ability to identify this fly predator using vi-bration cues, and use a specific set of behaviors againsttheir attacks. These abilities are of particular importancefor colonial Metepeira species, as they may have to dis-tinguish between prey, parasites and predators fromwithin the same taxa (Spiller and Schoener 1989; Hieberand Uetz 1990; Uetz and Hieber 1994, 1997; Rayor andUetz 1990, 1993; Uetz et al. 2002).

Spiders in several taxa, including araneid orb weavers,show sensory capabilities that allow detection and dis-crimination of airbone sound. These capabilities includedirectional sensitivity to near-field airborne vibration(Görner 1965; Görner and Andrews 1969; Harris andMill 1977), discrimination of sound at lower frequenciesaround 50–250 Hz (Frings and Frings 1966; Görner andAndrews 1969; Finck 1969; Reissland and Görner 1978),and the ability to perceive airborne sounds for up to20 cm in some cases (Görner and Andrews 1969; Barth1982). At these frequencies, the orb web plays no role intransducing sound (Frings and Frings 1966).

The sonic “fingerprint” of A. lindae has a number ofdefinite peaks, and the first two in particular have rela-

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tively large amounts of energy associated with themwhich may be useful for discrimination (see Fig. 4a).Our experiments with tethered flies, and the equal re-sponses of spiders to speakers playing a recorded flywingbeat or live tethered flies, suggest that some compo-nent of the fly wingbeat is the cue used for fly discrimi-nation. The spiders’ lack of response to dead flies pro-vides further evidence that sound alone, and not a visualor chemical cue, enables fly recognition. Spiders facedwith wasp predators hovering at the retreat, which gener-ate powerful and distinctly different wingbeat vibrationsthan either A. lindae or Musca domestica, (53–115 Hz;Uetz et al. 2002) behave completely differently (Rayor1997; Uetz et al. 2002).

Once spiders have identified the fly, they use a specif-ic set of behaviors to prevent this predator from larva-positing on the surface of the egg-sac, and during and af-ter an attack, to locate and remove any potential larvaethat have been deposited. The fly is able to circumventspider guarding and grooming behavior with a preymimicry ploy to lure the spider off its egg-sac. At somepoint during most natural fly attacks, the fly landed oneither the web-hub or the connecting signal line and vi-brated it. The effect was always the same; the spidermoved off her egg-sac(s) and down the signal line,sometimes all the way to the orb-web, checking for prey.In many cases, once a spider had moved onto her web,we observed the fly to immediately move onto the egg-sac, presumably to larvaposit.

Circumvention of egg-guarding by prey mimickingbehavior in flies may have evolved because of a “cruelbind” in cost-benefit trade-offs faced by female spiders.Females must incur guarding costs to protect current re-productive investment, but if they do so exclusively, theyreduce future fitness gains by not catching prey. If theyinvest in capturing prey to the exclusion of egg-sacguarding, they may increase future fitness benefits bygaining biomass (subsequent egg production), but losetheir current reproductive investment through loss toegg-sac predators. In addition to these trade-offs, spidersmust be vigilant against other predator species and con-specifics stealing prey from their webs, and respond toneighbors attempting to encroach on or evict them fromtheir territory. Presumably, it is this conflict that flies ex-ploit in their attempt to gain access to the egg-sac.

The spider, however, should be under strong selectionpressure to counter the fly’s behavioral ploy, and appar-ently has, with the behavioral counter-ploy of signalthread cutting. While signal thread cutting is not seen inall spider-fly interactions, our experiments demonstratehow effectively this behavior eliminates potentially con-flicting vibratory information, as all spiders responded toweb vibration until we cut the signal thread. During nat-ural attacks, prey-mimicking behavior is used by the flyuntil the spider cuts the signal line, at which point thebehavior is abandoned. Once the attack is over andgrooming of the egg-sac has ceased, the signal line is al-ways reconnected to the orb-web by the spider. The sig-nal line cutting experiment further supports an airborne

rather than web borne vibrational cue for predator identi-fication; cutting the signal line had no effect on recogni-tion – spider response to fly attacks was the same.

While coevolved relationships are common inhost–parasite systems and host-brood parasite systems,true coevolution appears to be extremely rare in preda-tor–prey systems for a number of reasons (Endler 1991):(1) most predators prey on several species which makesit difficult to specialize on any one species; (2) predatorsmay switch to alternate prey at low prey densities; (3)predators usually encounter generalized defenses andevolve to counteract them since more specialized preydefenses do not show up until late in the process; (4) thelife/dinner principle (Dawkins and Krebs 1979) impliesstronger selection for prey defense than for predatorcounter-defense; (5) the fact that predators have lowerpopulation densities than prey, resulting in slower ratesof response to directional selection than the prey; and (6)a high generation rate of the prey relative to the predator,with resulting faster rates of evolution for the prey.

This predatory fly/spider host system differs in anumber of ways from those suggested by Endler (1991).While A. lindae is referred to as an egg-sac predator, it isthe fly larva, and not the fly itself, which is the actualpredator. As such, the system more resembles a host-par-asitoid system, where the adult parasitoid makes hostchoices, but it is the developing larvae which ultimatelykills the host. Equally important, this fly appears to bespecialized on this spider species (or at least on colonialMetepeira in general), as it is a recently-described spe-cies only found in this context. We have never observedthis fly harassing spiders of other species or attackingegg-sacs of any other species (except colonial Metepeiraatascadero in desert/mesquite grassland; Hieber andUetz 1990). It is doubtful that prey are ever at lowenough densities to cause prey switching. Metepeiraincrassata reproduces continuously, and all colonies (except very new ones) contain some females with egg-sacs. In addition, the flies are adept at locating and utiliz-ing small colonies (Hieber and Uetz 1990). There is nolife/dinner asymmetry (Dawkins and Krebs 1979), asboth are gambling off-spring (fly larva vs spider eggs) inthe interaction rather than themselves [flies do not para-sitize spiders, and are almost never captured (1 in 600+observed attacks)], suggesting that selection pressureshould be approximately equal for both participants. Fi-nally, the generation time for the spider is 4–5 times thatof the fly. While the population density of the flies isconsiderably lower than their prey, their rapid generationtime coupled with continuous host reproduction and thesubsequent constant availability of egg-sacs in the envi-ronment suggests that this predator has the capability toevolve at least as fast as its more common host.

As mentioned before, testing specific hypotheses re-garding specialization and coevolution of these speciesand attempts at historical reconstruction of the relation-ship require examination of interactions from a phylo-geographic perspective (Thompson 1999) well beyondthe scope of this study. However, there are some insights

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available from other studies that allow us to speculateabout the level of specialization seen in this predatorprey interaction. The egg predator fly species A. lindaehas only been collected from egg-sacs of two colonialweb-building species in this genus, M. incrassata and M. atascadero (Hieber and Uetz 1990), which belong tothe same phylogenetic clade (Piel 2001). While one of us(G.W.U.) has observed several fly attacks on egg-sacs ofM. atascadero, most of the defense behaviors exhibitedby M. incrassata in this study (i.e., shuttle, groom) werenot seen, although “attack fly” behavior was noted. Wehave never observed attacks on nor collected A. lindaefrom egg-sacs of Metepeira spinipes, another extensive-ly-studied colonial orb weaver from this genus (see Uetz1985, 2001). Playback of the same audiotape of A. lindaewingbeat vibrations that elicited “shuttle” and “groom”behavior in M. incrassata resulted only in “startle” reac-tions from California populations of M. spinipes, butnone of the other defensive behaviors seen in this study(Uetz, unpublished data). As M. spinipes is a member ofthe Metepeira labyrinthea clade, which is basal to the M. incrassata clade (Piel 2001), and is widely distribut-ed throughout Mexico and California (and overlaps the geographic range of A. lindae, M. atascadero, and M. incrassata), this observation provides additional sup-porting evidence for some degree of specialization in the relationship between the predator and prey populationsin this study.

Predation and parasitism are important selection pres-sures on group-living species, and several group level at-tack-abatement strategies are well-documented (Hamilton1971; Turner and Pitcher 1986; Inman and Krebs 1987;Elgar 1989; Spiller and Schoener 1989; Wrona and Dixon1991; Brown and Brown 1996). A number of attack-abatement mechanisms are operative against a diversityof predators and parasitoids of the colonial web-buildingspider M. incrassata (“selfish herd” – Rayor and Uetz1990, 1993; “encounter/dilution” effects – Hieber andUetz 1990; Uetz and Hieber 1997; “early warning” ef-fects – Uetz et al. 2002). Previous observations havesuggested that predator species using specialized attackbehaviors elicit specific individual defense behaviors(Rayor 1997; Uetz et al. 2002). While further study willbe required before we can conclude with any degree ofcertainty that this novel spider and fly interaction is theresult of a coevolutionary arms race, results of this studyhave clearly demonstrated that selection pressures fromhighly specialized predators may result in evolution ofpredator-specific prey responses.

Acknowledgements We are terribly sad to have to inform readersof the untimely passing of the senior author and our dear friend,Dr. Craig Hieber, who deserves most of the credit for this re-search. As he passed away during the review process, the final re-visions were made by co-author Uetz, who accepts all responsibil-ity for any errors. The research reported here was supported byfunds from National Science Foundation grants BSR-8615060 andBSR-9109970 to G.W.U., a National Geographic Society grantno.4428–90 to G.W.U, C.S.H., and R.S.W, and a California Acad-emy of Science Exline-Frizzell grant to C.S.H. We thank the Mex-ican Government, Direccion Conservacion y Ecologia de los Re-

cursos Naturales, Direccion de Flora y Fauna Silvestres for per-mission to conduct this work in Mexico. We are grateful to AnaValiente de Carmona and family, Blanca Alvarez, and Lolita Alva-rez-Garcia for allowing us to conduct research on their properties.We are also appreciative of the work and assistance given by David Kroeger, Steve Leonhardt, Beth Jakob, Alison Mostrom,Andrea McCrate, Veronica Casebolt, and Rebecca Forkner in datacollection in the field. We are also grateful to Mark Elgar and twoanonymous reviewers for comments on the manuscript. Most im-portantly, we are grateful to our wives and families for toleratingour absence during periods of fieldwork.

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