Anim. Behav., 1979,27,515-521

BEHAVIOUR OF THE KLEPTOPARASITIC SPIDER ARGYRODES ELEVATUS (ARANEAE, THERIDIIDAE)

BY FRITZ VOLLRATH Smithsonian Tropical Research Institute, P.O. Box 2072, Balboa, Canal Zone, Panama

Abstract. The theridiid spider Argyrodes elevatus lives as a kleptoparasite in the webs of two araneid spiders, Argiope argentata and Nephila clavipes, where it steals small insects from the orb web as well as pillages large prey-items that have already been caught and often predigested by the host spider. The prey-catching behaviour of the two host spiders and the stealing by the kleptoparasites are described. The success rate of raids on stored prey is given. Two raid strategies were observed in the klepto- parasites: either A. elevatus moved onto the hub during the first prey-catching sequence of the host (strategy PxP) or it moved onto the hub during the subsequent (second) prey-catching sequence (strategy PPx). Strategy PxP was employed more often than strategy PPx and the stealing succes with strategy PxP was superior. The host spiders react to raid-strategy PxP by reducing the duration of prey-catching sequences. The advantage to the kleptoparasites of switching between strategies is discussed.

Most araneid spiders build webs much larger than themselves which they operate in favourable locations for considerable periods of time (Enders 1975; Robinson & Robinson 1976). Such webs have a relatively high cost in raw materials and construction energy (Peakall & Witt 1976; Prestwich 1977). The capture of entangled insects, however, requires little further energy expenditure (Peakall & Witt 1976). The spiders that operate webs may catch more prey than they can consume at once. This surplus food is stored for later use.

The relative immobility of the predatory unit (web and spider) and the storage of prey items, create conditions favourable to food piracy, i.e. kleptoparasitism (from Greek kleptein, to steal). A wide range of organisms steal from spiderwebs, either to supplement their diet or exclusively as specialists. Some scorpionflies (Mecoptera, Thornhill 1975), wasps (Hymen- optera, Jeanne 1972) and damselflies (Zygoptera, Vollrath 1977) may prey on the spiders in the web, but they may also remove large prey items and fly off with them. Michiliid flies (McMillan 1975; Robinson & Robinson, 1977) and the spider Curimagua bayano (Vollrath, in press) often sit perched on the body of the host, waiting for him to catch and predigest prey. They will then share his meal.

Theridiid spiders of the genus Argyrodes have evolved into kleptoparasites of a wide range of araneid spiders (Exline & Levi 1962; Brignoli 1966). Being spiders themselves, they are pre-adapted for walking on silken threads and have highly developed vibration receptors (see Witt 1975). Argyrodes living as klepto- parasites on araneid webs may: (i) remove small

insects from the web that have not been attacked by the host spider (because they are either ignored or undetected); (ii) feed on the same prey-item as the host (share food); or (iii) remove from the host-web stored insects that have been attacked and immobilized by the host. All three types of behaviour have been mentioned for the kleptoparasites of Argiope and Nephila sp. (Robinson & Robinson 1973).

Physically removing large prey-items that have been attacked and subdued by the host may well be the most advanced type of food piracy. It is certainly the type that most merits the appellation kleptoparasitism and it is the subject of this paper. I studied the behaviour of the Theridiid kleptoparasite Argyrodes elevatus in relation to two hosts, the araneids Argiope argentata and Nephiia clavipes. The two host species differ markedly in the nature of their webs and in their predatory behaviour. In particular Argiope argentata stores prey in the prey-capture area of its web, whereas Nephila clavipes stores it only at the hub. This difference led Robinson et al. (1969) to suggest that Argiope might be more susceptible to klepto- parasitic attack than Nephila.

Materials and Methods The behaviour of stealing stored prey-items was studied in Panama with adult females of Argy- rodes elevatus Taczanowski (1872), which seem to be restricted to webs of female Argiope argentata Fabricius (1775) and Nephila clavipes L. (1767) (both Araneidae). The host spiders were housed in rectangular wooden frames (A. argentata: 50 x 35 x 15 cm; N. clavipes: 70 x 60 x 20 cm; for host sizes, see Fig. 4, Plate VII)

515

516 ANIMAL BEHAVIOUR, 27, 2

that could be covered with glass doors. Argyrodes elevutus females of unknown experience were collected from Nephila webs in the wild and were maintained separately for three experiments in host webs built in the frames. In addition, 12 A. elevutus were kept for 6 to 10 experiments on a single host-web to investigate the sequences of stealing behaviour over several trials.

The experiments were conducted daily and consisted of two presentations of prey to the host spaced 15 min apart. For the purpose of studying the stealing success of A. elevatus, the host spider was often lured from its resting position at the centre of the web (the hub) for a third prey-run.

Since the prey-insects presented were often digested by the host in 30 min or less, it was necessary to present the prey in rapid succession. This is not unnatural, since spiderwebs in the wild often contain more than one stored prey- item. At times the main prey-insects for Nephila and Argiope are Hymenoptera of the same size as the experimental prey (Robinson t Robinson 1970; Robinson & Robinson 1973).

In each experiment either live prey (Polybiu occident& wasps, weight 2 = 7.6 f 1 mg) were allowed to fly into the orb web, or dead wasps were presented on the tip of an electric vibrator moving at a frequency of 50 Hz with an ampli- tude of l-5 mm (Vollrath 1977). The moment the spider attacked, the vibrator needle was withdrawn. While the host spider caught the prey, the stealing behaviour of A. elevates was watched and the stealing process was noted. A videorecorder (Sony AV 3400) was employed for finer analysis of the stealing behaviour. The kleptoparasites were not allowed to feed on stolen prey longer than 15 min, to prevent satiation. The laboratory studies were supple- mented with field observations carried out in tropical monsoon forest (Barr0 Colorado Island) and second-growth habitat (Arraijan) (Vollrath 1976).

The terminology for describing the parts of an orb web follows Bristowe (1941, page 244). Accordingly, the viscid spiral zone, the capture area, of the web will be called the orb, and the web centre covered by non-sticky spiral, the hub.

Prey-catching Behaviour of the Host Spiders The webs of both host spiders are only similar insofar as both use the orb as the capture area. Adult Argiope females build a simple wheel-like structure, comprised of ca. 30 radii and 40 turns of the sticky spiral within a diameter of 30 to

60 cm. The Argiope web is a fairly loose, open- spaced web. The Nephila orb in contrast is fine meshed, with about 150 radii and 160 spiral- turns in 40 to 65 cm diameter. The hub of the Nephik web is located eccentrically in the upper third of the orb and is surrounded by an exten- sive space- or barrier-web, consisting of irregular threads (Wiehle 1931). The prey-catching beha- viour of the two host spiders is, according to Robinson (1969), Robinson et al. (1969), Robinson & Olazarri (1971), Robinson & Mirick (1971) and Robinson 8z Robinson (1973), as follows: Argiope argentata aims at rapid immobilization of the struggling prey. Lepi- doptera are attack-bitten, but as a rule prey- insects (like the wasps) are attack-wrapped and then bitten, cut from the web and carried to the hub. Here the prey-item is wrapped again briefly, and hung on a thread from the hub. Additional prey items are not carried to the hub but left tied to the capture site after having been wrapped and bitten (Fig. 1, left). Prey the size of the wasps are carried to the hub in the chelicerae. The wasps are wrapped at the hub and fixed to it, hanging on a thread. Large prey-items are wrapped in the orb prior to transportation. In contrast, Nephila clavipes initially bites the en- meshed prey-insects, which are then cut or pulled from the web. Nephila, as opposed to Argiope, stores all prey at the hub. The potential prey of Nephila are insects that fly above the herb layer. This includes large- and medium- sized Coleoptera, Lepidoptera, Diptera and Hymenoptera (Robinson & Robinson 1973),

Fig. 1. The prey-catching and storing behaviour of Argiope argentata and Nephila clavipes (with two prey the size of a housefly) and the stealing raids of klepto- parasitic A. elevatus (K) are shown in a diagrammatic fashion (the diagrams are not drawn to scale). 6 = stored prey, + = prey is transported by the host to the hub, -= prey is not transported by the host to the hub but left at the capture site, wl = long wra , ws = short wrap, b = bite, Pt = first prey-item o ff ered, Ps = second prey-item offered.

VOLLRATH : K~PTOPARASITIC SPIDERS 517

but small and tiny Hymenoptera were also frequently observed in large numbers in the webs of both host spiders. The main prey-items of Argiope are (depending on the season): Hymenoptera (Trigona), Orthoptera, and Lepi- doptera (Robinson & Robinson 1970).

Pillaging of Argymfes elevatus The kleptoparasitic A. elevates builds no web for prey capture, but depends exclusively upon the host web for procuring food. They hang at rest outside the capture area (orb) of the host web either between the frame threads (Argiope) or in the barrier web (Nephilu) (Fig. 1). Fine signal threads laid down by the kleptoparasite connect A. elevatus to several radii and the hub. It can be assumed that vibrations in the web are transmitted to the kleptoparasite since I have shown that the prey-wrapping movements of the host trigger raids by A. elevatus females (Vollrath, in preparation).

Raids are adjusted to the prey-storing beha- viour of the host species: in Nephila webs the pillaging kleptoparasite searches at the hub, where all prey are stored; in Argiope webs the search is conducted both at the hub and in the orb web. The prey-searching behaviour of A. elevates is distinct from its other activities: the long front legs wave forward and sideways, continuously tapping the host threads while the animal moves along slowly.

Although experiments show A. elevatus to be perfectly palatable to the host spiders, they are rarely attacked and never caught. Violent vibrations like those generated by the host plucking the web or lunging forward caused the kleptoparasite to drop out of the web on its dragline. Thus an alerted host succeeds only in temporarily driving the kleptoparasite away from prey-packets, which it may later reclaim. Presumably to avoid stimulation of the host spider, the kleptoparasites move very slowly and carefully when the host is motionless. How- ever, when the host moves around in the web, they proceed more quickly. This is evident when the kleptoparasite is in the actual capture area of the Argiope web searching for stored prey. After locating a prey-packet in the Argiope orb, and after attaching securing threads to it, the kleptoparasite carefully cuts the sticky spiral connecting the prey-item to the adjacent radii. The radius or radii incorporated into the wrapped prey-packet by the host are also cut (for details of wrapping behaviour of Argiope see Robinson 8z Olazarri 1971). To avoid a

sudden release of tension in any cut thread, the kleptoparasite first fixes its own dragline to the web radius. Then this radius is cut while being held by the first legs. These legs are then slowly stretched, gradually releasing the tension in the thread. This behaviour may prevent the host being alerted to the kleptoparasite’s action.

Prey stored by the host at the hub is mostly stolen while the host spider is away catching more prey at the orb. At this time the host is unlikely to respond even to strong vibrations elsewhere in its web. In the Nephila web, experi- enced A. elevatus often search for prey-packets at the border between hub (non-sticky spiral) and orb (sticky spiral).

(In 70 measurements of stored Nephilu prey- items sampled during the experiments, 36 (51%) of the prey-items were suspended at the hub-orb border, 28 (40 %) between 0.5 and 2 cm off to either side, and only 6 (9%) further than 3 cm off. Since the legspan of female A. elevatus is 1.8 cm, the chance of detecting a suspended prey-packet at the hub is high when the klepto- parasite searches in this fashion.)

Kleptoparasites seem to sense the presence of live prey or prey packets from the vibrations generated by the prey or by the host during its capture (Vollrath, in preparation). These vibra- tions guide the kleptoparasite toward the hub or the prey. The prey-items are identified chemo- tactically (orientation by airborne chemical cues seems unlikely). Several times I observed the waving legs of a searching A. elevatus to miss the partly liquified prey-packet by only fractions of a millimetre without detecting it. If, however, the kleptoparasite touched the prey packet with its front legs, it moved onto the prey and fixed a thread to it. More securing threads were drawn, leading to points outside the plane of the orb. Host threads were then cut and the prey packet was transported along these securing threads. On several occasions, A. elevatus females took no longer than 1 to 2 s to find stored prey at the hub and 4 to 10 additional seconds to complete the theft. Prey up to ten times the weight of the kleptoparasite may be stolen in this way.

Both Argiope and Nephila respond as though they sense the loss of prey from the storage places (as observed by Robinson & Robinson 1973). They start searching around on the hub, constantly plucking the radii. Several times Nephilu was observed climbing into the barrier web after an intensive search at the hub, wildly shaking the threads. Occasionally NephiZa succeeded in recovering a stolen prey-item that

518 ANIMAL BEHAVIOUR, 27, 2

was thus set into swinging motion. However, in general, prey is only retrieved when the host spider perceives and intercepts the transporta- tion activities of the kleptoparasites prior to the cutting of all host-web threads.

The Stealing Success of A. elevafus The number of kleptoparasites in a single host web varies considerably with the season and the density of the host spider population (Robinson & Robinson 1973; Vollrath 1977), and at times it may be as high as 40 to 45 A. elevatus in one Nephila web, although there are usually 2 to 5 kleptoparasites per web, and not all Nephila or Argiope webs in the study area contained A. elevatus.

Due to the large size difference (see Plate VII) and difference in web-building activities between host and parasites, the energy requirements of the parasites are less than those of the host spiders and may be fulfilled easily by a few Drosophila-sized insects a day. But since A. elevatus also steal prey already caught by the host, often large amounts of food are removed in a single raid, comprising far more than the kleptoparasite is able to ingest. When a stolen prey-item has not been at least partly predigested by the host spider, A. elevatus is likely to aban- don it and search for another prey-packet. It seems that the size of the kleptoparasite does not allow it to predigest insects much larger than Drosophila; its reserves of digestive enzymes may not be enough to liquefy more than one limb of a bigger prey-item.

Argiope

Fig. 2. Circle diagrams (100%) showing the relative success of pillagin argentata and Nep x*

A. elevatus m the webs of Argiope da clavipes: 0 = prey-item was not

found by A. elevatus, T and TX = prey-item was stolen from the hub, TY = prey-item was stolen from the orb, S = prey was shared without an attempt to steal it, F = prey was found but the theft was prevented by the host, n indicates the sample size. Based on experiments with live and dead prey.

After a raiding kleptoparasite moves onto the hub and makes contact with the prey, it will no always attempt to carry off the prey-packe (Fig. 2), but might start feeding immediate11 at the storage site, thus often sharing it with the host (Fig. 2, S; see also Fig. 4, Plate VII).

Under the experimental conditions when deac as well as live prey were introduced into the webs, the kleptoparasites successfully locatec the prey-packets in 67 % of all trials (Fig. 2, 0) The actual stealing success was equally high il the webs of both host spiders (Fig. 2, T, TX, TY) although Argiope was able to recover sigmfi cantly (P < 0~001, ~2 2 x 2 contingency table more prey-items removed by the kleptoparasite than was Nephila (Fig. 2, F). In Argiope web, the kleptoparasites stole more often from the hub than from the orb web (Fig. 2, TX, TY) ant usually reached the prey-item stored in the or1 web only after first moving onto the hub (7: of 80 observations). Only rarely (8 times) did the kleptoparasite move directly from its resting posi tion in the frame threads into the Argiope orb il search of stored prey, without first orientating towards the prey from the hub. Nephila neve stored prey in the orb area of its web.

Experiments with Live Prey In 54 of the 247 experiments with Argiope a host, live prey were introduced to investigatl the influence of vibrations generated by the pre: insect which might aid A. elevatus in locating it, booty in the Argiope orb. Due to the wrap-bit! attack strategy of Argiope, the prey insect i active for several minutes after being caughl In these experiments with live prey, the ratio o prey stolen from the orb to prey stolen from th hub was reversed (Table I), indicating tha A. elevatus does i?nd live prey stored in the or1 more easily than it finds dead prey. The result of this experiment suggest that the stealin: success of A. elevates from Argiope in naturi (where all prey are initially alive) may be highe than in my experiments where dead prey wer usually given (192 out of 247).

Table I. Comparative Stealing S- of A. elevans ir the Argiope Web, De.pedng upon Whetk the Pre

Given to Argiope was PresenW Dead or Alive

me~~om TheZom

Live prey 10 (36’%) 18 (64%) Dead prey 70 (74 %I 25 (26%)

VAN DER POEL: STRETCHED ATTENTION IN RATS

PLATE VI

Fig. 1. Rat displaying stretched attention.

Van der Poel, Anion. Behov., 27. ‘7

VOLLRATH: IUEPTOPARASIIIC SPIDERS 519

Host webs in the wild that contain klepto- parasites often have more than a single in- dividual (Vollrath 1976), thus competition for food among the kleptoparasites might also lead to a higher rate of stealing.

Raid Strategies of A. e&w&s Two different strategies (called PxP and PPx) were observed when kleptoparasites are raiding webs. No apparent correlation between the use of either strategy by the kleptoparasite and the behaviour of the host spider was found in the experiments.

The strategies differ as to the timing of the kleptoparasite’s movement onto the hub with regard to subsequent prey-runs of the host spiders. In strategy PxP, the kleptoparasite proceeds carefully toward the hub after the host spider has carried and wrapped the first prey insect (PI). The kleptoparasite then waits for the host to leave the hub for another prey-item (Pz) before searching for the first prey (PI) on the hub. In strategy PPx, the kleptoparasite is alerted by the 8rst prey-capture (PI) of the host spider, but waits outside the orb near its resting position for the second prey-run (Pz) of the host. As the host moves for Pz, the kleptoparasite rushes onto the hub where it immediately searches for the first prey-packet (PI). Thus, in PxP it moves onto the hub after the first prey run of the host, in PPx it proceeds onto the hub after the second prey run.

The experiments showed that the klepto- parasites more often use strategy PxP than strategy PPx on a raid; this is significant (bino- mial probabilities table) in both host webs (P Q O-05) in the Nephila web, n = 87, and P < 0.01 in the Argiope web, n = 96). Equally, the stealing process of the kleptoparasites was higher when strategy PxP was used (Fig. 3). This was significant (P < O-05, n = 35) in the Nephila web; in the Argiope web the differ- ence was significant (P < 0.01, n = 13) only for the prey stolen from the hub. It was not significant for prey stolen from the storage site in the orb (n = 7, NS).

The sequence of strategies used by a typical kleptoparasite in successive experiments was : ABaaaAbAAbAa(PxP=A,PPx=B; the capitals indicate a successful raid). Table II shows data obtained in raid sequences of 12 A. elevatus females, maintained one per host web. The individual variability was too high and the sample size too small for firm conclusions, but the trend indicates that following an unsuccess-

ful raid strategy PxP is more likely to be used than strategy PPx, regardless of which strategy was used in the preceding raid. Only one of the 12 kleptoparasites used strategy PPx more often

%

60

50

LO

30

20

10

0

PXP PPX PXP PPX

Fig. 3. Raid strategies and stealing success of A. elevates in the webs of the host spiders Nephila ciavipes (n = 201) and Argiope argentata (n = 247) are shown. The columns above the letters PxP and PPx represent the relative ocau-rence of each strategy in the experiments. The striped bars show the relative stealing success of A. elevatus in the Nephila web, the stippled bars show the success in the Argiope web (tine grain-prey stolen from the orb, coarse grain-prey stolen from the hub); the numbers show the percentage of success with each strategy in the webs of either host spider.

Table II. Raiding Strategies PxP and PPx Employed by 12 A. elevutus in Sequenth~eqwjmenperlments in the Argiope

Strategy in previous

Strategy in subsequent experiments

experiments hrP Ppx

The exclamation mark indicates success in the previous experiment. The probabilities of the employment of either strategy in the subsequent experiment were calculated with the sign teat.

520 ANIMAL BEHAVIOUR, 27, 2

than strategy PxP and this animal had the lowest stealing success of all.

Disamion The lower stealing success of A. elevatus in Argiope webs as opposed to Nephila webs can partly be explained by the higher percentage of prey-items recovered by Argiope. The higher recovery rate may be due to a better monitoring by Argiope of its web which contains fewer radii than a Nephila web. In addition, in the Argiope web the radii are connected by fewer spiral threads and thus transmittance of vibratory signals should be better since there is less inter- ference by interconnecting fibres. Therefore, a comparison of the security of the two prey- storing methods (in the orb versus the hub) against theft cannot be made easily.

The complication is obvious when examining and comparing the results of feeding experiments with dead and live prey. In the Argiope orb, A. elevatus was more successful when live prey were given than when dead prey were given (Table I) indicating that struggling of the wrapped prey- insect facilitates the search of the klepto- parasite. Similarly, if either host fails to respond to prey moving in the capture web, kleptoparasites will inspect it. They occasionally even attack such prey, up to the size of the common housefly. The higher stealing success in the Nephila web and the differential raiding on orb or hub in the Argiope web might in part be attributed to the fact that the experimental A. elevates had all been collected from Nephila webs. Previous experience may thus have influenced the per- formance of the kleptoparasites, especially in the experiments with dead prey, where no vibrational clues indicated the presence of prey in the Argiope web.

The distinct prey-stealing behaviour and the high success rate of A. elevates indicate that they are kleptoparasites well adapted for living in the webs of the two host spiders. Assuming that traits which reduce the fitness of the individual are eliminated from the population through natural selection, we have to ask: ‘What is the survival value of strategy PPx which, in its immediate stealing success, seems to be inferior to strategy PxP?‘.

Preliminary time measurements of the prey- catching sequences of both host spiders were taken. They indicate that the mean time of an entire prey-capture by either host spider is shorter when a kleptoparasite is active on the hub before the host leaves it for a prey-run

(as it is when strategy PxP is employed). This might indicate that the host perceives the action of kleptoparasites on the hub and accordingly shortens the time it is away from the hub (where food is stored), thus cutting the time limit the kleptoparasites have for a raid. Observations of naive host spiders originally free of klepto- parasites showed that these behave more ‘ner- vously’ towards introduced kleptoparasites and often run to and fro between the struggling prey and the hub where the kleptoparasite is active. This behaviour occasionally leads to the loss of entangled prey. Not only the disappearance of stored (and remembered) prey affects the behaviour of the host spider, but vibrations generated by kleptoparasites moving in the web and on the hub might give the host an indi- cation of the state of the kleptoparasitic com- munity, the ‘load’, in its web. The kleptoparasites seem to control the size of the community by intrageneric as well as intraspecific aggression (Vollrath, in preparation). It is disadvantageous to a kleptoparasite to be in a web with too many conspecifics, because the antiparasitic behaviour of the host spider goes further than just shorten- ing its prey-capture timing: a study of the kleptoparasite load in several Nephila webs in the wild suggests that Nephila leaves its web-site as a response to a high number of Argyrodes (Vollrath 1977), thus drastically reducing the number of kleptoparasites in its new web (see also Robinson & Robinson 1976). The klepto- parasites then have to find a new host web. It can be assumed that the predation on hostless and wandering A. elevatus is fairly high, since they are not cryptic.

The experiments described in this paper only cover the stealing behaviour and success of a single A. elevates per host web. Since the klepto- parasite ‘load’, at least in a Nephila web, gene- rally consists of more than one A. elevates female, it is possible (and preliminary experi- ments with two A. elevates females in one host web confirm this idea) that the different strategies may be used alternating among several indivi- duals. Thus, in a web containing two klepto- parasites, usually both will not wait on the hub for the host to leave (and invariably chase each other in hostile encounters, thus alerting the host). Alternating strategies make it more difficult for the host to evaluate the number of associated kleptoparasites, thus reducing the frequency of web-site changes. The overall life- time success of animals using a mixed long-term strategy (varying between PxP and PPx) would

VOLLRATH: KLEPTOPARASITIC SPIDERS 521

be greater than those who used all PxP or all PPX.

Acknowledgments This paper is dedicated to the memory of Dr. W. S. Bristowe. I want to thank Dr Michael Robinson and Professor Dr P. Weygoldt for their encouargement during the project. Dr Robinson and Dr Donald Windsor carefully read and amiably commented on the manuscript. I am indebted to Professor Dr H. Levi for identifying the Argyrodes. The work was carried out while I held a GRAFt)F Fellowship to the University of Freiburg and was partly supported by funds of the Smithsonian Tropical Research Institute.

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(Received 2 May 1978; revised 2 July 1978; MS. number: 1758)