Michael H. Robinson and Heath Mirick.
The Predatory Behavior of the Golden-Web Spider Nephila clavipes (Araneae: Araneidae).
Psyche 78:123-139, 1971.

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PSYCHE
Vol. 78 September, 1971 No. 3
THE PREDATORY BEHAVIOR OF
THE GOLDEN-WEB SPIDER NEPHILA CLAVIPES
(ARANEAE : ARANEIDAE)
BY MICHAEL H. ROBINSON
AND HEATH MIRICK
Smithsonian Tropical Research Institute, P.O. Box 2072.. Balboa, Canal Zone (Panama)
Details of the role of wrapping behavior in the predatory activities of Nephila clavipes (Linnaeus) were given by Robinson, Mirick & Turner (1969). Their account also gave broad details of the total behavior of this species. At that time, the publication of an exhaustive account of the oredatorv behavior of N. clavites was in- tended and anticipated. Since then, however, the senior author has carried out studies of the behavior of other species of Nephila in Africa and Asia, and with co-workers is currently engaged in a study of the behavior and ecology of Nephila maculata (Fabricius) in New Guinea. It now seems appropriate for us to leave details of part of our work on N. clavipes for inclusion in a broad compara- tive paper and publish here those aspects which relate most directly to the main points cited in outline by Robinson, Mirick & Turner (1969).
This paper therefore presents a summary model of the predatory behavior of Nephila clavipes, based on the investigations of the present authors in the summer of 1968 and further experiments carried out by the senior author in 1969. "We give emphasis to the investigations and experiments that led to establishing some of the major aspects of the model and leave detailed descriptions of be- havior units and behavior sequences for inclusion in the projected comparative paper. We have also left consideration of the temporal aspects of the behavior sequences for inclusion in the later paper. Materials and Methods
Our basic observations and some of our experiments were car- ried out with captive adult female spiders. The spiders were not confined to cages but were released in a large screened insectary at



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124 Psyche [September
the Barro Colorado Island research station of the Smithsonian Trop- ical Research Institute. We maintained a minimum of fifteen spiders in captivity during the period of study in 1968. The experiments carried out in 1969 required a larger number of spiders and we used free-living spiders in the Barro Colorado forest as well as a number of captive spiders. Even with access to twenty or more non-captive spiders it was necessary to use some spiders more than once in an experimental series ; details of how these were manipulated to obviate the possible effects of experience is given below in the appropriate section. Captive spiders were collected from the same areas as the non-captives and all were of unknown age and previous experience.
We made repeated presentations of a number of different prey items in order to establish the basic patterns of the spider's behavior. This involved fifty presentations of each of seven prey types. These were chosen for their relevance to the natural diet of the spider (see later) and because they presented large differences in size, weight and type of activity after striking the web. We used the data obtained from these observations to prepare ethograms of the type used by Robinson & Olazarri
( 1971 ) and then used these etho-
grams as a basis for the integrated model. Fifty presentations of
each prey item meant that a proportion of the spiders received the same type of prey more than once. In general we presented but one prey per spider each day. In an attempt to avoid any possible effects of experience we avoided successive presentations of the same type of prey. In all cases at least two days (usually more) inter- vened between one presentation and the next presentation of the same type. Usually another type of prey, or several other types of prey, would be presented between presentations of the same type. In addition we made presentations of all the prey types to a large number of free-living spiders as a check on the behavior of our captive spiders.
In the course of our initial observational work we presented the following prey items: moths (living & dead), grasshoppers (living), crickets (living & dead), Tenebrio beetles (living), dragonflies (liv- ing), Trigona sp. (living), pentatomids (living), and blowflies (living). Later when we attempted to elucidate the stimuli which the spiders were capable of detecting at various stages in the process of predation we used a number of experimental techniques involving modified insects in the form of 'dummies'. These techniques are de- tailed in the appropriate section below. All the insects that we used were weighed and measured before being presented to the spiders. Where insects were presented dead



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19711 Robinson 3 Mirick - Nephila cla-vipes 125 they were killed by freezing to avoid contamination by chemical killing agents.
Behavioral observations were supplemented by cine- photography and the duration of behavior units was recorded on a Rustrak multi-channel chart recorder.
NATURAL HISTORY
In Panama, and wherever we have encountered Nephila clavipes in South America (Colombia & Venezuela), the spider is most fre- quently found at the edges of forest clearings, alongside forest trails and
across forest streams and watercourses. It thus apparently ex- ploits areas within the forest that are in all probability flightpaths of insects.
The structure, size, and siting of N. clmipes webs is consistent with the view that the spider specialises on prey that are in flight above the herb layer rather than moving about in it. Al- though the web may be sited with part of the prey capture area within the herb layer it is most frequently above this or stretched across gaps in the vegetation. The structure of the web has been described by Gertsch (1948) and Peters (1954, 1955). For its area, which is large, the web has a very fine mesh which is far less penetrable by small insects than the much smaller but coarse-meshed web of Argiope argentata (Fabricius), as was shown by Robinson, Mirick & Turner (1969).
The web of the adult spider is not a
complete orb but is U-shaped with the hub very close to the upper bridge thread.
Kaston & Kaston (1953, p. 176) give an excellent figure of the N. clavipes web. Webs of several adult spiders are often built in close proximity and fairly large aggregations of the spider may occur in apparently favourable areas (Shear 1970, has commented briefly on this phenomenon).
The adult web is frequently equipped with a barrier web con- sisting of a complex of strong lines arranged in a non-symmetrical manner above and/or below the main plane of the orb. There is considerable variation in the structure of barrier webs. Robinson & Robinson (1970) have suggested that they may function as early warning devices enabling the spider to detect the approach of pos- sible predators. Certainly the spider often responds to manipulation of the barrier web by escape or other forms of defensive behavior. Web renewal is not a daily occurrence, and in captivity the spider may only renew part of the web at a time. (This is also certainly
the case with free-living N. maculata but we have not made exten- sive observations on web renewal by free-living N. clavipes). Our observations on the natural prey of N. clavipes, although limited in scope, confirm the deductions based on web structure and siting i.e. that flying prey of small to medium size may be the



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126 Psyche [September
speciality of this species. We found flies, bees, wasps and small lepi- dopteran~ to be predominant in the prey found in webs and amongst the corpses that are occasionally suspended from the barrier web after they have been consumed by the spider.



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Robinson C5" Alirick - Nephiia cimif)e.s I. The basic pattern
Figure I is a summary diagram of the complex of ~ossible se- quences of predatory behavior that we have observed to be given by adult female N. ciavipes to a variety of prey presented in several ways. It has the same summary function that Figure 2 of Robinson (1969) had in relation to the behavior of Argiope argentata and is not meant to be a flow diagram or relate to any cybernetic con- ventions. It does, however, illustrate the temporal sequence and relationships between behavior units, their degree of association and the effects of some stimuli on the course of the sequence. Like most models it is certainly much more simple than reality. In the predatory behavior of N. clavipes alternative behavior units are available at several functional stages and the 'decision' to employ one or other of these is shown, on the diagram, to be the result of a discrimination. We recognise that in some cases our assumption of the basis (or bases) for the discrimination may be an oversimpli- fication; this matter is discussed at the appropriate place in the text. The diagram employs simple conventions. The behavior units are shown as circles connected by lines. The arrows on the lines
show the direction of change from one behavior unit to the next. In the upper half of the diagram two conventions are used to denote 'choiceJ points. Some of the circles are divided into two halves by vertical lines so that two behaviors may follow the behavior shown by the circle.
In addition, here and throughout the diagram, small square boxes on the lines connecting behavior circles represent places vrhei-e behavior may be switched from one course to another. Dotted lines from oblong boxes suggest imputs to the system that are de- pendent on stimulus properties of the prey. Behavior occurring up to first contact with the prey is shown in a much simplified form. The activity spider on hub (which could
be amplified as 'spider at hub in predatory position waiting for prey') is represented as an ongoing activity by a vertical line beneath the behavior circle. If a prey item, or in fact any item cbove a certain weight, strikes the web the spider is alerted (the square box has an iinput from prey strikes web and an output to spider alert, this represents a diversion from the ongoing activity spider at hub). The overt behavioral change following impact may be a momentary in- crease in the flexion of the spider's legs prior to an almost immediate movement towards the prey or a more sustained adoption of this alert position.




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I 28 Psyche [September
After the spider has been alerted two things may occur. If the prey is in a state of sustained vibration the spider almost always makes an immediate approach to it. If on the other hand, the prey is immobile or merely making spasmodic movements the spider usually plucks the web. If the prey has remained in the web until the spider plucks, this behavior is likely to be followed by an approach to the prey (during which the plucking movement may be repeated). When the prey escapes after the initial alerting impact, the spider usually returns to its pre-alerted state after plucking. As examples of the simplification involved in our model we can cite the following examples that occur in the fairly complex section that we have traced so far. Thus a further alternative behavior that can occur on the impact of prey is not shown. Instead of the spider being alerted it may show an immediate escape response. This usually takes the form of the spider running upwards from the hub, out of the web and onto the support lines (or even onto nearby vegetation). Similarly, if it did not result in an illegibly complex diagram, we could show at least three distinct approach-to-prey behaviors. There is a rapid, unhestitant, approach that is made to rapidly vibrating insects and a hesitant, much interrupted by pluck- ing, approach to non-vibrating insects. In addition very large or very heavy insects are approached in a slow 'deliberate' manner, in which legs I & I1 are flexed far back over the prosoma in a very characteristic 'cautious' gait.
After arrival at the prey the spider may immediately attack with- out a perceptible pause, or may touch (with the tarsi) and palpate (with the pedipalps) the prey, before attacking. We have not shown these 'investigative' stages in our diagram. There are three basic attack behaviors, all of which involve the use of the chelicerae. In no case have we ever seen NephiZa clavipes (or any other species of Nephila, for that matter) use the strategy of attack wrapping. This matter is extensively discussed by Robin- son, Mirick & Turner (1969). The three basic forms of biting attack are as follows:
I. An attack similar to the seize and pull out behavior of Argiope argentata (see Robinson & Olazarri, 1971). This is given to very small or light prey.
2. A long bite in which the bite is not immediately followed by pulling out movements but is sustained in sit,u. This long bite may be accompanied by a special posture in which legs I & I1 are raised off the web and the opisthosoma is raised at its apex so that the body presents a concave dorsal aspect.




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19711 Robinson ‰â Mirick - Nep hila cZaz4pe.s 129 3.
The third attack strategy we call bite &? back off. It consists of a rapid forward lunge, a short-duration bite and a rapid with- drawal to a distance at which the prosoma is well away from the prey. We have shown this behavior as a repeatable unit since the lunge, bite, retreat sequence may be repeated a large number of times
(we have records of over 12 repetitions) before a sustained long bite ensues. In many cases, when this form of attack is em- ployed, the spider can be seen to open the chelicerae until they are almost held horizontally, before making the rapid forward lunge that terminates in the bite.
The type of attack strategy that the spider employs seems to be largely determined by parameters of weight and/or size of prey; our experimental analysis of these factors is described later. We have not been able to determine what factor or factors mediate the decision to cease repetitions of bite & back-off and commence sus- stained biting. This process is not dependent on reduction of activity by the prey (that might be consequent on a series of bites) since it occurs in the spider's behavior towards large dead prey. Behavior following the initial attack phase is somewhat more complex than is the case with araneids that are efficient at enswathing their prey in silk. Attack is almost always terminated by pull-out movements. The body of the spider is lowered, on flexed legs, during the biting attack and pulling out consists of strong extensions of the leg pairs. These result in the spider pushing down on the web and pulling up on the prey. Very small prey, adhering to a small area of viscid spiral, are quickly freed, as are lepidopterans which do not adhere strongly because of their loose wing scales (see Eisner et al 1964). Other prey may be subjected to repeated pull out movements before being freed. Prey that are not readily freed by pulling movements are wrapped in the web and then cut out by the spider. Robinson, Mirick & Turner ( 1969) called this wrapping at the capture site, Type I. We have been able to show that the spicier can be induced to wrap prey (in this way) if its pulling out attempts are blocked experimentally (see later). It seems possible that the pull out movements enable the spider to gauge the degree of adhesion of the prey to the web and that the 'decision' to continue pulling, or to wrap in situ and then cut out, is influenced by this information. A further complication arises from the fact that the spider may wrap, at the capture site, prey that have already been freed from the web by pulling (Post-im7nobiZization wrapping at the capture site. Type 2, of Robinson, Mirick & Turner 1969).



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1 30 Psyche [September
(The functional interpretation of these behaviors is that pulling prey from the web results in less web damage than wrapping in situ and subsequent cutting out. This can easily be substantiated by com- paring the web damage resulting from the two techniques of prey- removal. Wrapping after removal from the web has a trussing effect which reduces the bulk of the prey and may facilitate its transportation to the feeding site.)
After the prey is freed from the web, and trussed in some cases, it is transported to the hub of the web. Here the spider employs one of two techniques. It may either carry the prey held in the cheh'cerae, perhaps supported by one or both of the first legs, or it can carry the prey package on a silk thread hanging from the spin- nerets and supported by one or both of the fourth legs. Prey carried to the hub in the jaws are wrapped on arrival at the hub and then suspended from the hub silk as the spider turns to assume its preda- tory posture. Prey that are carried suspended on silk are not wrapped on arrival at the hub but are suspended. Nephila cZavipes does not store prey at the capture site but carries all prey to the hub where it is hung until previously caught prey are consumed. Very small prey, carried to the hub in the jaws, may not be wrapped on arrival (but prey as small as stingless bees - 10-30mg in weight - are regularly wrapped on arrival at the hub). The return to the hub from the capture site is carried out in a forwards direction after the spider has turned to face the hub at the capture site. (Nephila nzacuZata frequently backs slowly up the web, without turning, when carrying prey in its jaws.) When the spider has assumed its normal head-down predatory position it may undertake more or less extensive grooming activities before taking up the prey in its jaws and anterior legs. These are very similar to those that Robinson & Olazarri (1971 ) described for Argiope argentata. Prior to the commencement of feeding the spider often carries out extensive manipulations of the prey during the course of which small bites are given to region after region of the prey body.
We have a few records of prey being wrapped after removal from the web and then being transported in the jaws. Most prey that are trussed in this way are then carried suspended on silk behind the spider. Heavy prey are also carried in this way, but as in the case of Argiope argentata, the weight threshold for the changeover from rarry in jaws to carry on silk varies from individual to individual, and from time to time within individuals (see Robinson 1969, p. 170-1).




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19711 Robinson S" Mirick - Nrpft-la dnviprs 13 1 2. Experimental investigations
To investigate the possibility that certain aspects of the predatory pattern are responses to relatively simple stimuli we carried out a number of investigations using 'modified' insects as dummies. We manipulated the size and weight of some insects and also modified such parameters as the strength of their adhesion to the web. We were able to compare the responses of the spiders, at various stages in the predatory process, to such dummies, using adequate controls or using their response to unmodified insects as a baseline. {rt)
The bite & hack-off attack behavior
Our observation's on the responses of N. clmipes to a range of prey items (st c page 123, above) showed that the bite & back-off attack behavior was only given to large and heavy prey. Since all of these that were presented to the spiders were alive there was a possibility that the response could be to size, weight or specific activity of large heavy prey, or a combination of any of these factors. Preliminary tests showed that the response was given to dead (im- mobile) prey so that although it remained possible that activity could enhance the response it was not the important stimulus. We then decided to manipulate the parameters of size and weight. Using small acridiicis (25-30mm, +p0-550mg) as prey, we added lead shot to some to double the weight (approximately), increased the length of others fay inserting a wooden tooth pick in line with the long axis, and with the third group we increased both the length and the weight. The dead dummies Were presented at right angles to the radii of the web. This ensured that the weight of prey was dis- tributed over as wide an area of web as was covered by the length of the insect (or the insect + toothpick). In fact it meant that the maximum dimension of the prey was at right angles to the spider as it approached across the web. The insects were vibrated elec- trically- at 250 cps until the spider left the hub on its predatory excursion. At that stage the vibrator was switched off so that the prey was motionless when the spider came in contact with it. The results of this experiment are shown in Table I. In addition to the form of biting attack we noted whether the spider raised legs I & II off the web during the attack (see page 127, above), and whether the prey was wrapped in situ, or free wrapped, after the attack. There was a significant increase in leg raising during the attack in the case of the weighted insects. There was a slight numerical, but not a statistically significant, increase in the number of attacks on the long dummies that involved leg raising. There were three attacks out of ten, on heavy insects, that involved the bite and back off behavior.



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132 Psyche
[September
Table 1.
Acridiids weighted and unweighted, lengthened and normal. Prey A Prey B Prey C
Behaviors.
Yes 4 1 1
1. Bite and back off.
no 6 9 9
Yes 9 4 2
2. Bite, legs raised.
no 1 6 8
Yes 7 7
3. Wrap in web.
no 3 3
Ten spiders chosen at random from wild population receive A, ten captive spiders B ten wild spiders C. A is acridiid with weight added and lengthened (total weight ca. 1000 mg length 70 mm). B is acridiid lengthened to 70 mm weight 400-550 mg. C is unmodified length 25-30 mrn weight 400-550 mg.
Statistical analysis - Fisher's exact probability: differences between A & B, A & C, B & C for Bite and back off are not significant; difference between A & B significance level 0.05, difference between A & C significance level 0.005, no signficant difference between B & C, all for Bite, legs raisled; for wrap in web difference between A & B and B & C level of significance 0.05.
All levels one-tailed
Although this number is not statistically significant we regarded the occurrence of this behavior in attacks on the weighted insects as being highly suggestive. We then carried out a further series of experiments, using similar sized acridiids, in which we quadrupled their weight.
We found that very heavy dummies of small size often dropped out of the webs before the spider reached them. We therefore in- creased the length of both experimental (very heavy) dummies and controls (normal weight) by adding the toothpick to the insect. This worked in a perfectly satisfactory way to distribute the weight ..over a greater number of web members.
As before, we vibrated
the dummies until the spider commenced its predatory excursion. The results are given in Table 2. There is a statistically significant effect of weight on the occurrence of the bite & back-off attack behavior. Note that we did not induce thi,s attack behavior in all the presentations; we consider that with large, heavy active insects there may be some heterogeneous summation. We attempted to test for the possible additive effect of activity, at the moment of contact



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19711 Robinson &? Mirick - LNephiZa clavipes 133 Tablse 2. Acridiids weighted and unweighted all lengthened to 70 mm. Behaviors.
Yes
1. Bite and back off
no
Yes
2. Bite, legs raised
no
yes
3. Wrap in web
no
4. Quit the web
Prey A
8
4
Prey B
2
10
Prey A weights 1700-1800 mg.
Prey B weights 400-550 mg.
Statistical analysis - Fisher's exact probability: Difference between A & B for bite
and back off, level of probability 0.025; difference between A & B for bite, legs raised, level of probability 0.01; difference between A & B for wrap in web not significant; difference between A & B for quit the web not significant.
All levels one-tailed
with the prey, by tapping weighted and control dummies from behind the web. (We did this when the spider was in tarsal contact with the prey and before it had attacked). This led to a numerical, but non statistically significant, increase in the number of bite & back