Predatory behavior of the zebra spider, Salticus scenicus (Araneae: Salticidae) LAWRENCEM. DILL' Department of Biology, York University, Toronto, Ontario Received March 27, 1975 DILL, L. M. 1975. Predatory behavior of the zebra spider, Salticus scenicus (Araneae: Sal- ticidae). Can. J. Zool. 53: 1284-1289. The zebra spider (Salticus scenicus), a small jumping spider, orients towards prey detected by its lateral eyes whenever the angle subtended by such prey exceeds 5.5". The velocity of the prey is not involved in the determination of reactive distance, but only moving objects elicit orienta- tion. The probability that orientation is followed by stalking is a function of both prey size and velocity, but the effects of these parameters on reactive distance for stalking were not deter- mined. The zebra spider's stalk velocity declines progressively as it nears its (stationary) prey, and the probable optimality of this behavioral tactic is discussed. DILL,L. M. 1975. Predatory behavior of the zebra spider, Salticus scenicus (Araneae: Sal- ticidae). Can. J. Zool. 53: 1284-1289. Le saltique harlequin (Salticus scenicus) petite araignee sauteuse, s'oriente vers une proie repCree au moyen de ses yeux lateraux, lorsque I'angle soustendu par la proie depasse 5.5". La vitesse de la proie n'hfluence pas la distance ii laquelle I'araignke reagit, mais seuls les objets en mouvement entrainent I'orientation. La probabilite que I'orientation soit suivie d'une poursuite est fonction de la taille de la proie et de sa vitesse; cependant, les effets de ces parametres sur la distance a laquelle I'araignee commence sa poursuite n'ont pas kt6 determines. La vitesse de poursuite de I'araignee diminue progressivement lorsqu'elle s'approche de sa proie (station- naire); on discute des avantages probables de cette tactique. [Traduit par le journal] Introduction substrate, and pounces at the prey with its The zebra spider (Salticus scenicus) is a forelegs extended anterolaterally. If the strike jumping spider commonly observed hunting on is successful the prey is bitten, rapidly im- the exterior walls of buildings on sunny days. mobilized, and masticated. Having detected a potential prey anywhere in The objectives of the experiments described its extensive visual field, the spider wheels here were twofold. Firstly, an attempt was made around to face the object (a behavior referred to to define quantitatively the characteristics of as "orientation"), and fixates it with the large the stimulus situation which elicit orientation anteromedian eyes. It is likely that retinal and stalking by zebra spiders. Characteristics For personal use only. scanning movements are then carried out examined were prey size, prey velocity, and (Land 1969), enabling the spider to distinguish distance between predator and prey. The data edible prey from conspecifics and inedible ob- were collected to test the hypothesis that orienta- jects. A decision is made at this point either to tion occurs whenever the angle subtended by an attack the object, or to ignore it and resume object at the predator's eye exceeds some thres- searching. If the former, the spider stalks hold level, as found for the mantid Hierodula (=approaches) the prey, flattening itself to the crassa by Holling (unpublished data). This substrate during the later stages of the ap- emphasis on reactive distance for orientation proach. Stalks are usually oriented directly distinguishes the study from Drees' (1952) toward the prey, but if the prey is very large earlier attempt to define the key stimuli for relative to the spider, the approach may be prey capture in this species. circular, ending behind the prey (also noted by Secondly, filmed records of stalks were Gardner 1965, in Phidippus coccineus, another analyzed to determine the way in which stalk species of jumping spider). Once very close velocity varies with distance between predator Can. J. Zool. Downloaded from www.nrcresearchpress.com by Simon Fraser University on 12/03/12 to the prey, the spider attaches a drag line to the and prey during the course of the stalk. A model of avoidance behavior by prey (Dill 'Present address: Department of Biological Sciences. 1974) suggests that an optimally behaving Simon Fraser University, Burnaby, British Columbia predator should move progressively more slowly V5A 1S6. as it nears its prey. DILL: ZEBRA SPIlDER BEHAVIOR Methods All of the spiders used in this study were males, about 8-10mm in length. They were collected from buildings and fences at Black Creek Pioneer Village, Toronto, during a 5-day period in late June, 1974. Between experiments the spiders were housed individually in plastic petri dishes (supplied with water via a wick), and fed one vestigial-winged Drosophila melanogaster daily. The apparatus shown in Fig. 1 was used to present model prey to the spiders. A bead, painted flat black, was towed upwards, adjacent to (but not quite touching) the pole, towards a spider situated at the top. The size of the bead could be varied, and its velocity controlled by a transformer attached to a 6-V electric motor. At the beginning of a test the spider was placed into the chamber on top of the pole. The inside walls of this chamber were coated with Fluon, a teflon-like material (C-I-L, Toronto, Ont.) which prevents climbing. To leave the chamber, the spider had no choice but to exit via a hole in the bottom, adjacent to the front of the pole. As the underside of the chamber bottom was also fluon-coated, the spider invariably ended up on the pole immediately in line with the path of the model prey. In addition, owing to the placement of the chamber exit, the spider also ended up facing about 45" from the vertical. Thus, prey could be presented at about the same FIG. 1. Apparatus used to present model prey to the angular position relative to the spider's midline in each spiders. Legend: (1) chamber (coated with fluon) in trial. (According to Land (1971) the probability of an which spider was placed at start of experiment; (2) hole object at fixed distance being oriented to by jumping in chamber floor allowing spider to move onto (3) spiders of the genus Metaphidippus is greatest when the 0.95-cm-diameter pole; (4) black bead towed by (5) stimulus is presented 40-90" from the spider's midline.) lateral monofilament threads passing through (6) Emerging from the chamber to the pole the spider nylon bushings in (7) white plexiglass floor and ceiling usually remained stationary for a few seconds. It was of apparatus; (8) pulleys on (9) free axle (above) and drive during this period that the motor was turned on and the shaft (below) attached to (10) 6-V electric motor. "prey," previously situated 12cm below the top of the pole, began to move towards the predator. As soon as a recognizable orientation occurred the motor was stopped 2.1-mm prey at 1.0 cm/s, and 10 with a visually more and the bead ceased to move. There was no obvious complex "prey", a crude model of a fly (7.1-mm overall response to the sound of the motor. width) moving at same speed. The latter prey elicited The main experiment was conducted over a period of nine stalks and these data were pooled with the data For personal use only. 2 days, after the spiders had been in the laboratory collected earlier for the purpose of analyzing stalk 6-12 days. The 80 spiders used were deprived of food for velocity. 3 days before the start of the experiment. On day 1, 20 spiders each were presented with a 3.6- Results and Discussion mm-diameter bead at velocities of 0.5, 1.0, 1.5, and 2.0 cm/s. The velocities were presented in random order Orientation during the course of the experiment (0900-1700 h). On The size and velocity of the bead had no day 2, 20 spiders each were presented with beads of 2.1, consistent effect on the probability that the 3.6, 4.8, and 6.2 mm diameter, approaching at 1.0 cm/s. Owing to the difficulty of changing beads between trials, zebra spider would orient towards it during its all of the trials with one "prey" size were presented approach (Table 1). The overall proportion of sequentially, though the order of presentation of the orientations, compared with the results of four sizes was randomized (i.e. 20 x 3.6, 20 x 2.1, other studies of salticid spiders presented with 20 x 6.2, 20 x 4.8). Every trial was filmed from the model prey (Drees 1952; Gardner 1966; Land side with a Sankyo Super-8 camera and orientation distances measured from the films, using a PDQ I1 1971), indicates the adequacy of the present Motion Analyzer (Photographic Analysis Ltd., Toronto). stimulus situation. The same films were analyzed to determine the manner The distance from the prey at which orienta- Can. J. Zool. Downloaded from www.nrcresearchpress.com by Simon Fraser University on 12/03/12 in which stalk velocity varied with predator-prey distance tion occurred was not significantly affected by (estimated to the nearest 0.1 mm, aided by the 2-mm reference marks on the pole). However, since few stalks model velocity (p > 0.05)' lending support to were observed in the main experiment, an additional the angular size hypothesis. The angle (a) 20 trials were conducted on day 3: 10 trials with the subtended by the prey at the spider's eye at CAN. J. ZOOL. VOL. 53, 1975 TABLE 1 The effect of model size and approach velocity on percentage orientation, and on the percentage of orientations which were followed by stalks Diameter, Velocity, Percentage Percentage Day nun cm/s orientation stalks 3 3.6 0.5 35 33 1.O 75 27 1.5 50 30 2.0 70 7 2 2.1 1.O 80 73 3.6 45 0 4.8 65 0 6.2 75 0 Total 61.9 17.6 For personal use only.