Fred Punzo.
The biology of the spider wasp, Pepsis thisbe (Hymenoptera: Pompilidae) from Trans Pecos, Texas. II. Temporal patterns of activity and hunting behavior with special reference to the effects of experience.
Psyche 101(3-4):243-256, 1994.

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THE BIOLOGY OF THE SPIDER WASP, PEPSZS THISBE (HYMENOPTERA: POMPILIDAE) FROM TRANS PECOS, TEXAS. 11. TEMPORAL PATTERNS OF ACTIVITY AND HUNTING BEHAVIOR WITH SPECIAL REFERENCE TO THE EFFECTS OF EXPERIENCE
Dept. of Biology
Box 5F
University of Tampa
Tampa, FL 33606
The temporal pattern of activity and hunting behavior of the spi- der wasp, Pepsis thisbe, from Trans Pecos, Texas are described. Females exhibited a bimodal pattern with peak periods of hunting activities occurring between 0800-0959 and 1600-1759 hr. The lowest levels of activity (1100-1359 hr) occurred when ambient temperatures exceeded 38OC. Activity declined abruptly after 1800 hr indicating a strong preference for diurnal activity in this species. The behavioral components of the hunting sequence consist of an initial approach and antennation of the host, grooming, antennation and paralyzation followed in some cases by lapping, oviposition, burial of the host, and closure of the nest. The time required to complete the hunting sequence was recorded for both field and lab- oratory encounters between a wasp and spider. The amount of time required by a female wasp to complete the initial approach and antennation-paralyzation sequences decreased significantly as a function of increasing encounter experience. The spider is stung most frequently (SO%) through the intersegmental membrane between the sternum and coxa of the first leg. Whether or not a wasp exhibits lapping behavior is determined by the amount of time spent in flight searching for a host burrow. Manuscript received 22 August 1994.




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244 Psyche [vo~. 101
The hunting behavior of female tarantula hawk wasps of the genus Pepsis (Hymenoptera: Pompilidae) has captured the imagi- nation of scientists and amateur naturalists since the early descrip- tions of Buckley (1862), Petrunkevitch (1926, 1952), and Passmore (1936). Wasps of this genus, as well as Hemipepsis, are common representatives of the desert arthropod fauna of the southwestern United States (Hurd, 1952; Punzo, 1995). The hunting behavior of Pepsis and Hemipepsis wasps has been traditionally viewed as an excellent example of instinctive behav- ior (Petrunkevitch, 1926; Williams, 1956; Cazier and Mortenson, 1964) comprised of genetically pre-programmed, rigid behavioral acts. However, recent studies on Pepsis formosa (Punzo and Gar- man, 1989; Punzo, 1991) have shown that there is some behavioral plasticity exhibited by the female wasp during an encounter with its host. Its hunting behavior can be analyzed in terms of discrete behavioral acts (Punzo, 1991): 1) initial approach and antennation of the spider (AP); 2) moving away and grooming (MG); 3) anten- nation and paralyzation (AnP); burial and oviposition (BO); and closure of the nest (C). The amount of time required by the wasp to complete the entire hunting sequence decreased significantly with increasing number of encounters; however, only the AP and AnP components improved with experience (Punzo and Garman, 1989). The purpose of the present study was to describe the hunting behavior and temporal pattern of activity for Pepsis thisbe Lucas from the Trans Pecos Region of Texas. Little information is avail- able for this species. In this region, Pepsis thisbe females selec- tively hunt the theraphosid spider, Aphonopelma echina Chamberlin (Orthognatha: Theraphosidae). This spider is primarily sedentary and occupies burrows constructed in sandy or adobe soils (Punzo, 1988, 1989).
Studies were conducted on wasps located within a 7-km radius of Study Butte (Brewster Co., Texas; 32åÁ24'N 103' 12'W; eleva- tion: 920m), located within the northern region of the Chihauhuan Desert. A detailed description of the study area is given by Punzo (1995).




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19941 Punzo 245
Adult wasps were collected using light traps and sweep nets and maintained individually in clear plastic Drosophila vials where they were fed a mixture of honey and water. Host spiders were trapped at night at the entrances to their burrows as described by Punzo (1989). Some spiders were transported back to my labora- tory and housed separately in plastic cages. They were maintained on a diet of grasshoppers and crickets.
I collected eggs and pupae of P. thisbe from the burrows of par- asitized spiders as described previously by Punzo and Garman (1989). Emergent adult males and females were placed together in 100-gal aquaria and allowed to mate in order to obtain gravid females. All female wasps used in encounter experiments under field and laboratory conditions had no prior experience with host spiders and were described as naive. Statistical methods used throughout this study follow those described by Sokal and Rohlf (1981).
I determined the temporal pattern of activity for hunting in this study area. Whenever a female wasp was observed actively search- ing on the ground, it was collected with a sweep net and the time of day (CST) was recorded. Similar data were recorded for wasps observed in an actual encounter with a host spider. Data were col- lected for a total of 315 wasps and the results expressed as the per- centage of wasps engaged in hunting behavior at various times of the day (2 hr intervals; i.e., 0400-0559, etc.). To describe the hunting behavior of this species, encounters between naive female wasps and their hosts were recorded on film, both in the field and laboratory, using a Nikon FE2 35mm camera and a Panasonic L3 video recorder. I used female wasps (body length: 3 1.3-35.1 mm that had no previous encounter with a spider reared from larvae. For field studies, a previously located spider burrow was covered with a spherical, dome-shaped cone 60 cm in diameter and 30 cm in height, constructed of fine-mesh aluminum window screening and provided with an opening on top into which a Drosophila vial housing a wasp could be inserted. The dome was positioned so that the entrance to the burrow was located in the center of the dome. The presence of silk covering the burrow entrance is a reliable indication that the burrow is occupied by a spider (Punzo, 1989). To initiate an encounter, a vial containing one female wasp was placed in the opening at the top of the dome



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246 Psyche [VOI. 101
and the wasp was allowed to enter. A Timex stopwatch (L210) was used to determine the amount of time required for the wasp to per- form various behavioral acts of the hunting sequence. Ongoing studies had indicated that most wasps in this area exhibit hunting behavior between 0800-0959 and 1600-1759 CST and so encoun- ters were staged during these time intervals. Each wasp was sub- jected to encounters with 10 hosts to determine if there was any improvement in performance as a function of increasing encounter experience. Analysis of data was restricted to only those encoun- ters between wasps (N = 27) and hosts within a certain size range (7.4-8.1 g) in order to minimize any confounding effect of host size on hunting behavior.
The chamber and procedure used to analyze encounters in the laboratory have been previously described by Punzo and Garman (1 989). All observations were conducted through a one-way mirror to minimize extraneous disturbances. A burrow was constructed at one end of the chamber which ran along one of its transparent sides so that events within the burrow could be observed. A female spider (mean weight: 7.4 å 0.6 g) was placed within the burrow 24 hr prior to an encounter sequence. Each encounter was initiated by introducing a naive female wasp into the chamber and the time required to complete various behavioral components of hunting was recorded with a stopwatch. Each wasp was subjected to encounters with 10 hosts. Data were collected for 65 wasps (body length: 31.8-33.5mm).
For field and laboratory encounters, hunting behavior was ana- lyzed in terms of the following components: initial approach and antennation of the host (AP); moving away and grooming (use of mouthparts to clean antennae, (MG); antennation and paralyzation (AnP); burial and oviposition (BO); and closure of the burrow (C). Analysis of films and photographs also allowed me to collect data on preferred sites for insertion of stinger and the percentage of wasps exhibiting what I will refer to as lapping behavior (drinking of host body fluids emanating from the wound site where the sting had been inserted, or from the buccal cavity of the host). I also conducted experiments to determine whether or not the degree of physical activity of the wasp prior to an encounter had any effect on the frequency of lapping behavior. Preliminary obser- vations had indicated that wasps exhibit lapping in fewer than 40%



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of their encounters with a host. Perhaps this behavior is conducted to replenish nutrients and water lost during unusually lengthy for- aging flights which may encompass distances of over 3 km (per- sonal observation). If so, then the frequency of lapping should increase as a function of increased flight time. To test this, I forced female wasps to perform fixed flight bouts of 0 (controls), 1, 2, 3, 4 or 5 min (N = 20 wasps at each time interval). Prior to testing, each wasp was anesthetized with carbon dioxide. A drop of warm paraffin wax was affixed to the dorsal surface of the prothorax and allowed to harden. An insect pin (size 1) was inserted into the ball of wax. The wasp was then suspended by the pin from a horizontal metal rod attached to ring stand and allowed to recover until it was alert (moving its head, legs and antennae). The legs of the wasp were directed downward onto a spherical piece of styrofoam (2 cm in diameter). The suspended wasp was positioned facing an air hose at a distance of 10 cm, so that a gentle stream of compressed air could be directed at the wasp. When the styrofoam sphere was gently pulled away from the legs with forceps, the wasp would begin to exhibit flight behavior. After the allotted time, the air was turned off and the sphere positioned beneath the wasp which grasped it and then ceased flight behavior. The wax and pin were removed using a dilute acetone solution and forceps. The wasp was then placed into the encounter chamber as described previously and allowed to interact with its host. Its hunting behavior was observed and the following data recorded: (1) the presence or absence of an attack response; (2) if an attack occurred, did the wasp exhibit lapping behavior?
The behavioral acts which comprise the hunting behavior of P. thisbe are listed in Table 1. No meaningful differences were observed between encounters in the field versus those staged in the laboratory. In the field, female wasps were observed to leave perch sites and exhibit what appear to be random flight search patterns which lasted 1-2 min. The wasp then alights on the ground and begins to walk rapidly over the surface while tapping the substrate in rapid fashion with the antennae. In some cases, a wandering male spider is encountered as it moves along the ground. This occurs most frequently during those months (July-August) in



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248 Psyche [VOI. 101
which A. echina males seek out females for mating. More fre- quently? a burrow occupied by a spider is located, In these cases? the wasp uses its mandibles to cut through the silk covering at the burrow entrance. It has been suggested that the wasp relies primar- ily on alfactory cues to ascertain the suitability of the host (Petrunkevitch, 1952; Punzo, 19907 1991). The wasp then enters the burrow and forces the spider to the surface. Apparently the physical dimensions of the burrow are too confining for the wasp to attack the spider in its retreat. Once at the surface, the wasp begins to explore the surface of the spider with its antennae (anten- nation). The host may exhibit a threat display in which it raises its front legs and exposes the fangs. However? the spider makes no attempt to strike at the wasp. The wasp continues antennation for a variable period of time and then moves a short distance away (9-15 cm) to groom. During grooming, both antennae are alter- nately passed through the mandibles for cleaning purposes. During this time? the spider makes no attempt to escape. The wasp returns to its host and resumes antennation. After a variable period of time? it suddenly darts under the spider and seizes one of the rear legs with its mandibles while bracing its own metathoracic legs against the pedipalps of the host making a strike by the spider more difficult. The spider is usually flipped over on its back at this time. The sting is inserted and the host is paralyzed very rapidly (1.5-2.5 sec). The effect of the wasp venom can be readily seen as the legs of the spider begin to curl inward and all movement ceases.
The sites at which the sting is inserted can vary? although the most frequent site (81-86%) is the intersegmental membrane between the sternum and coxa of the first leg (contingency test? x2 = 34.6? p < 0.001). This is the region in which the large subesophageal ganglion which supplies innervation to the muscles of the legs7 pedipalps and chelicerae is located (Foelix? 1979). Unlike many other solitary wasps whose venom has only transitory effects (Ross et al., 1988), the venom of Pepsis wasps paralyzes the host permanently. The venoms of many solitary wasps have been shown to exhibit strong neuromuscular effects (Piek? 1981). For A. echina, the paralysis is permanent. The spider shows no visible signs of recovery and remains paralyzed until it is finally killed by the developing wasp larva. In no case did P thisbe sting its host



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Punzo
Table 1. Sequence of behavioral acts comprising the hunting behavior of the spider wasp, Pepsis thisbe, and associated behaviors of its host, Aphonopelma echina. Female wasp behavior
Host spider behavior
1. Search for occupied spider
burrow
2. Enters burrow and forces
spider to the surface
3. Emerges from burrow
4. Antennation of the host
5. Assumes defensive
posture
6. Continues exploration of
body surface of the host
spider (antennation)
7. Cleaning of the antennae
(grooming)
8. Resumes antennation of
spider
9. Seizes leg of spider
10. Attempts to escape
1 1. Presses tarsi against legs
of spider
12. Insertion of sting
13. Paralysis
14. Resumes grooming
15. Excavation of burrow (if
spider burrow is not
available)
16. Drags spider backwards
into burrow
17, Oviposition of a single
egg on the abdomen of
the spider
18. Closure of burrow entrance
more than once. With a host potentially capable of delivery a toxic and fatal bite of its own, the adaptive significance of a fast-acting venom is obvious. Wasps showed no preference for the right vs. the left side of the host body, and the frequency of sting insertion sites was the same for male and female hosts. Less frequently (12-16%), the sting was inserted into the mem- brane between the sternum and coxa of leg 2 or the labium and



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250 Psyche [vo~. 101
sternum (2-3%). Individual wasps showed some variation in behavior regardless of whether or not it had previously attacked a host.
Following paralyzation, the wasp may drink (lapping) from the host body fluids oozing from the wound site at which the sting was inserted or lap at moisture located around the buccal cavity, The frequency of occurrence for lapping was 34.7% at the wound site, and 3.6% at the mouth.
The wasp then moves away from the paralyzed host and again grooms its antennae (Table 1). If a male spider had been attacked while wandering over the surface of the ground, the wasp will excavate a shallow, curvilinear burrow (length: 16-25 cm; diame- ter: 22-28 mm). Most frequently, the burrow of the spider (male or female) is used as a nest. The wasp grasps one of the hind legs of the host and drags the spider backwards into the burrow. As stated earlier, laboratory staged encounters allowed me to observe events inside the burrow. A single egg is attached at one end to the ventral surface of the abdomen just above the posterior opening to the book lungs. The free end of the egg extends laterally along the abdomen. Once the egg is laid, the wasp leaves the burrow and seals the entrance. Soil near the entrance is loosened with the mandibles or prothoracic legs and compacted with the abdomen. The amount of time required to complete all components of the hunting sequence and the effects of encounter experience on over- all performance are shown in Table 2. The results indicate that the hunting behavior of naive I? thisbe females improves with experi- ence. In field observations, the time required for the entire hunting sequence decreased from a mean of 208.1 min for the first encounter with a host to 150.1 min after 10 encounters (t = 8.67, df = 49, p < 0.01). In laboratory experiments, the time required decreased from 165.3 to 117.2 min (t = 7.32, df = 49, p < 0.01). The results also indicate that only certain behavioral components of the hunting sequence improve with experience (AP and AnP) whereas others do not (MG, BO, and C). Thus, AP and AnP exhibit a degree of behavioral plasticity characteristic of more open behav- ioral programs influenced by learning whereas MG, BO, and C can be characterized as rigid, closed behavioral programs. Similar results were reported for Z? formosa (Punzo and Garman, 1989) which occurs sympatrically with l? thisbe and can utilize the same



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Table 2. Effect of encounter experience on the time (min) required for naive Pep- sis thisbe females to complete the various behavioral components of the hunting sequence. Values represent means (k S.D.) for wasps under both field (N = 27) and laboratory (N = 65) conditions1.
Behavioral components
Encounter AP MG AnP BO C Total
Field
1 28.3 (7.1) 12.5 (2.5)
19.8 (4.3) 49.6 (6.8)
2 31.1(5.5) 13.4(3.7)
22.7(2.7) 54.7(8.3)
3 21.5 (2.7) 10.1 (2.1)
17.7 (4.4) 49.9 (6.7)
4 18.6 (2.3) 12.5 (3.5)
13.6 (1.9) 57.2 (7.1)
5 10.3 (1.9) 10.9 (2.6) 7.7 (1.1) 47.8 (5.6) 6 7.2(0.8) 12.6(2.1) 5.9(0.9) 43.2(5.9)
7 5.9 (0.5) 10.2 (1.9) 6.8 (0.4)
50.1 (6.8)
8 6.2 (0.4) 11.7 (1.7)
5.3 (0.5) 48.1 (5.5)
9 5.7(0.6)
13.1(2.2) 4.7(0.2)
43.6(6.1)
10 6.4(0.4)
ll.g(l.7) 5.5(0.3) 46.1(7.2)
Laboratory
1 25.9 (3.9) 15.8 (5.2) 17.3 (4.1) 41.7 (6.8) 2 21.4(5.1) 21.1(7.2)
15.7(2.8) 60.3(9.8)
3 25.2 (4.8)
14.8 (5.6) 18.4 (5.1) 43.8 (6.2)
4 20.4 (3.7) 11.9 (4.2)
12.9 (2.9) 59.9 (7.7)
5 11.3 (4.1)
12.5 (2.8) 10.1 (2.1)
51.3 (6.1)
6 8.3 (1.9)
13.9 (4.5) 7.7 (1.4) 40.9 (6.6)
7 6.2 (1.2) 15.1 (3.3) 4.7 (0.8) 34.4 (5.6) 8 5,4(0.6)
16.5(3.6) 3.2(0.3)
40.1(6.2)
9 5.1 (0.5) 12.2 (2.4) 4.4 (0.3) 39.1 (4.4) 10 5.7 (0.3) 13.1 (4.5)
5.1 (0.4) 41.2 (6.7)
ANO OVA showed the overall effect of encounter experience on the total time required to complete hunting behavior to be significant under both field (F = 82.8; df = 9, 133; p < 0.001) and laboratory (F = 74.2; df = 9, 227; p < 0.001) condi- tions. A Duncan multiple range test (Sokal and Rohlf, 1981) showed no signifi- cant improvement in performance after encounter 7. Encounter experience had no significant effect on MG, BO, and C components of behavior. host. However, f! formosa females required a longer period of time (237 min) to complete the hunting sequence in initial encounters. The amount of time spent in flight before an encounter has a significant effect (x2 = 35.7, p < 0.01) on the probability of an attack. Upon the first presentation of a host, newly inseminated control wasps (no forced flight) attack at a frequency of 74%. A



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252 Psyche [vo~. 101
forced flight time of one rnin has no significant effect (p > 0.5) on the attack rate (68%). However, after 2 and 3 rnin of forced flight, the frequency of attack decreases to 29% and 5%, respectively (p c 0.01). Wasps forced to fly 4-5 min do not exhibit any attack behav- ior until given a rest period of at least 60 rnin (personal observa- tion).
Flight time was also found to affect the frequency of lapping behavior. Naive control wasps exhibit lapping very infrequently (3-7%). This increases to 10% (mouth site) and 56% (wound site), and 20% and 73%, after 2 and 3 rnin of flying, respectively. When lapping behavior is exhibited, the wound site is preferred to a much greater extent than the mouth of the host. The temporal pattern for hunting behavior by female wasps is shown in Fig. 1. Females exhibit a bimodal activity pattern with the highest percentages of wasps searching for burrows between 0800-0959 (25.3%, N = 3 15) and 1600-1759 hr (33.3%). Applica- tion of log frequency analysis (Sibly et al., 1990) indicated that searching behavior increases significantly during these time peri- ods (F = 15.7, p c 0.01). From 1400-1559 hr, 13.9% of the total number of wasps observed were active. Over 39% of the wasps exhibited searching behavior between 1400 and 1759 hr. Lowest peaks of activity occurred between 2200-1559 hr or at times when ambient temperatures exceeded 38OC (1 100-1359). Activity declined abruptly after 1800 hr (from 33.3% at 1600-1759 hr to 7.9% at 1800-1959 hr). This indicates a strong preference for diur- nal activity in this species.
It is becoming increasingly evident that many behavioral acts previously considered to be instinctive can exhibit varying degrees of plasticity (Punzo, 1985, 1991). This is certainly the case for the hunting behavior of Pepsis wasps. In f? thisbe, specific behavioral components of the hunting sequence improved with practice whereas others did not. This improvement in performance as a function of encounter experience suggests that learning may con- tribute to the overall fitness of these spider wasps. In terms of energy budgets and optimal foraging theory (Krebs, 1978; O'Brien et al., 19901, time spent in locating and handling a required resource is rendered unavailable for other important activities such



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Fig. 1. Temporal patterns of activity for hunting in the spider wasp, Pepsis thisbe (N = 315 females). Data represent percent activity at various intervals over a 24-hr period. CST = Central Standard Time.
as feeding and mating. By decreasing the amount of time required to subdue its prey, I? thisbe can minimize its foraging costs and increase fitness. Selection should favor the evolution of adaptive behavioral programs that allow for a certain degree of plasticity (Punzo, 1985). It is also interesting to note that those specific behavioral components that improve with experience (AP and AnP) are precisely those that are most closely related with poten- tially unpredictable behavioral acts of the host. During AP and AnP, the wasp is in close proximity to its formidable host which is still alert and active. However, no unpredictable interactions are associated with the MG (wasp is out of striking range of the host) or BO and C (host is immobilized) components of hunting. Since olfactory cues leading to paralyzation apparently play an important role in Pepsis hunting strategies (Petrunkevitch, 1952; Punzo, 19911, the grooming of the antennae (MG) is very important. The wasp is deliberate and meticulous in its cleaning movements which



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254 Psyche [vo~. 101
are under the influence of certain biomechanical constraints. A cer- tain minimum amount of time is required to complete the mechani- cal movements associated with dragging the host into the burrow, oviposition, and closure of the burrow entrance. Under these bio- mechanical constraints, the capacity for plasticity would not be particularly adaptive.
The results also indicate a strong site preference for insertion of the sting by I? thisbe. Stinging patterns in other species of wasps are species-specific and can also vary considerably between species (see review by Steiner, 1986). However, the sting is usually inserted at a site very close to a major nerve center affecting the locomotor activities of the host. In many insect hosts, the mem- brane between the head and thorax near the thoracic ganglia is chosen. In other cases, the wasp may sting the host in several loca- tions. In spiders, a site close to the subesophageal ganglion is usu- ally selected. This was the case in the present study. It was also demonstrated for the first time that lapping behavior is influenced by the degree of physical activity by the wasp prior to an encounter with a host. Although this behavior had been described by early investigators (Petrunkevitch, 1926; Passmore, 19361, the reason(s) why it occurs in some encounters and not in others has never been satisfactorily explained. The energetic demands of insect flight are well known. Also, the xeric conditions which frequently confront diurnal desert arthropods can lead to significant evaporative water loss and osmotic stress (Cloudsley- Thompson, 1975). Presumably, lapping allows the wasp to replen- ish nutrients and water required to meet the energetic demands of flight. The frequency of this behavior should be related to the amount of energy used. The results of this study support this view. The mouth of the host represents a potential source of water whereas hemolymph oozing from the wound site can provide both water and nutrients. It is not surprising that P. thisbe showed a preference for the wound site.
I thank T. Punzo, J. Bottrell, and T. Ferraioli for assistance in collecting and observing wasps in the field, Dr. B. Garman (Dept. of Mathematics) for assistance in statistical analyses, and anony- mous reviewers for helpful comments on an earlier draft of the



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19941 Punzo 255
manuscript. I also thank the University of Tampa for Faculty Development Grants from 1986-1993 and for a sabatical leave in 1986 which made much of this work possible. Buckley, S. B. 1862. The tarantula (Mygale hentzii Girard) and its destroyer (Pom- pilus formosus Say). Proc. Ent. SOC. Philadelphia 1:138-139. Cazier, M. A. and M. Mortenson. 1964. Bionomical observations on tarantula hawks and their prey (Hymenoptera : Pompilidae): Pepsis. Ann, Ent. SOC. Amer. 57:533-541.
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Punzo, F. and B. Garman. 1989. Effects of encounter experience on the hunting behavior of the spider wasp, Pepsis formosa (Say) (Hymenoptera: Pompilidae). Southwest. Nat. 34~513-5 18.
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