Predator foraging behavior in response to perception and learning by its prey: interactions between orb-spinning spiders and stingless bees

Although rewarded bees learn and remember colors and patterns, they have difficulty in learning to avoid negative stimuli such as decorated spider webs spun by Argiope argentata. A. argentata decorates its web with silk patterns that vary unpredictably (Fig. 1) and thus foraging insects that return to sites where spiders are found encounter new visual cues daily. Stingless bees can learn to avoid spider webs but avoidance-learning is slowed or inhibited by daily variation in web decorations (Figs. 3,4; Tables 1,2). In addition, even if bees learn to avoid decorated webs found in one location, they are unable to generalize learned-avoidance responses to similarly decorated webs found at other sites. A. argentata seems to have evolved a foraging behavior that is tied to the ways insects perceive and process information about their environment. Because of the evolutionary importance of bee-flower interdependence, the predatory behavior of web-decorating spiders may be difficult for natural selection to act against.     

Predation risk makes bees reject rewarding flowers and reduce foraging activity

In the absence of predators, pollinators can often maximize their foraging success by visiting the most rewarding flowers. However, if predators use those highly rewarding flowers to locate their prey, pollinators may benefit from changing their foraging preferences to accept less rewarding flowers. Previous studies have shown that some predators, such as crab spiders, indeed hunt preferentially on the most pollinator-attractive flowers. In order to determine whether predation risk can alter pollinator preferences, we conducted laboratory experiments on the foraging behavior of bumble bees (Bombus impatiens) when predation risk was associated with a particular reward level (measured here as sugar concentration). Bees foraged in arenas containing a choice of a high-reward and a low-reward artificial flower. On a bee’s first foraging trip, it was either lightly squeezed with forceps, to simulate a crab spider attack, or was allowed to forage safely. The foragers’ subsequent visits were recorded for between 1 and 4 h without any further simulated attacks. Compared to bees that foraged safely, bees that experienced a simulated attack on a low-reward artificial flower had reduced foraging activity. However, bees attacked on a high-reward artificial flower were more likely to visit low-reward artificial flowers on subsequent foraging trips. Forager body size, which is thought to affect vulnerability to capture by predators, did not have an effect on response to an attack. Predation risk can thus alter pollinator foraging behavior in ways that influence the number and reward level of flowers that are visited.     

Comparisons of forager distributions from matched honey bee colonies in suburban environments

We conducted experiments designed to examine the distribution of foraging honey bees (Apis mellifera) in suburban environments with rich floras and to compare spatial patterns of foraging sites used by colonies located in the same environment. The patterns we observed in resource visitation suggest a reduced role of the recruitment system as part of the overall colony foraging strategy in habitats with abundant, small patches of flowers. We simultaneously sampled recruitment dances of bees inside observation hives in two colonies over 4 days in Miami, Florida (1989) and from two other colonies over five days in Riverside, California (1991). Information encoded in the dance was used to determine the distance and direction that bees flew from the hive for pollen and nectar and to construct foraging maps for each colony. The foraging maps showed that bees from the two colonies in each location usually foraged at different sites, but occasionally they visited the same patches of flowers. Each colony shifted foraging effort among sites on different days. In both locations, the mean flight distances differed between colonies and among days within colonies. The flight distances observed in our study are generally shorter than those reported in a similar study conducted in a temperate deciduous forest where resources were less dense and floral patches were smaller.     

Odour transfer in stingless bee marmelada (Frieseomelitta varia) demonstrates that entrance guards use an “undesirable–absent” recognition system

In group-level recognition, discriminators use sensory information to distinguish group members and non-members. For example, entrance guards in eusocial insect colonies discriminate nestmates from intruders by comparing their odour with a template of the colony odour. Despite being a species-rich group of eusocial bees closely related to the honey bees, stingless bee nestmate recognition is a relatively little-studied area. We studied Frieseomelitta varia, a common Brazilian species of stingless bee known as marmelada. By measuring the rejection rates of nestmate and non-nestmate worker bees by guards, we were able to show that guards became significantly less accepting (from 91 to 46%) of nestmates that had acquired odour cues from non-nestmate workers; however, guards did not become significantly more accepting (from 31 to 42%) of non-nestmates that had acquired equivalent amounts of odour cues from the guard’s nestmates. These data strongly suggest that guards use an “undesirable–absent” system in recognition, whereby incoming conspecific workers are only accepted if undesirable cues are absent, despite the presence of desirable cues. We suggest that an undesirable–absent system is adaptive because robbing by conspecifics may be an important selective factor in F. varia, which would lead to selection for a non-permissive acceptance strategy by guards.     

Division of labor between undertaker specialists and other middle-aged workers in honey bee colonies

A primary determinant of colony organization in temporally polyethic insect societies is inter-individual variation in behavior that is independent of worker age. We examined behavioral repertoires, behavioral correlates of adult development, and spatial distributions within the hive to explore the mechanisms that produce behavioral variation among middle-age honey bees (Apis mellifera). Individually labeled undertakers, guards, food storers, and wax workers exhibited a broad range of task-related behavior, but bees tagged as undertakers were more likely to subsequently remove a corpse from the hive and handle a corpse compared to other middle-aged bees. The activity level of undertakers was similar to other task groups, suggesting that undertaking specialists were neither hyper-active “elites” nor quiescent “reserves” that become active only when a dead bee stimulus is present. Undertakers also were more likely to remove debris and to remain in the lower region of the hive or near the entrance, even when not engaged in corpse removal; both preferences may promote colony efficiency by reducing inter-task travel times. Guards and undertakers were less likely to perform behavior normally associated with young bees compared to food storers and wax workers. Undertakers and guards also initiated foraging at earlier ages than the other task groups. These results suggest that undertakers and guards may be slightly developmentally advanced compared to food storers and wax workers. There also was evidence for lifetime differences in behavioral preferences which could not be explained by differences in adult development. Bees tagged as undertakers were more likely to subsequently remove a dead bee during their entire pre-foraging career compared to other task groups or members of their general age cohort. Differences in both the rate of adult development and individual behavioral preferences, both of which may be subject to genetic and environmental influences, are important determinants of inter-individual variation among honey bees of middle age. Received: 5 February 1997 / Accepted after revision: 27 May 1997     

Does interaction between bumblebees (Bombus ignitus) reduce their foraging area?: bee-removal experiments in a net cage

To examine whether the interaction between bumblebees, Bombus ignitus, reduces their foraging area, we conducted bee-removal experiments in a net cage. In the cage, we set potted Salvia farinacea plants, allowed bumblebees to forage freely on those plants, and followed their plant-to-plant movements to identify a bee with a relatively small foraging area. We then removed all the other foraging bees, except for the bee with a small foraging area, and observed the change of the foraging area of the focal bee under conditions of no interaction with other bees. After the removal of the other bees, all five bees tested enlarged their foraging areas, suggesting that the interaction between bees is an important determinant of their foraging areas. The result also means that bumblebees are able to adjust their foraging areas in response to other foragers, indicating the necessity for future studies to clarify what cues bees use to interact with other bees. Moreover, after the removal treatments, all five bees showed temporary increases in the number of flower probes per plant. This can be explained by their optimal foraging according to the old average intake rate for the plant population and by the delayed changes in response to the new high average energy intake rate after the bee-removal treatments.Communicated by M. Giurfa     

Honey bee foragers as sensory units of their colonies

Forager honey bees function not only as gatherers of food for their colonies, but also as sensory units shaped by natural selection to gather information regarding the location and profitability of forage sites. They transmit this information to colony members by means of waggle dances. To investigate the way bees transduce the stimulus of nectar-source profitability into the response of number of waggle runs, I performed experiments in which bees were stimulated with a sucrose solution feeder of known profitability and their dance responses were videorecorded. The results suggest that several attributes of this transduction process are adaptations to enhance a bee's effectiveness in reporting on a forage site. (1) Bees register the profitability of a nectar source not by sensing the energy gain per foraging trip or the rate of energy gain per trip, but evidently by sensing the energetic efficiency of their foraging. Perhaps this criterion of nectar-source profitability has been favored by natural selection because the foraging gains of honey bees are typically limited by energy expenditure rather than time availability. (2) There is a linear relationship between the stimulus of energetic efficiency of foraging and the response of number of waggle runs per dance. Such a simple stimulus-response function appears adequate because the range of suprathreshold stimuli (max/min ratio of about 10) is far smaller than the range of responses (max/min ratio of about 100). Although all bees show a linear stimulus-response function, there are large differences among individuals in both the response threshold and the slope of the stimulus-response function. This variation gives the colony a broader dynamic range in responding to food sources than if all bees had identical thresholds of dance response. (3) There is little or no adaptation in the dance response to a strong stimulus (tonic response). Thus each dancing bee reports on the current level of profitability of her forage site rather than the changes in its profitability. This seems appropriate since presumably it is the current profitability of a forage site, not the change in its profitability, which determines a site's attractiveness to other bees. (4) The level of forage-site quality that is the threshold for dancing is tuned by the bees in relation to forage availability. Bees operate with a lower dance threshold when forage is sparse than when it is abundant. Thus a colony utilizes input about a wide range of forage sites when food is scarce, but filters out input about low-reward sites when food is plentiful. (5) A dancing bee does not present her information in one spot within the hive but instead distributes it over much of the dance floor. Consequently, the dances for different forage sites are mixed together on the dance floor. This helps each bee following the dances to take a random sample of the dance information, which is appropriate for the foraging strategy of a honey bee colony since it is evidently designed to allocate foragers among forage sites in proportion to their profitability.     

Reward rate and forager activation in honeybees: recruiting mechanisms and temporal distribution of arrivals

We analyzed the foraging and recruitment activity of single foragers ( Apis mellifera), exploiting low reward rates of sucrose solution. Single employed foragers (test bees) were allowed to collect 2.0 m sucrose solution delivered by a rate-feeder located at 160 m from the hive for 2 h. Flow rates varied between 1.4 and 5.5 µl/min. The individual behavior of the test bees was registered both at the hive and the food source, and the social output was calculated as the number of incoming bees arriving at the feeder per hour (henceforth: arrival rate). Incoming bees were captured once they landed at the feeder and assigned to one of three categories according to their foraging experience and hive interactions with the test bee: inspector, reactivated, or inexperienced bees. Both the waggle-runs performed per hour of foraging by test bees and the social output attained, increased with the reward rate. Also the number of hive-stays and the trophallactic-offering contacts performed by test bees were positively correlated with the arrival rate. For the highest reward rates, the duration of Nasonov-gland exposure at the feeding place was higher, and the arrival of most of the incoming bees occurred shortly after the test bee landed at the feeding platform. Thus, in addition to hive-interactions, landing of incoming bees at the food source is promoted by olfactory and/or visual information provided by the test bees. The proportions of inspector, reactivated, and inexperienced bees changed depending on the reward rate offered. Therefore, not only the occurrence and intensity of the recruitment-related behaviors performed by the test bees, but also the stimulation required by each category of incoming bees, determined the social output observed.     

Directional change in a suite of foraging behaviors in tropical and temperate evolved honey bees (<Emphasis Type="Italic">Apis mellifera</Emphasis> L.)

The concept of a suite of foraging behaviors was introduced as a set of traits showing associative directional change as a characterization of adaptive evolution. I report how naturally selected differential sucrose response thresholds directionally affected a suite of honey bee foraging behaviors. Africanized and European honey bees were tested for their proboscis extension response thresholds to ascending sucrose concentrations, reared in common European colonies and, captured returning from their earliest observed foraging flight. Race constrained sucrose response threshold such that Africanized bees had significantly lower sucrose response thresholds. A Cox proportional hazards regression model of honey bee race and sucrose response threshold indicated that Africanized bees were 29% (P<0.01) more at risk to forage over the 30-day experimental period. Sucrose response threshold organized age of first foraging such that each unit decrease in sucrose response threshold increased risk to forage by 14.3% (P<0.0001). Africanized bees were more likely to return as pollen and water foragers than European foragers. Africanized foragers returned with nectar that was significantly less concentrated than European foragers. A comparative analysis of artificial and naturally selected populations with differential sucrose response thresholds and the common suite of directional change in foraging behaviors is discussed. A suite of foraging behaviors changed with a change in sucrose response threshold that appeared as a product of functional ecological adaptation.Communicated by R.F.A. Moritz     

Brood pheromone modulates honeybee (Apis mellifera L.) sucrose response thresholds

Foraging behavior and the mechanisms that regulate foraging activity are important components of social organization. Here we test the hypothesis that brood pheromone modulates the sucrose response threshold of bees. Recently the honeybee proboscis extension response to sucrose has been identified as a ”window” into a bee’s perception of sugar. The sucrose response threshold measured in the first week of adult life, prior to foraging age, predicts forage choice. Bees with low response thresholds are more likely to be pollen foragers and bees with high response thresholds are more likely to forage for nectar. There is an associated genetic component to sucrose response thresholds and forage choice such that bees selected to hoard high quantities of pollen have low response thresholds and bees selected to hoard low quantities of pollen have higher response thresholds. The number of larvae in colonies affects the number of bees foraging for pollen. Hexane-extractable compounds from the surface of larvae (brood pheromone) significantly increase the number of pollen foragers. We tested the hypothesis that brood pheromone decreases the sucrose response threshold of bees, to suggest a pheromone- modulated sensory-physiological mechanism for regulating foraging division of labor. Brood pheromone significantly decreased response thresholds as measured in the proboscis extension response assay, a response associated with pollen foraging. A synthetic blend of honeybee brood pheromone stimulated and released pollen foraging in foraging bioassays. Synthetic brood pheromone had dose-dependent effects on the modulation of sucrose response thresholds. We discuss how brood pheromone may act as a releaser of pollen foraging in older bees and a primer pheromone on the development of response thresholds and foraging ontogeny of young bees. Received: 24 May 2000 / Revised: 26 September 2000 / Accepted: 15 October 2000     

Choosing rewarding flowers; perceptual limitations and innate preferences influence decision making in bumblebees and honeybees

Flowers exhibit great intra-specific variation in the rewards they offer. At any one time, a significant proportion of flowers often contain little or no reward. Hence, foraging profitably for floral rewards is problematic and any ability to discriminate between flowers and avoid those that are less rewarding will confer great advantages. In this study, we examine discrimination by foraging bees among flowers of nasturtium, Tropaeolum majus. Bee visitors included carpenter bees, Xylocopa violacea, which were primary nectar robbers; honeybees, Apis mellifera, which either acted as secondary nectar robbers or gathered pollen legitimately and bumblebees, Bombus hortorum, which were the only bees able to gather nectar legitimately. Many flowers were damaged by phytophagous insects. Nectar volume was markedly lower in flowers with damaged petals (which were also likely to be older) and in flowers that had nectar-robbing holes. We test whether bees exhibit selectivity with regards to the individual flowers, which they approach and enter, and whether this selectivity enhances foraging efficiency. The flowers approached (within 2 cm) by A. mellifera and B. hortorum were non-random when compared to the floral population; both species selectively approached un-blemished flowers. They both approached more yellow flowers than would be expected by chance, presumably a reflection of innate colour preferences, for nectar standing crop did not vary according to flower colour. Bees were also more likely to accept (land on) un-blemished flowers. A. mellifera gathering nectar exhibited selectivity with regards to the presence of robbing holes, being more likely to land on robbed flowers (they are not able to feed on un-robbed flowers). That they frequently approached un-robbed flowers suggests that they are not able to detect robbing holes at long-range, so that foraging efficiency may be limited by visual acuity. Nevertheless, by using a combination of long-range and short-range selectivity, nectar-gathering A. mellifera and B. hortorum greatly increased the average reward from the flowers on which they landed (by 68% and 48%, respectively) compared to the average standing crop in the flower population. Overall, our results demonstrate that bees use obvious floral cues (colour and petal blemishes) at long-range, but can switch to using more subtle cues (robbing holes) at close range. They also make many mistakes and some cues used do not correlate with floral rewards.     

Generalist cuckoo bees (Hymenoptera: Apoidea: Sphecodes) are species-specialist at the individual level

Intensive and incessant arms races between a parasite and its host are generally expected to lead to parasite specialization. Nevertheless, some parasitic species still successfully attack wide spectra of hosts. One of the solutions to the evolutionary enigma of the long-term existence of generalist parasites is their specialization at an individual level, a phenomenon well known, e.g., in European common cuckoo. Over its range, it parasitizes a number of bird species; however, individual females are mostly specialists possessing adaptations to a particular host species. In this study, we test the possibility of individual specialization in generalist cuckoo bees, the insect counterparts of avian cuckoos. Females of cuckoo bees lay each egg into a single brood cell in the nests of other bee species. The host’s offspring is destroyed by the parasitic female or later by her larvae, which feed on pollen supplies accumulated by the host. Both studied cleptoparasitic bees (Sphecodes ephippius and Sphecodes monilicornis) are widely distributed in Europe, where they have been reported to use broad host spectra. We recorded several host species (including some previously unknown) for both cuckoo bee species, and confirmed that these parasites are indeed generalist even at a small local scale. However, we demonstrate that exactly as in the avian cuckoos, each female in both species of generalist bee parasites tends to attack just one host species.     

The honey bee shaking signal: function and design of a modulatory communication signal

This study explores the meaning and functional design of a modulatory communication signal, the honey bee shaking signal, by addressing five questions: (I) who shakes, (II) when do they shake, (III) where do they shake, (IV) how do receivers respond to shaking, and (V) what conditions trigger shaking. Several results confirm the work of Schneider (1987) and Schneider et al. (1986a): (I) most shakers were foragers (at least 83%); (II) shaking exhibited a consistent temporal pattern with bees producing the most signals in the morning (0810–1150 hours) just prior to a peak in waggle dancing activity; and (IV) bees moved faster (by 75%) after receiving a shaking signal. However, this study differs from previous work by providing a long-term, temporal, spatial, and vector analysis of individual shaker behavior. (III) Bees producing shaking signals walked and delivered signals in all areas of the hive, but produced the most shaking signals directly above the waggle dance floor. (IV) Bees responded to the signal by changing their direction of movement. Prior to receiving a signal, bees selected from the waggle dance floor moved, on average, towards the hive exit. After receiving a signal, some bees continued moving towards the exit but others moved directly away from the exit. During equivalent observation periods, non-shaken bees exhibited a strong tendency to move towards the hive exit. (V) Renewed foraging activity after food dearth triggered shaking signals, and, the level of shaking is positively correlated with the duration of food dearth. However, shaking signal levels also increased in the morning before foraging had begun and in the late afternoon after foraging had ceased. This spontaneous afternoon peak has not previously been reported. The shaking signal consequently appears to convey the general message “reallocate labor to different activities” with receiver context specifying a more precise meaning. In the context of foraging, the shaking signal appears to activate (and perhaps deactivate) colony foraging preparations. The generally weak response elicited by modulatory signals such as the shaking signal may result from a high receiver response threshold which allows the receiver to integrate multiple sources of information and which thereby increases the probability that receiver actions will be appropriate to colony needs. Received: 21 March 1997 / Accepted after revision: 30 August 1997     

Effect of bombardier beetle spray on a wolf spider: repellency and leg autotomy

Summary. Data are presented on the repellency of the spray of a bombardier beetle (Pheropsophus aequinoctialis) to a lycosid spider (Lycosa ceratiola). The secretion is shown to cause the spider to desist from its assault on the beetle within, on average, 58 ms of onset of the beetle’s secretory emission, a reaction time that is at a par with latencies previously reported for startle, escape, and avoidance reactions of cockroaches, flies, and moths. Spray ejections by the beetle, are shorter in duration (43 ms, on average) than the response time of the spider, an indication that the beetle does indeed pack a formidable “punch” into its ejection. After being hit by a beetle’s spray, L. ceratiola were found occasionally to autotomize one or two of their legs. It is argued, but not proven, that this unusually severe effect from exposure to an arthropodan defensive secretion may be caused by the high temperature of the bombardier beetle spray.     

Do social spiders cooperate in predator defense and foraging without a web?

Nearly all social spiders spin prey-capture webs, and many of the benefits proposed for sociality in spiders, such as cooperative prey capture and reduced silk costs, appear to depend on a mutually shared web. The social huntsman spider, Delena cancerides (Sparassidae), forms colonies under bark with no capture web, yet these spiders remain in tightly associated, long-lasting groups. To investigate how the absence of the web may or may not constrain social evolution in spiders, we observed D. cancerides colonies in the field and laboratory for possible cooperative defense and foraging benefits. We observed spiders’ responses to three types of potential predators and to prey that were introduced into retreats. We recorded all natural prey capture over 447 h both inside and outside the retreats of field colonies. The colony’s sole adult female was the primary defender of the colony and captured most prey introduced into the retreat. She shared prey with younger juveniles about half the time but never with older subadults. Spiders of all ages individually captured and consumed the vast majority of prey outside the retreat. Young spiders benefited directly from maternal defense and prey sharing in the retreat. However, active cooperation was rare, and older spiders gained no foraging benefit by remaining in their natal colony. D. cancerides does not share many of the benefits of group living described in other web-building social spiders. We discuss other reasons why this species has evolved group living.     

The honey bee’s tremble dance stimulates additional bees to function as nectar receivers

If a forager bee returns to her hive laden with high-quality nectar but then experiences difficulty finding a receiver bee to unload her, she will begin to produce a conspicuous communication signal called the tremble dance. The context in which this signal is produced suggests that it serves to stimulate more bees to function as nectar receivers, but so far there is no direct evidence of this effect. We now report an experiment which shows that more bees do begin to function as nectar receivers when foragers produce tremble dances. When we stimulated the production of tremble dances in a colony and counted the number of bees engaged in nectar reception before and after the period of intense tremble dancing, we found a dramatic increase. In two trials, the number of nectar receivers rose from 17% of the colony’s population before tremble dancing to 30–50% of the population after the dancing. We also investigated which bees become the additional nectar receivers, by looking at the age composition of the receiver bees before and after the period of intense tremble dancing. We found that none of the bees recruited to the task of nectar reception were old bees, most were middle-aged bees, and some were even young bees. It remains unclear whether these auxiliary nectar receivers were previously inactive (as a reserve supply of labor) or were previously active on other tasks. Overall, this study demonstrates that a honey bee colony is able to rapidly and strongly alter its allocation of labor to adapt to environmental changes, and it further documents one of the communication mechanisms underlying this ability. Received: 31 May 1996/Accepted after revision: 9 August 1996     

Predator recognition and evasive behavior by sweat bees, <Emphasis Type="Italic"> Lasioglossum umbripenne </Emphasis>(Hymenoptera: Halictidae), in response to predation by ants, <Emphasis Type="Italic"> Ectatomma ruidum </Emphasis>(Hymenoptera: Formicidae)

Ectatomma ruidum is an abundant soil-nesting Neotropical ant, which displays extensive behavioral flexibility during foraging activities. We studied here one unusual element of their behavioral repertoire: ambush predation. A worker of E. ruidum waits near a nest of a social sweat bee, Lasioglossum umbripenne, lunging at incoming bees, or less frequently, at departing bees. However, bees detected ambushing ants and modified their behavior. Dead ants placed at bees' nest entrances significantly decreased bee activity, indicating that bees recognized dead ants as potential predators. Neither simple black models (square and rectangle) nor olfactory cues had any effect on overall bee activity. A returning bee usually approached her entrance and immediately entered, but if an ant was waiting at the nest, a bee was significantly more likely to abort the first approach flight and then to re-approach the nest on the side opposite the ant's position. As models became increasingly ant-like, returning bees more frequently aborted their first approach flight, expressing other behaviors before entering nests. These behaviors included withdrawal followed by an approach from a different direction; zigzagging flights, either from a distance or close to the entrance or even a close inspection; landing a short distance from the nest, then approaching on foot or waiting for several seconds before entering. Ants responded with effective counter-behaviors. Behavioral flexibility in nest entering/exiting by L. umbripenne and in hunting strategy by E. ruidum shows the complexity of this predator-prey relationship, and illustrates the importance of information processing by both species involved in determining the outcome of the interspecific interaction.     

The role of internal and external information in foraging decisions of Melipona workers (Hymenoptera: Meliponinae)

Social insect foragers have to make foraging decisions based on information that may come from two different sources: information learned and memorised through their own experience (“internal” information) and information communicated by nest mates or directly obtained from their environment (“external” information). The role of these sources of information in decision-making by foragers was studied observationally and experimentally in stingless bees of the genus Melipona. Once a Melipona forager had started its food-collecting career, its decisions to initiate, continue or stop its daily collecting activity were mainly based upon previous experience (activity on previous days, the time at which foraging was initiated the day(s) before, and, during the day, the success of the last foraging flights) and mediated through direct interaction with the food source (load size harvested and time to collect a load). External information provided by returning foragers advanced the start of foraging of experienced bees. Most inexperienced bees initiated their foraging day after successful foragers had returned to the hive. The start of foraging by other inexperienced bees was stimulated by high waste-removal activity of nest mates. By experimentally controlling the entries of foragers (hence external information input) it was shown that very low levels of external information input had large effect on the departure of experienced foragers. After the return of a single successful forager, or five foragers together, the rate of forager exits increased dramatically for 15 min. Only the first and second entry events had large effect; later entries influenced forager exit patterns only slightly. The results show that Melipona foragers make decisions based upon their own experience and that communication stimulates these foragers if it concerns the previously visited source. We discuss the organisation of individual foraging in Melipona and Apis mellifera and are led to the conclusion that these species behave very similarly and that an information-integration model (derived from Fig. 1) could be a starting point for future research on social insect foraging. Received: 16 April 1997 / Accepted after revision: 30 August 1997     

Why do nocturnal orb-web spiders (Araneidae) search for light?

The nocturnal orb-web spider Larinioides sclopetarius lives near water and frequently builds webs on bridges. In Vienna, Austria, this species is particularly abundant along the artificially lit handrails of a footbridge. Fewer individuals placed their webs on structurally identical but unlit handrails of the same footbridge. A census of the potential prey available to the spiders and the actual prey captured in the webs revealed that insect activity was significantly greater and consequently webs captured significantly more prey in the lit habitat compared to the unlit habitat. A laboratory experiment showed that adult female spiders actively choose artificially lit sites for web construction. Furthermore, this behaviour appears to be genetically predetermined rather than learned, as laboratory-reared individuals which had previously never foraged in artificial light exhibited the same preference. This orb-web spider seems to have evolved a foraging behaviour that exploits the attraction of insects to artificial lights. Received: 8 June 1998 / Received in revised form: 18 January 1999 / Accepted: 19 January 1999     

The appeasement effect of sterility signaling in dominance contests among Bombus terrestris workers

The establishment of dominance hierarchies through aggressive interactions is very common in insect societies. In many cases, it is also mediated through pheromone emissions that enable individuals to evaluate the reproductive quality and level of aggressiveness of the dominant individual, thereby reducing the number and intensity of costly fights. Here, we studied these processes in the primitively eusocial bee Bombus terrestris, using a paired bee system. Specifically, we investigated the behavioral, reproductive, and pheromonal correlates of dominance establishment. Workers were shown to establish dominance hierarchies using overt aggression within 3–4 days. Thereafter, the aggression drastically decreased, and dominance was maintained mostly by ritualized agonistic behavior. The behaviorally dominant bee lost the ester compounds that workers produce in their Dufour's gland (the so-called “sterility signal”) concomitantly with the development of her ovaries. The other bee announced as subordinate by continuously producing high amounts of those esters. The hypothesis that sterility signaling serves as an appeasement signal to pacify the dominant bees is supported by the negative correlation found between the proportion of these esters and the level of aggression that the subordinate received from the dominant worker. Physical interactions, and presumably also the ensuing overt aggression between the bees, were essential for the above pheromonal change to take place and enabled the dominant workers to develop their ovaries and to lay eggs. The subordinate bee’s signaling of non-reproductive status may minimize energy expenditure in costly fights and help stabilize the reproductive division of labor among workers.