Learning,retention and coding of nest-associated visual cues by the Australian desert ant, <Emphasis Type="Italic">Melophorus bagoti</Emphasis>

A variety of social insects use visual cues for homing. In this study, we examine the possible factors affecting the learning and retention of nest-associated visual cues by the Australian desert ant Melophorus bagoti and the manner in which such cues are encoded by foraging ants. We placed four prominent cylindrical landmarks around a nest and trained foragers from that nest to a food source. Ants were tested with the landmark array in a distant testing field after (1) a known number of exposures to the landmarks (1, 3, 7 or 15 trials, spread over a period of 1 day, 2 days or ≥3 days) and (2) after a known period of delay (0, 24, 48, 96 or 192 h). The results show that a combination of an increase in training trials and an increase in number of training days affected the acquisition of landmark memory. Moreover, once the landmarks were learnt, they became a part of long-term memory and lasted throughout the ants’ foraging lifetime. To examine visual cue encoding behaviour, ants trained under similar conditions for 4 days were tested with (1) an identical landmark array, (2) landmarks of the same size used in training, but placed at twice the distance from each other, and (3) landmarks whose dimensions were doubled and placed at twice the distance from each other. In conditions (1) and (3), the ants searched extensively at the centre of the four landmarks, suggesting that, similar to the Saharan ant (genus Cataglyphis) and the honeybee, M. bagoti too uses a snapshot to match the view of the landmarks around the nest. But contrary to the snapshot model, in condition (2), the ants did not search extensively at the centre of the landmarks, but searched primarily 0.5 m from the landmark, the distance from each landmark to the nest during training. We discuss how various search models fare in accounting for these findings.     

Desert ants adjust their approach to a foraging site according to experience

In the habitat of desert ants, Cataglyphis fortis, a constant wind is usually blowing during the daytime. When visiting a familiar food source, the ants steer some distance downwind of the feeder, rather than attempting a direct approach that might miss small food sources, in particular. In the downwind area, the ants pick up the odor plume emanating from the food and follow it upwind to the prey. This strategy saves considerable walking distance and time. The additional path necessitated by the downwind strategy is only about 0.75 to 2 m, depending on nest–feeder distance, while missing the food on the upwind side results in much longer search trajectories. During the initial three to five visits to a feeding site, downwind distance and length of the approach path are shortened notably, and the approach trajectory is straightened. Desert ants further exhibit considerable short-term flexibility in their approach. Experienced individuals are evidently able to decide upon leaving the nest which direction to choose toward the feeder, depending on current wind direction (that fluctuates slightly during the day). Notable changes in wind direction occur primarily overnight. For larger nest–feeder distances, the animals adjust their approach en route to the altered wind direction during their first foraging trip in the morning.     

Effect of substrate roughness on load selection in the seed-harvester ant Messor barbarus L. (Hymenoptera, Formicidae)

We investigated the effect of substrate roughness on load selection in the seed-harvester ant Messor barbarus. Ants were forced to travel either on sand or on gravel to reach a seed patch containing seed fragments of different weights. We hypothesized that foragers travelling on a rough substrate could either increase their load as a result of the increased distance travelled (due to a more sinuous path and an increase in the vertical component of the path) or decrease their load because of the anticipated difficulty of moving with a heavy load on a rough surface. Our results were consistent with neither of these hypotheses: Load selection by ants did not depend on the roughness of the substrate encountered during their outbound trip. The main effect of substrate roughness was to slow down the progression of the ants and increase their probability of dropping or transferring heavy seeds on their way back to the nest, thus resulting in an overall reduction of the rate of seed return to the nest.     

Both evaluation of resource quality and speed of recruited leaf-cutting ants (Acromyrmex lundi) depend on their motivational state

Summary During recruitment, running velocity of both outbound and laden workers of the leaf-cutting ant Acromyrmex lundi depended on the information about resource quality they received from the first successful recruiter. In independent assays, single scout ants were allowed to collect sugar solutions of different concentrations and to recruit nestmates. Recruited workers were presented with standardized paper discs rather than the sugar solution given to the original recruiting ant. Outbound recruited workers were observed to run faster the more concentrated the solution found by the recruiter. Speed of disc-laden workers also depended on the concentration of the solution found by the recruiter, i.e. on the information about food quality they received, since they had no actual contact with the sugar solution. Disc-laden workers ran, as intuitively expected, slower than outbound workers. The reduction in speed, however, could not be attributed to the effects of the load itself, because workers collecting discs of the same weight, but with added sugar, ran as rapidly as outbound, unladen workers. Workers collecting standardized sugared discs reinforced the chemical trail on their way to the nest. The percentage of trail-layers was higher when workers were recruited to 10% than to 1% sugar solution, even though they collected the same kind of discs at the source. Their evaluation of resource quality, therefore, depended on their motivational state, which was modulated by the information they received during recruitment. Using previously published data on energetics of locomotion in leaf-cutting ants, travel costs of A. lundi workers recruited to sugar solutions of different concentration could be estimated. For workers recruited to the more concentrated solution, both speed and oxygen consumption rate increased by a roughly similar factor. Therefore, although workers ran faster to the high-quality resource, their actual energy investment per trip remained similar to that made by workers recruited to the low-quality resource. It is suggested that the more motivated workers reduced travel time without increasing energy costs during the trip. The adaptive value of these responses seems to be related to a rapid transmission of information about a newly discovered food source.     

Visual associative learning in two desert ant species

Visual cues are important navigational tools for many solitary foraging insects. In addition to information provided by path integration, desert ants learn and use visual cues for homing back to their nest. In this study, we compared the visually based learning of two desert ant species: the North African Cataglyphis fortis and the Australian Melophorus bagoti, each of which lives in ecologically similar but visually different environments. In our experiment, ants’ choice performance was measured by training foragers in a channel system. We used a decision box with two visual stimuli during their homebound trips, with one of the stimuli always being the correct one that provided thoroughfare. To determine any habitat effects on learning, we examined intraspecific comparisons in C. fortis with different nest surroundings. The intraspecific comparison in C. fortis revealed no differences in learning the task. In general, C. fortis showed little learning in our task and the results were similar for ants from barren and cluttered environments. Overall, M. bagoti learned the task faster and had a higher level of accuracy than C. fortis. One explanation for this species-specific difference could be that the cluttered habitat of M. bagoti favours the evolution of visual associative learning more so than the plain habitat of C. fortis.     

Efficiency and regulation of recruitment during colony emigration by the ant Temnothorax curvispinosus

Recruitment helps insect societies by bringing individuals to places where work needs to be done, but it also imposes energetic and opportunity costs. The net effect depends both on recruitment efficiency and on the ease with which insects can find work sites on their own. This study examined both of these factors for colony emigration by the ant Temnothorax curvispinosus. Emigrations were organized by a corps of active ants who transported the rest of the colony. These active ants either found new sites independently or followed tandem runs led by successful scouts. Although most tandem runs broke apart before reaching their target, even lost followers found the new site faster than did unguided searchers. When the new site was near the old nest, tandem runs were rare and summoned only a small proportion of the transporter corps. When the new site was instead distant and inconspicuous, tandem runs were common and brought roughly one third of the transporters. This pattern likely results from the quorum rule used by individual scouts to decide when to switch from tandem runs to transports. By monitoring how many nestmates have already found the nest, the ants ensure that the costs of recruitment are born only when necessary.     

Blinkered teaching: tandem running by visually impaired ants

During tandem runs, one ant worker recruits another to an important resource. Here, we begin to investigate how dependent are tandem leaders and followers on visual cues by painting over their compound eyes to impair their vision. There are two ways in which Temnothorax albipennis might use vision during tandem running. First, the follower might track the movements of the leader by keeping it in sight. Our results suggest that the ants do not use vision in this way. For example, in all four classes of tandem run (those with either leader or follower, both, or neither of their participants with visual impairments) progress was most smooth at about 3 mm/s. This suggests that communication between leaders and followers during tandem runs is not based on vision and is purely tactile and pheromonal. Second, the leader and the follower might be using vision to navigate and our results support this possibility but also suggest that these ants have other methods of navigation. Ants with visual impairments were more likely to follow than to lead, but could occupy either role, even though they had many fully sighted nestmates. This might help to explain why the ants did not focus grooming on their most visually impaired nestmates. Wild-type tandem runs, with both participants fully sighted and presumably taking time to learn landmarks, were overall significantly slower, smoother, and a little less tortuous, than the other treatments. All four classes of tandem run significantly increased mean instantaneous speeds and mean absolute changes in instantaneous acceleration over their journeys. Moreover, tandems with sighted followers increased their speed with time more than the other treatments. In general, our findings suggest that eyesight is used for navigation during tandem running but that these ants also probably use other orientation systems during such recruitment and to learn how to get to new nest sites. Our results suggest that the ants’ methods of teaching and learning are very robust and flexible.     

Complexity in an obligate mutualism: do fungus-gardening ants know what makes their garden grow?

Fungus gardening ants make clear choices among fungal substrates (food for their fungus). It has been proposed, but never demonstrated, that these ants are collecting the best for their symbiotic fungus and the production of ant biomass (fitness). The goal of this study was to determine whether preferred substrates lead to higher fitness in the attine, Trachymyrmex septentrionalis. Preferences exhibited by foragers were established. Colonies were fed a single substrate or a mixture of substrates during the entire course of the experiment, which ended when sexual offspring appeared in the nest. The response variables were numbers and weights of ant offspring and the chitin content of fungus gardens. Preference was not strongly related to fitness. The preferred oak catkins produced the highest amounts of ant and fungal biomass, but the ants collected much more material than needed, which indicates that forager activity is decoupled from fitness. The preferred caterpillar feces were rejected shortly after the feedings began. The unpreferred oak leaves were just as effective at producing ant and fungal biomass as catkins. Leaves are possibly unpreferred because they are expensive to cut. The unpreferred huckleberry flowers were inferior but did not cause rejection behavior. The mixed diet was just as productive as catkins or leaves. This study indicates that foragers possess a default mechanism to prefer catkins and frass, which can be quickly changed if substrates are bad. In contrast, there does not appear to be a similar mechanism causing substrates to become preferred quickly.     

Foraging and spatiotemporal territories in the honey ant Myrmecocystus mimicus wheeler (Hymenoptera: Formicidae)

Summary The honey ant Myrmecocystus mimicus is a scavenger, forages extensively on termites, collects floral nectar, and tends homoptera. Individual foragers of M. mimicus usually disperse in all directions when leaving the nest, but there are also groups of foragers that tend to swarm out of the nest primarily in one direction. Such massive departues are usually at irregular intervals, which may last several hours. The results of field and laboratory experiments suggest that these swarms of foragers are organized by a group recruitment process, during which recruiting scout ants lay chemical orientation trails with hindgut contents and simultaneously stimulate nestmates with a motor display and secretions from the poison gland. Usually these columns travel considerable distances (4–48 m) away from the nest, frequently interfering with the foraging activity of conspecific neighboring colonies.To prevent a neighboring colony from access to temporal food sources or to defend spatiotemporal borders, opposing colonies engage in elaborate display tournaments. Although hundreds of ants are often involved during these tournaments almost no physical fights occur. Instead, individual ants confront each other in highly sterotyped aggressive displays, during which they walk on stilt legs while raising the gaster and head. Some of the ants even seem to inflate their gasters so that the tergites are raised and the whole gaster appears to be larger. In addition, ants involved in tournament activities are on average larger than foragers.The dynamics of the tournament interactions were observed in several colonies over several weeks-mapping each day the locations of the tournaments, the major directions of worker routes away from the nest, and recording the general foraging activities of the colonies. The results indicate that a kind of dominance order can occur among neighboring colonies. On the other hand, often no aggressive interactions among neighboring colonies can be observed, even though the colonies are actively foraging. In those cases the masses of foragers of each colony depart in one major direction that does not bring them into conflict with the masses of foragers of a neighboring colony. This stability, however, can be disturbed by offering a new rich food source to be exploited by two neighboring colonies. This invariably leads to tournament interactions.When a colony is considerably stronger than the other, i.e., with a much larger worker force, the tournaments end quickly and the weaker colony is raided. The foreign workers invade the nest, the queen of the resident colony is killed or dirven off, while the larvae, pupae, callow workers, and honey pot workers are carried or dragged to the nest of the raiders. From these and other observations we conclude that young M. mimicus queens are unlikely to succeed in founding a colony within approximately 3 m of a mature M. mimicus colony because they are discovered and killed, or driven off by workers of the resident colony. Within approximately 3–15 m queens are more likely to start colonies, but these incipient groups run a high risk of being raided and exterminated by the mature colony.Although populations of M. mimicus and M. depilis tend to replace each other, there are areas where both species overlap marginally. Foraging areas and foraging habitats of both species also overlap broadly, but we never observed tournament interactions between M. mimicus and M. depilis.The adaptive significance of the spatiotemporal territories in M. mimicus is discussed.     

Multiple mate choice criteria and the importance of age for male mating success in the microhylid frog, Cophixalus ornatus

We examined multiple mate choice criteria in Cophixalus ornatus, a terrestrial breeding, microhylid frog. Mate choice consisted of three stages: mate attraction (male calling), courtship (male behavior between the call site and the nest), and nest site selection by the female. For male C. ornatus, the possession of a call with low dominant frequency relative to calling neighbors increased the probability that they would attract females. Dominant frequency was negatively correlated with age independent of male mass and snout vent length. When escorting the female from the call site to their nest, males traveled along more convoluted paths than when returning to the nest alone. The convolution of the path was, therefore, considered an aspect of courtship. Females released eggs into nests with structural characteristics typical of nests constructed by older males. Thus, females increased their chances of locating an acceptable nest by preferentially approaching males with lower dominant frequencies. This study is the first to demonstrate that age, independent of mass or snout-vent length, can influence call characteristics in anurans, and it is also the first to demonstrate the importance of male age to female mate choice in an amphibian.     

Quorum sensing, recruitment, and collective decision-making during colony emigration by the ant Leptothorax albipennis

When its nest is damaged, a colony of the ant Leptothorax albipennis skillfully emigrates to the best available new site. We investigated how this ability emerges from the behaviors used by ants to recruit nestmates to potential homes. We found that, in a given emigration, only one-third of the colony's workers ever recruit. At first, they summon fellow recruiters via tandem runs, in which a single follower is physically led all the way to the new site. They later switch to recruiting the passive majority of the colony via transports, in which nestmates are simply carried to the site. After this switch, tandem runs continue sporadically but now run in the opposite direction, leading recruiters back to the old nest. Recruitment accelerates with the start of transport, which proceeds at a rate 3 times greater than that of tandem runs. The recruitment switch is triggered by population increase at the new site, such that ants lead tandem runs when the site is relatively empty, but change to transport once a quorum of nestmates is present. A model shows that the quorum requirement can help a colony choose the best available site, even when few ants have the opportunity to compare sites directly, because recruiters to a given site launch the rapid transport of the bulk of the colony only if enough active ants have been "convinced" of the worth of the site. This exemplifies how insect societies can achieve adaptive colony-level behaviors from the decentralized interactions of relatively poorly informed insects, each combining her own limited direct information with indirect cues about the experience of her nestmates.     

Global optimization from suboptimal parts: foraging sensu lato by leaf-cutting ants

Central-place foraging theory has been unable to explain the load selection behavior of leaf-cutting ants (Atta spp., Attini: Formicidae). We suggest that this is due to incomplete consideration of the sequence of behaviors involved in resource acquisition by these ants. Unlike most central-place foragers, leaf-cutting ants do not return to their nests with food. Instead, the leaf fragments they gather must be processed within the nest to convert them to substrate for fungal gardens. We have shown previously that leaf fragment size affects the rate of distribution and processing of leaf tissue inside laboratory nests of Atta colombica. Including these tasks in the calculation of foraging rate may help explain load selection and other features of central-place foraging by Atta colonies. Here we develop a mathematical model of the complete sequence of external and internal tasks that lead to addition of substrate to fungal gardens. Using realistic parameter values, the leaf fragment sizes predicted to maximize a colony's rate of foraging in this broad sense correspond well with the mean fragment sizes actually collected by Atta colonies in the field. The optimal fragment size for global performance in the model is below the size that would maximize the delivery rate by above-ground foragers. The globally optimal size also fails to maximize the rate of either fragment distribution or fragment processing within the nest. Our results show how maximum collective performance of an ensemble of linked tasks may require behavior that would appear suboptimal in a piecemeal analysis of tasks.     

Collective decisions in ants when foraging under crowded conditions

In this paper we examine the effect of crowding on the selection of a path in the mass-recruiting ant Lasius niger. In our experiment, ants had to go from their nest to a food source by crossing a diamond-shaped bridge, giving the choice between two paths. Two types of bridges were used: the first had two branches of equal length but different width while the second had two branches of different length and width. Experiments at high traffic volume always ended up with the selection of the wider branch, even if it was longer. This result shows that overcrowding on the narrow branch plays an essential role in the mechanism underlying the choice of route in ants. A mathematical model was developed to evaluate the importance of two mechanisms that could account for this result. The first is based on the difference in travel duration between the two paths. The second is based on the repulsive interactions between workers making head-on encounters. The model shows that travel duration per se is not sufficient to explain path choice. Rather, it is the interplay between trail following behaviour and repulsive interactions that allows ants to choose the path that minimizes their travel time. When choosing a path ants thus prefer to trade time against energy. Our results demonstrate that any environmental constraint that alters the dynamics of trail recruitment can lead to the emergence of adaptive foraging decisions without any explicit coding of information by the foragers at the individual level.     

Improvement in collective performance with experience in ants

We show that entire ant colonies can improve their collective performance progressively when they repeat the same process. Colonies of Leptothorax albipennis can reduce their total emigration times over successive emigrations. We show that this improvement is based on experience and some memory-like process, rather than a coincidental developmental change or an increased general level of arousal. We demonstrate that the benefits of experience can be lost (i.e. forgotten) if the interval between successive emigrations is too long. We also show that the benefits of experience are more likely to be retained over a longer period if the collective performance has been repeated several times. This is a new demonstration of a process akin to learning in ants and we briefly discuss how it may involve not only improvements in individual performance but also improvements in the ways in which the ants interact with one another.Communicated by L. Sundström     

The iridoid glycoside,catalpol, as a deterrent to the predatorCamponotus floridanus (Formicidae)

Summary We investigated the role of the iridoid glycoside, catalpol, as a deterrent to the predator,Camponotus floridanus. Four laboratory colonies of this ant were offered buckeye caterpillars (Junonia coenia: Nymphalidae) raised on diets with and without catalpol. The same colonies were offered sugar-water solutions containing varying concentrations of catalpol, in both no-choice and choice tests. Regardless of diet, buckeye caterpillars appeared to be morphologically protected from predation by the ants, possibly because of their large spines or tough cuticle. However, buckeyes raised on diets with catalpol had high concentrations of catalpol in their hemolymph; extracts of this high-catalpol hemolymph proved to be an effective deterrent to the ants. When starved ants were not given the choice of food items, they were more likely to consume sucrose solutions that contained 5 mg catalpol/ml or 10 mg catalpol/ml than they were to consume solutions with 20 mg catalpol/ml. When they were given a choice of sugar solution or a sugar solution containing catalpol, the ants avoided solutions with catalpol at any of these concentrations. Ant colony responses to catalpol in sucrose solutions varied considerably over time and among colonies.     

The behaviour of ant transporters at the old and new nests during successive colony emigrations

Colonies of the ant Temnothorax albipennis improve their collective performance over successive emigrations (Langridge et al. Behav Ecol Sociobiol 56:523–529, 2004, Behav Ecol Sociobiol 62:447–456, 2008). Here, by analysing the performance of individual transporters (workers that carry the brood, queen and a proportion of adults), we investigate whether they spend less time at the old and new nests during repeated emigrations. Transporters expedited choosing and picking up brood items at the old nest and depositing them in the new nest. Such improvements were not associated with adult transport. Generally, when carrying brood items, but not when carrying adults, transporters visited several locations in the new nest before depositing them. Transporters did not interact with other adults when depositing brood items. Consequently, reductions in depositing times are the sum of time savings made by individual transporters. By contrast, transporters spent most time interacting with other adults before picking up brood items at the old nest. As the frequency of these interactions did not decline, we suggest the behaviours of interacting adults were modified in a way that hastened their completion. Thus, reductions in picking-up times probably occur because of time saved during interactions.     

A South East Asian ponerine ant of the genus Leptogenys (Hym., Form.) with army ant life habits

Summary The emigration and raiding behavior of the SE Asian ponerine ant Leptogenys sp. 1, which resembles L. mutabilis, were observed in the field (Ulu Gombak, Malaysia). The ants formed monogynous colonies that consisted of up to 52 100 workers. The bivouac sites of this species were found in leaf litter, rotten logs, ground cavities, etc., and were rarely modified by the ants. The colonies stayed in these temporary nests for several hours to 10 days; afterwards, they moved to a new nest site. The emigration distances ranged from 5–58 m. Since nest changing takes place at irregular intervals, and pupae and larvae are always present in the nest relocations of Leptogenys sp. 1, the emigration behavior is not linked to a synchronized brood development. Leptogenys sp. 1 is a nocturnal forager; in our study, up to 42 600 workers participated in each raid. The ants move forward on a broad front; behind the swarm a fan-shaped network of foraging columns converges to form a main trunk trail. A new system of foraging trails is developed in each raid. The workers search for their prey collectively; they attack and retrieve the booty together. The diet of Leptogenys sp. 1 consists mainly of arthropods. Army ant behavior is characterized by (1) formation of large monogynous colonies, (2) frequent emigrations, and (3) mass raids in which all foraging activities are carried out collectively. Since Leptogenys sp. 1 performs these typical army ant behavior patterns, this species represents the army ant ecotype. However, this species differs considerably from army ant species that have synchronized broods and huge colonies with dichthadiiform queens.Dedicated to Professor Dr. M. Lindauer on the occasion of his 70th birthday     

The migrating herdsman Dolichoderus (Diabolus) cuspidatus: an ant with a novel mode of life

Summary The Malayan ant Dolichoderus cuspidatus lives in obligatory symbiosis with the pseudococcid Malaicoccus formicarii and other species of the same genus. The assemblies, which may be encountered up to 25 m away from the nest, are constantly covered with a great number of worker ants who protect them and receive honeydew. In the event of heavy rain the workers from a dense protective cluster, clinging to each other on top of the mealybugs. Neither hunting behavior nor active search for protein sources was observed in D. cuspidatus, although dead insects were accepted as food. When not searching for new plants, the activity of the ants outside the colony is limited to visiting the mealybugs. During the night and parts of the day the ants stay in their nest. Ant colonies deprived of their mealybugs are not viable due to their dependence on the symbiosis and because of the competition of other ants. Antless M. formicarii are likewise not viable. The mealybugs are extremely polyphagous and feed on many different monocotylous and dicotylous angiosperms. They feed exclusively on the phloem sap of young plant parts which are rich in amino acids. Dolichoderus cuspidatus workers carry the mealybugs to such locations. During the picking up and carrying process both partners display typical behavioral patterns. The colonization of new feeding sites takes place in well organized mass processions. During the foundation or disintegration of large feeding complexes, provisional depots with waiting mealybugs and ants are set up. The pseudococcids are carried not only while shifting the feeding sites, but also whenever the colony leaves its former nesting site and especially when any kind of disturbance occurs. They are even carried about without any apparent external cause, which leads to the fact that, at all times of trail activity, on average more than 10% of all ants using the trails carry mealybugs. Mealybugs are also present within the nest, especially adult females which are viviparous and give birth to their offspring there. Censused colonies each consisted of over 10 000 workers, about 4000 larvae and pupae, more than 5000 mealybugs and one ergatoid queen. Male winged ants were observed in large numbers during the dry season (January–February) and during the rainy season (September–October). The colonies form typical clumplike bivouac nests consisting of clusters of workers clinging to each other, thereby covering the brood and the mealybugs. The nesting site is in no way altered by constructive measures and is mostly found close to the ground. The preferred nesting sites are clusters of leaves, and cavities in wood or soil, although a freely hanging bivouac between a few branches may be set up as well. As soon as the distance between the nest and the feeding site is too great the colony moves to the feeding site, whereby the brood and the mealybugs are carried along in a well organized manner. During such nest-moving the establishment of intermediate depots can be observed. A shift of nest sites can also be induced by disturbances or by a change in the microclimate in the vicinity of the nest. Colonies multiply by budding. The tropical rain forest continuously offers different sprouting plants, the utilization of which requires extreme mobility on the part of the consumer. The unique behavioral strategy of D. cuspidatus, to carry constantly their polyphagous mealybug partners to new feeding sites and to take the whole colony there has enabled this ant and its symbiont to occupy this rich food niche. Dolichoderus cuspidatus is the first true nomad found in ants.     

Is the alkaloid in 2spot ladybirds (Adalia bipunctata) a defence against ant predation?

Garden black ants,Lasius niger L., in a laboratory colony, attacked three species of live ladybirds found near their nest, killing the smaller two species. A second colony was offered artificial diets containing crushed ladybirds of two species, and the ants' choice of feeding site noted. Both the diets were aversive compared to control, but that containing 7spot,Coccinella septempunctata L., was more aversive than the diet containing 2spot,Adalia bipunctata L. The implications of this lesser protection for 2spots in terms of the chemical defence of the species are discussed.     

Modulation of individual behavior and collective decision-making during aggregation site selection by the ant<Emphasis Type="Italic"> Messor barbarus</Emphasis>

Insect societies are often confronted with choices among several options such as food sources of different richness or potential nest sites with different qualities. The mechanisms by which a colony as a whole evaluates these situations and takes the appropriate decision are of crucial importance for its survival. Here we studied how collective decisions arise from individual behaviors when a group of workers of the ant Messor barbarus is given a choice between two aggregation sites. Two hundred ants were introduced into an arena and given a choice between two tubes connected to the arena. The tubes had different physical properties: dry and transparent (termed as dry), humid and transparent (termed as humid), or dry and dark (termed as dark). After 30 min, most ants were found to be aggregated in a humid tube when paired with a dry tube, or in a dark tube when paired with a humid one. When two humid tubes were in competition, ants aggregate more in one of the sites. The choice of ants was consistent throughout experiments. An analysis of individual behaviors shows that the probability of an ant recruiting and the intensity of its trail-laying behavior strongly depend on the quality of the tubes. Our study suggests that the selection of an aggregation site does not require that individual ants directly compare sites, but rather relies on the synergy between amplification processes involving recruitment by chemical trails, and a modulation of the individual resting time in a site as a function of its population.Communicated by L. Sundström