Nest Enlargement in Leaf-Cutting Ants: Relocated Brood and Fungus Trigger the Excavation of New Chambers

During colony growth, leaf-cutting ants enlarge their nests by excavating tunnels and chambers housing their fungus gardens and brood. Workers are expected to excavate new nest chambers at locations across the soil profile that offer suitable environmental conditions for brood and fungus rearing. It is an open question whether new chambers are excavated in advance, or will emerge around brood or fungus initially relocated to a suitable site in a previously-excavated tunnel. In the laboratory, we investigated the mechanisms underlying the excavation of new nest chambers in the leaf-cutting ant Acromyrmex lundi. Specifically, we asked whether workers relocate brood and fungus to suitable nest locations, and to what extent the relocated items trigger the excavation of a nest chamber and influence its shape. When brood and fungus were exposed to unfavorable environmental conditions, either low temperatures or low humidity, both were relocated, but ants clearly preferred to relocate the brood first. Workers relocated fungus to places containing brood, demonstrating that subsequent fungus relocation spatially follows the brood deposition. In addition, more ants aggregated at sites containing brood. When presented with a choice between two otherwise identical digging sites, but one containing brood, ants'' excavation activity was higher at this site, and the shape of the excavated cavity was more rounded and chamber-like. The presence of fungus also led to the excavation of rounder shapes, with higher excavation activity at the site that also contained brood. We argue that during colony growth, workers preferentially relocate brood to suitable locations along a tunnel, and that relocated brood spatially guides fungus relocation and leads to increased digging activity around them. We suggest that nest chambers are not excavated in advance, but emerge through a self-organized process resulting from the aggregation of workers and their density-dependent digging behavior around the relocated brood and fungus.     

Sex allocation in the polydomous leaf-cutting ant <Emphasis Type="Italic">Acromyrmex</Emphasis><Emphasis Type="Italic">balzani</Emphasis>

Logistic regression analysis was used to analyse sex allocation in a population of the leaf-cutting ant Acromyrmex balzani occurring in a pasture in southern Brazil. The field sample consisted of 151 fungus-garden chambers (18 queenright and 133 queenless), belonging to 50 nests with three vertically stacked chambers per nest on average. Taking nest chamber as the unit of analysis, seven predictor variables were considered: sampling date, chamber depth, chamber volume, weight of fungus garden, presence of a queen, number of large workers, and number of small to medium workers. The population-level numerical proportion of females was 0.548 and the inferred proportional energetic investment in females 0.672. The former was not significantly different from 0.5 (P=0.168), but the latter was (P=0.0003). The proportional investment in females per fungus garden increased with the number of large workers present (P=0.0002) and decreased with the dry weight of the fungus garden (P=0.012). This implies that resource acquisition through foraging is likely to be a major proximate determinant of sex allocation. The negative correlation between female bias and fungus garden weight might be due to developing adult females requiring more food than males, but this hypothesis could not be confirmed by direct statistical evidence.     

Compound nesting of <Emphasis Type="Italic">Strumigenys</Emphasis> sp. (Myrmicinae) and <Emphasis Type="Italic">Diacamma</Emphasis> sp. (Ponerinae), and other nesting symbioses of myrmicine and ponerine ants in Southeast Asia

Summary. A new type of compound nest, inhabited by the small Strumigenys sp.1 (Myrmicinae) and the relatively big Diacamma sp.1 (Ponerinae), was found in Java, Indonesia. Field records as well as results of experimental studies were used to describe this association. The compound nests were relatively frequent in the research area: 75% of the examined Diacamma sp.1 nests were also inhabited by Strumigenys sp.1. Strumigenys sp.1 is specialized on nesting in small, excavated nest chambers, in close vicinity to Diacamma sp.1, while Diacamma sp.1 does not depend on Strumigenys sp.1. The rather small workers of this myrmicine species forage for food inside the Diacamma nests as well as outside, favouring mites and collembolans as prey, but principally not rejecting any other nitrogen source. The refuse pile of Diacamma sp.1 inside the nest houses small living invertebrates and insect parts, preyed upon by Strumigenys sp.1. Given the choice between larvae of Diacamma sp.1 and mites, Strumigenys sp.1 clearly preferred the mites. Thus, the association seems to be beneficial to Strumigenys sp.1, and does not harm Diacamma sp.1. We found two more similar compound nests: In Java, a different Strumigenys species was frequently found nesting within the nest of a Pachycondyla (Ponerinae) species. In Sabah, East Malaysia, we recorded for the first time a Pheidole (Myrmicinae) species nesting in chambers coming out of the walls of Diacamma nest chambers. Existing classifications for compound nest associations (i.e. xenobiosis, cleistobiosis etc.) are too restricted, because they were based on a few cases. Hence, we present a list of non-normative traits describing the various types of nest symbioses by ants.     

Prevalence and impact of a virulent parasite on a tripartite mutualism

The prevalence and impact of a specialized microfungal parasite (Escovopsis) that infects the fungus gardens of leaf-cutting ants was examined in the laboratory and in the field in Panama. Escovopsis is a common parasite of leaf-cutting ant colonies and is apparently more frequent in Acromyrmex spp. gardens than in gardens of the more phylogenetically derived genus Atta spp. In addition, larger colonies of Atta spp. appear to be less frequently infected with the parasite. In this study, the parasite Escovopsis had a major impact on the success of this mutualism among ants, fungi, and bacteria. Infected colonies had a significantly lower rate of fungus garden accumulation and produced substantially fewer workers. In addition, the extent of the reduction in colony growth rate depended on the isolate, with one isolate having a significantly larger impact than two others, suggesting that Escovopsis has different levels of virulence. Escovopsis is also spatially concentrated within parts of ant fungus gardens, with the younger regions having significantly lower rates of infection as compared to the older regions. The discovery that gardens of fungus-growing ants are host to a virulent pathogen that is not related to any of the three mutualists suggests that unrelated organisms may be important but primarily overlooked components of other mutualistic associations.     

Waste of leaf-cutting ants: disposal,nest structure,and abiotic soil factors around internal waste chambers

Leaf-cutting ants produce large quantities of waste that harbor bacteria and fungi that are harmful to the colony. To be protected from these pathogens, the workers of Atta species present a sophisticated organization to manage harmful material, which can be deposited outside the nest or in internal chambers. However, little is known about the behavior of Acromyrmex species in handling and disposal of waste. Due to some observations, we assume that the same species of Acromyrmex can deposit waste outside the nest and into internal chambers and raise the following question: what determines the occurrence of internal waste chambers in Acromyrmex? To address this question, we verified whether nest depth influences the waste-chamber occurrence. We also verified the nest structure and the abiotic factors of soil beside each waste-chamber: pH and water content of the soil. For this, eight nests were excavated for Acromyrmex balzani and Acromyrmex rugosus rugosus. We verified that not only can the same leaf-cutting ant species deposit debris both outside and inside the nest but also the same nest can present internal chambers and external waste deposit. The soil beside the waste chamber always presented an acidic pH, while the humidity varied widely. Our results showed that the nest depth was highly correlated with the depth of the waste chamber (p = 0.0003) and probably has some influence on waste disposal. The characteristics of the nest and the role of depth in the choice of waste chamber location are discussed.     

Soil temperature, digging behaviour, and the adaptive value of nest depth in South American species of Acromyrmex leaf-cutting ants

In leaf-cutting ants, workers are expected to excavate the nest at a soil depth that provides suitable temperatures, since the symbiotic fungus cultivated inside nest chambers is highly dependent on temperature for proper growth. We hypothesize that the different nesting habits observed in Acromyrmex leaf-cutting ants in the South American continent, i.e. superficial and subterranean nests, depend on the occurrence, across the soil profile, of the temperature range preferred by workers for digging. To test this hypothesis, we first explored whether the nesting habits in the genus Acromyrmex are correlated with the prevailing soil temperature regimes at the reported nest locations. Second, we experimentally investigated whether Acromyrmex workers engaged in digging use soil temperature as a cue to decide where to excavate the nest. A bibliographic survey of nesting habits of 21 South American Acromyrmex species indicated that nesting habits are correlated with the soil temperature regimes: the warmer the soil at the nesting site, the higher the number of species inhabiting subterranean nests, as compared to superficial nests. For those species showing nesting plasticity, subterranean nests occurred in hot soils, and superficial nests in cold ones. Experimental results indicated that Acromyrmex lundi workers use soil temperature as an orientation cue to decide where to start digging, and respond to rising and falling soil temperatures by moving to alternative digging places, or by stopping digging, respectively. The soil temperature range preferred for digging, between 20°C and maximally 30.6°C, matched the range at which colony growth would be maximized. It is suggested that temperature-sensitive digging guides digging workers towards their preferred range of soil temperature. Workers’ thermopreferences lead to a concentration of digging activity at the soil layers where the preferred range occurs, and therefore, to the construction of superficial nests in cold soils, and subterranean ones in hot soils. The adaptive value of the temperature-related nesting habits, and the temperature-sensitive digging, is further discussed.     

Leaf‐cutting ants use relative humidity and temperature but not CO2 levels as cues for the selection of an underground dumpsite

1. Leaf‐cutting ants remove copious amounts of colony waste, a potential pathogen source for workers and reared symbiotic fungus, to above‐ground heaps or deep underground chambers. However, the dumpsite may also contain information about plants initially harvested and disposed of because of unsuitability for the fungus. 2. The underground environment presents climatic gradients across the soil profile and it is an open question whether leaf‐cutting ants use microclimatic cues to choose suitable sites for waste disposal, as displayed for other in‐nest tasks. 3. Climatic preferences in leaf‐cutting ants were investigated for the deposition of colony waste. In the laboratory, deposition of waste particles by workers of Atta laevigata was quantified by offering them, in different experiments, a binary choice of temperatures (range, 15–30 °C), levels of air humidity (range 10–98%), and CO₂ concentrations (range, atmospheric values to 10%). 4. Leaf‐cutting ants used temperature and air humidity, but not CO₂ levels, as cues for the deposition of their waste. They consistently preferred a dry (≤ 33% air humidity) environment. Less consistent, temperature preferences varied depending on colony (15–25 °C for one colony and 25–30 °C for the other). Although workers showed clear preferences for high levels of CO₂ for themselves, they were CO₂‐indifferent for waste deposition. 5. It is argued that the observed climatic preferences for underground waste disposal might aid nest hygiene by providing unsuitable dry conditions for pathogen growth, with thermal preferences that do not hinder worker activities for further waste management and inspection of discarded plants.     

Ventilation of the giant nests of Atta leaf-cutting ants: does underground circulating air enter the fungus chambers?

Nest ventilation should be particularly relevant for the huge colonies of leaf-cutting ants, genus Atta. Considerable amounts of O₂ are consumed and CO₂ produced by both the fungus gardens and the ants inside nest chambers, which are located at deep soil layers characterized by high CO₂ and low O₂ concentrations. In this work, passive nest ventilation was investigated in field Atta capiguara and Atta laevigata nests, first, by evaluating air movements through the nest using propane as tracer gas as well as the CO₂ and O₂ concentrations of the circulating air, and second, by exposing the internal nest morphology with the use of cement casts and excavations. Results showed that even though outflow of CO₂-rich air and inflow of O₂-rich air occurred at high-placed and low-placed openings, respectively, supporting a wind-induced interpretation of air movements through the nest, circulating air was never detected inside fungus chambers. The CO₂ and O₂ levels inside the fungus chambers increased and decreased with increasing soil depth, respectively, and were in the range observed in the soil phase. Based on the underground nest architecture, it is concluded that although the external shape of the nest induces underground air circulation, the inflowing air is unable to directly reach the fungus chambers. It is argued that colony respiration completely depends on diffusive flows between the chamber air and the adjacent nest and soil atmospheres. Circulating air, although not directly renewing the air inside the nest chambers, may contribute to colony respiration by increasing the capacity of the nest and soil airs to act as an O₂-source and a CO₂-sink, because of the decrease in the CO₂ and the increase in the O₂ levels in the underground air phase. Possible adaptations of both ants and fungus to the high CO₂ and low O₂ concentrations usually found in soils are discussed.     

Substrate distribution in fungus chambers in nests of Atta bisphaerica Forel, 1908 (Hym., Formicidae)

Abstract:  Substrate distribution was studied in three adult colonies of the leaf-cutting ant Atta bisphaerica Forel, 1908 using dye. Some supply holes were mapped using baits made from small plastic straws impregnated with citric pulp. Two holes, equidistant from one another, were then selected in each nest. The colonies were excavated completely 24 h after bait placement. During excavation, fungus chambers were checked for dye. We observed that the dyed baits were distributed in all sectors and at all depths regardless of where the baits had been placed. This supported the hypothesis that toxic baits placed in a single supply hole are uniformly distributed throughout the colony.     

Individual complexity and self-organization in foraging by leaf-cutting ants

Leaf-cutting ants cut vegetation into small fragments that they transport to the nest, where a symbiotic fungus cultivated by the ants processes the material. Since the harvested leaf fragments are incorporated into the fungus garden and not directly consumed by the workers, it is expected that foraging workers select plants by responding to those physical or chemical traits that promote maximal fungal growth, irrespective of the potential direct effects of these leaf features on them. In this paper I summarize experimental work focusing on the decision-making processes that occur at the individual level, and discuss to what extent individual complexity contributes to the emergence of collective foraging patterns. Although some basic features of self-organizing systems, such as the existence of regulatory positive and negative feedback loops, are expected to be involved in the collective organization of leaf-cutting ant foraging, I contend that they are combined with complex individual responses that may result from the integration of local information during food collection with an assessment of colony conditions.     

Optional gamergates in the queenright ponerine ant Gnamptogenys striatula Mayr

Summary: Ponerine ants display a number of social structures to which particular behaviours are associated. In the ponerine ant species Gnamptogenys striatula Mayr, queens occur and queenright colonies are functionally polygynous. However, some workers are capable to mate and to produce their own worker offspring. These gamergates appear several days after the queens are experimentally removed, allowing some workers to adopt a sexual calling posture inside the nest. At that time, other workers get outside the nest to collect males in the arena and carry them back into the colony. There, these males are groomed before they can mate with sexual calling workers. As for queens, several gamergates may coexist in a same colony. The social profile of gamergates is similar to those of nurses and they stay closer to the egg piles.     

Parasitism and biology of Myrmosicarius grandicornis (Diptera, Phoridae) in relationship to its host, the leaf-cutting ant Atta sexdens (Hymenoptera, Formicidae)

Summary. The parasitic phorid Myrmosicarius grandicornis Borgmeier is commonly found around nest entrances of the leaf-cutting ant Atta sexdens (L.) in Brazil, but there is no information about the importance of this fly for A. sexdens. We evaluated the parasitic capability of female M. grandicornis collected in the field and released in laboratory nests of A. sexdens and compared ants' foraging rhythm before and after M. grandicornis were released. We also determined biological characteristics of the parasitoid. Presence of M. grandicornis elicited foragers to abandon their loads and return to the nest, an effect previously described for the phorid Neodohrniphora sp., which is a more abundant A. sexdens parasitoid. Both phorid species occur at the same foraging trails during part of the year and attack ants of different size classes. Therefore, they may have a combined effect on the reduction of A. sexdens foraging. Myrmosicarius grandicornis pupariated inside hosts' head capsules, which are detached from their bodies and deposited in the nest's refuse pile. Adults emerged through the hosts' mouth cavity about a month after oviposition. The characteristics of M. grandicornis biology suggest that this parasitoid completes its development inside A. sexdens nests.     

Head-on encounter rates and walking speed of foragers in leaf-cutting ant traffic

Summary. Trail traffic of the leaf-cutting ant Atta cephalotes involves intermingled flows of outbound and returning foragers. Head-on encounters between workers from the opposite flows are a common occurrence in this traffic. Each encounter momentarily delays the two ants involved, and these small delays might pose a significant cost to the colony's foraging performance when summed over thousands of workers along many metres of trail. We videotaped outbound and returning foragers over a 1 m course, and measured the encounter rates they experienced and their velocity. Our analysis indicates that locomotion speed is diminished by increasing encounter rate, but that the effect is small relative to the effects of ant body size and load mass. Head-on encounters allow exchange of information and leaf fragments between workers, and we consider how the benefits of such encounters may make this form of traffic organization superior to segregated outbound and returning lanes, despite the measurable c ost of encounters in mixed traffic.     

Sequential Soil Transport and Its Influence on the Spatial Organisation of Collective Digging in Leaf-Cutting Ants

The Chaco leaf-cutting ant Atta vollenweideri (Forel) inhabits large and deep subterranean nests composed of a large number of fungus and refuse chambers. The ants dispose of the excavated soil by forming small pellets that are carried to the surface. For ants in general, the organisation of underground soil transport during nest building remains completely unknown. In the laboratory, we investigated how soil pellets are formed and transported, and whether their occurrence influences the spatial organisation of collective digging. Similar to leaf transport, we discovered size matching between soil pellet mass and carrier mass. Workers observed while digging excavated pellets at a rate of 26 per hour. Each excavator deposited its pellets in an individual cluster, independently of the preferred deposition sites of other excavators. Soil pellets were transported sequentially over 2 m, and the transport involved up to 12 workers belonging to three functionally distinct groups: excavators, several short-distance carriers that dropped the collected pellets after a few centimetres, and long-distance, last carriers that reached the final deposition site. When initiating a new excavation, the proportion of long-distance carriers increased from 18% to 45% within the first five hours, and remained unchanged over more than 20 hours. Accumulated, freshly-excavated pellets significantly influenced the workers'' decision where to start digging in a choice experiment. Thus, pellets temporarily accumulated as a result of their sequential transport provide cues that spatially organise collective nest excavation.     

Yeasts and filamentous fungi carried by the gynes of leaf-cutting ants

Insect-associated microbes exhibit a wide range of interactions with their hosts. One example of such interactions is the insect-driven dispersal of microorganisms, which plays an essential role in the ecology of several microbes. To study dispersal of microorganisms by leaf-cutting ants (Formicidae: Attini), we applied culture-dependent methods to identify the filamentous fungi and yeasts found in two different body parts of leaf-cutting ant gynes: the exoskeleton and the infrabuccal pocket. The gynes use the latter structure to store a pellet of the ants’ symbiotic fungus during nest founding. Many filamentous fungi (n = 142) and yeasts (n = 19) were isolated from the gynes’ exoskeleton. In contrast, only seven filamentous fungi and three yeasts isolates were recovered from the infrabuccal pellets, suggesting an efficient mechanism utilized by the gynes to prevent contamination of the symbiotic fungus inoculum. The genus Cladosporium prevailed (78%) among filamentous fungi whereas Aureobasidium, Candida and Cryptococcus prevailed among yeasts associated with gynes. Interestingly, Escovopsis, a specialized fungal pathogen of the leaf-cutting ant-fungus symbiosis, was not isolated from the body parts or from infrabuccal pellets of any gynes sampled. Our results suggest that gynes of the leaf-cutter ants Atta laevigata and A. capiguara do not vertically transmit any particular species of yeasts or filamentous fungi during the foundation of a new nest. Instead, fungi found in association with gynes have a cosmopolitan distribution, suggesting they are probably acquired from the environment and passively dispersed during nest foundation. The possible role of these fungi for the attine ant–microbial symbiosis is discussed.     

Colony Dehydration and Water Collection by Specialized Caste in the Leaf-Cutting Ant <Emphasis Type="Italic">Atta Sexdens Rubropilosa</Emphasis>

Social organization enables leaf-cutting ants to keep appropriate micro-ecological nest conditions for the fungus garden (their main food), eggs, larvae and adults. To maintain stability while facing changing conditions, individual ants must perceive destabilising factors and produce a proper behavioral response. We investigated behavioral responses to experimental dehydration in leaf-cutting ants to verify if task specialization exists, and to quantify the ability of ant sub-colonies for water management. Our setup consisted of fourteen sub-colonies, ten of which were randomly assigned to different levels of experimental dehydration with silica gel, whereas the remaining four were controls. The ten experimental sub-colonies were split into two groups, so that five of them had access to water. Diverse ant morphs searched for water in dehydrated colonies, but mainly a caste of small ants collected water after sources had been discovered. Size specialization for water collection was replicable in shorter experiments with three additional colonies. Ants of dehydrated colonies accumulated leaf-fragments on the nest entrance, and covering the fungus garden. Behaviors that may enhance humidity within the nests were common to all dehydration treatments. Water availability increased the life span of dehydrated colonies.     

Collective control of nest climate parameters in bumblebee colonies

We examined two aspects of the social control of nest climate in bumblebee colonies: which parameters of nest climate bumblebees actively down-regulate by fanning and the dynamics of the colony response as colony size increased. Colonies of Bombus terrestris were exposed to an increase in carbon dioxide, temperature or relative humidity. We performed 70 temperature trials (six colonies), 58 CO₂ trials (four colonies) and four humidity trials (two colonies). An increase in CO₂ concentration and temperature elicited a fanning response whereas an increase in relative humidity did not. This is the first report of fanning in bumblebee colonies to control respiratory gases. The number of fanning bees increased with stimulus intensity. The colony response to a CO₂ concentration of 3.2% was comparable to the colony response to a temperature of 30°C. A marked fanning response occurred at 1.6% CO₂, a concentration never exceeded in a large field nest during a pilot measurement of 10 days. We investigated the colony response over a wide range of colony sizes (between 10 and 119 workers). The proportion of the total workforce invested by colonies in nest ventilation did not change significantly; thus, the number of fanning workers increased with colony size. Furthermore, as colony size increased, the dynamics of the colony response changed: colonies responded faster to perturbations of their environment when they were large (60 or more individuals) than when they were small. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Polymorphism and Division of Labor During Foraging Cycles in the Leaf-cutting Ant <Emphasis Type="Italic">Acromyrmex octospinosus</Emphasis> (Formicidae; Attini)

While division of labor within leaf-cutting ant nests has been well-characterized in the context of the collection and processing of leaf material, environmental factors such as day-night cycles and heavy rainfall limit the time during which leaf-cutting ant workers leave the nest to gather forage. Using a novel “flat panel” nest design, we studied how patterns of within-nest task performance changed when a colony of the leaf-cutting ant Acromyrmex octospinosus was and was not provided access to forage. We conducted scan samples of individuals working within the nest under both conditions and compared task allocation patterns across provisioning regimes and between workers of different sizes. When labor was compared between worker size groups, “minor” workers (head width ≤2.0 mm) and “major” workers (head width >2.0 mm) showed significantly different task performance patterns when forage was available: minors performed mostly brood-care and garden maintenance, while majors were mostly involved in the handling of freshly-cut leaf fragments. In contrast, when the colony was deprived of forage, the task performance patterns of minor and major workers converged and did not significantly differ. Marked major workers known to be foragers tended to remain idle within the nest when the colony was deprived of forage, while non-foragers of similar head width engaged in a variety of within-nest tasks, suggesting polyethism in majors may be based on factors other than size.     

Digging effort in leaf-cutting ant queens (<Emphasis Type="Italic">Atta sexdens rubropilosa</Emphasis>) and its effects on survival and colony growth during the claustral phase

Nest foundation in the leaf-cutting ant Atta sexdens is claustral, and the single queen completely relies on its body reserves throughout, approximately, 9 weeks until the first workers emerge and initiate foraging. Nest digging is much time- and energy-consuming, and it is an open question how queens decide on the length of the tunnel they dig and therefore the depth of the initial chamber. Shallow founding nests may be energetically cheaper to dig, but queens may be more exposed to changing environmental variables. Deeper nests, on the other hand, may be climatically more stable and suitable, but more expensive to dig. We hypothesized that the maximal nest depth excavated by Atta founding queens may represent the outcome of an evolutionary trade-off between maximizing nest depth and minimizing energy expenditure during digging, so as to save energy for the long claustral phase. We tested this hypothesis by comparing the fitness consequences of increased digging effort in queens that were experimentally stimulated to excavate a complete founding nest either once, twice or three times consecutively compared to control queens that did not dig. Fitness was quantified as mortality rates, rates of egg-laying and offspring production, and size of the fungus garden until the emergence of the first workers. Results showed that, in contrast with the initial expectations, fungus growth, egg-laying rates and offspring production were not affected by the increased digging effort in the experimentally induced successive excavations. However, a significant higher mortality was observed in queens with increased digging effort, i.e., those that dug two or three nests consecutively. It is argued that in queens a behavioral mechanism for the control of nest depth has evolutionary been selected for as a trade-off between maximizing nest depth, to favor protection of the queen against unsuitable environmental variables, and minimizing energy expenditure during digging, which significantly affects survival.     

Nutritional function of replete workers in the pharaoh's ant, Monomorium pharaonis (L.)

Summary: Queens of the pharaoh's ant Monomorium pharaonis (L.), like several other ant species, feed on larval secretions as their main nourishment and their fecundity is positively correlated with the number of large larvae present in the nest. The surplus of secretions produced by larvae is stored in a temporary caste of replete workers, which comprises young workers who remain in the nest and store liquid nourishment. Repletes are characterised by a conspicuously large gaster, caused by large amounts of liquid food stored in the crop, from which it may be regurgitated and distributed among colony members. In this study, repletes of pharaoh's ants were demonstrated to be functioning as buffers, smoothing fluctuations in availability of high quality food to the reproductive queens when larvae are scarce or missing, thus temporarily keeping up the egg production of queens.¶In undisturbed two-queen colonies with 20 large worker larvae and 30 workers (15 young and 15 old workers), approximately 10 repletes developed (one replete per two larvae). Development of older workers into repletes, when some or all repletes had been removed from the colonies, demonstrated that their temporal polyethism exhibits great plasticity in this trait.¶This study confirmed that, in pharaoh's ants, the regulation of fecundity depends not only on the food flow to the queen from larvae or from repletes but also on an unknown larval stimulus.¶The term crop repletes is suggested for replete workers which use their crop to store nourishment, as opposed to fat-body repletes, which store nourishment in their fat body.¶The presence of brood tending crop repletes in nests in several European ant species of Leptothorax, Myrmica, and Lasius, show that repletism is a common trait in ants, and that it may play an important role in regulation of nutrition in ant colonies, as demonstrated in Monomorium pharaonis.