Just follow your nose: homing by olfactory cues in ants

How is an ant-equipped with a brain that barely exceeds the size of a pinhead-capable of achieving navigational marvels? Even though evidences suggest that navigation is a multimodal process, ants heavily depend on olfactory cues-of pheromonal and non-pheromonal nature-for foraging and orientation. Recent studies have directed their attention to the efficiency of pheromone trail networks. Advances in neurophysiological techniques make it possible to investigate trail pheromone processing in the ant's brain. In addition to relying on pheromone odours, ants also make use of volatiles emanating from the nest surroundings. Deposited in the vicinity of the nest, these home-range markings help the ants to home after a foraging run. Furthermore, olfactory landmarks associated with the nest enhance ants' homing abilities.     

Visual experience and age affect synaptic organization in the mushroom bodies of the desert ant Cataglyphis fortis

Desert ants of the genus Cataglyphis undergo an age‐related polyethism from interior workers involved in brood care and food processing to short‐lived outdoor foragers with remarkable visual navigation capabilities. The quick transition from dark to light suggests that visual centers in the ant's brain express a high degree of plasticity. To investigate structural synaptic plasticity in the mushroom bodies (MBs)—sensory integration centers supposed to be involved in learning and memory—we immunolabeled and quantified pre‐ and postsynaptic profiles of synaptic complexes (microglomeruli, MG) in the visual (collar) and olfactory (lip) input regions of the MB calyx. The results show that a volume increase of the MB calyx during behavioral transition is associated with a decrease in MG numbers in the collar and, less pronounced, in the lip. Analysis of tubulin‐positive profiles indicates that presynaptic pruning of projection neurons and dendritic expansion in intrinsic Kenyon cells are involved. Light‐exposure of dark‐reared ants of different age classes revealed similar effects. The results indicate that this structural synaptic plasticity in the MB calyx is primarily driven by visual experience rather than by an internal program. This is supported by the fact that dark‐reared ants age‐matched to foragers had MG numbers comparable to those of interior workers. Ants aged artificially for up to 1 year expressed a similar plasticity. These results suggest that the high degree of neuronal plasticity in visual input regions of the MB calyx may be an important factor related to behavior transitions associated with division of labor. © 2010 Wiley Periodicals, Inc. Develop Neurobiol 70: 408–423, 2010     

Light exposure leads to reorganization of microglomeruli in the mushroom bodies and influences juvenile hormone levels in the honeybee

Honeybees show a remarkable behavioral plasticity at the transition from nursing inside the hive to foraging for nectar and/or pollen outside. This plasticity is important for age‐related division of labor in honeybee colonies. The behavioral transition is associated with significant volume and synaptic changes in the mushroom bodies (MBs), brain centers for sensory integration, learning, and memory. We tested whether precocious sensory exposure to light leads to changes in the density of synaptic complexes [microglomeruli (MG)] in the MBs. The results show that exposure to light pulses over 3 days induces a significant decrease in the MG density in visual subregions (collar) of the MB. Earlier studies had shown that foragers have increased levels of juvenile hormone (JH) co‐occurring with a decrease of vitellogenin (Vg). Previous work further established that RNAi‐mediated knockdown of vg and ultraspiracle (usp) induced an upregulation of JH levels, which can lead to precocious foraging. By disturbing both Vg and JH pathways using gene knockdown of vg and usp, we tested whether the changes in the hormonal system directly affect MG densities. Our study shows that MG numbers remained unchanged when Vg and JH pathways were perturbed, suggesting no direct hormonal influences on MG densities. However, mass spectrometry detection of JH revealed that precocious light exposure triggered an increase in JH levels in the hemolymph (HL) of young bees. This suggests a dual effect following light exposure via direct effects on MG reorganization in the MB calyx and a possible positive feedback on HL JH levels. © 2014 Wiley Periodicals, Inc. Develop Neurobiol 74: 1141–1153, 2014     

Evidence for olfactory learning in procellariiform seabird chicks

Burrow nesting procellariiform seabirds use olfactory cues for both foraging and nest recognition. As chicks, burrow nesters develop in the dark, but are exposed to both prey‐related and individual‐specific scents through contact with their parents. This exposure suggests that chicks may have the opportunity to learn odours while still in the nest. In this study, we examined whether exposure to odourants during development might influence olfactory search behaviour expressed later in life. To test this idea, we exposed eggs of thin‐billed prions Pachyptila belcheri to a rosy‐scented novel odour (phenyl ethyl alcohol, PEA) or a control (water) just before hatching; chicks were then tested with these odours in a simple wind tunnel. Prior to fledging, subjects who had received pre‐exposure to PEA displayed head sweeps nearly twice as frequently as control birds did when presented with PEA. This study demonstrates that under natural rearing conditions, procellariiforms learn odour characteristics of their rearing environment in the nest.     

Horizontal transfer of insecticides in laboratory colonies of the Argentine ant (Hymenoptera: Formicidae)

Five insecticides used by urban pest management professionals for ant control and three experimental insecticides were tested to determine whether these insecticides were horizontally transferred among individuals in colonies of Argentine ants, Linepithema humile (Mayr) (Hymenoptera: Formicidae). Ants were exposed to insecticide-treated sand for 1 min and then placed in a colony of untreated ants. Ants exposed to 20 and 40 ppm fipronil readily transferred the insecticide to other individuals in the colony, resulting in high mortality. Most of the transfer and subsequent mortality occurred within 4 d after exposure to treated ants. The other insecticides were not transferred, and ants exhibited mortality rates similar to that of the controls. Experiments in large foraging arenas demonstrated that necrophoresis was an important behavior facilitating the horizontal transfer of fipronil. When ants contacted contaminated corpses in the process of removing them to refuse piles, they received a lethal dose of fipronil and subsequently died. Fipronil-contaminated dead ants that were placed in the vicinity of the nest resulted in significantly higher mortality than did corpses placed in a distant foraging arena (30 cm away). Most of the dead ants accumulated in the vicinity of the nest rather than in the foraging arena, workers retrieving dead ants to refuse piles from the foraging arena. The position effect of insecticide-contaminated corpses relative to the nest and its implication for Argentine ant control are discussed.     

Desert ants learn vibration and magnetic landmarks

The desert ants Cataglyphis navigate not only by path integration but also by using visual and olfactory landmarks to pinpoint the nest entrance. Here we show that Cataglyphis noda can additionally use magnetic and vibrational landmarks as nest-defining cues. The magnetic field may typically provide directional rather than positional information, and vibrational signals so far have been shown to be involved in social behavior. Thus it remains questionable if magnetic and vibration landmarks are usually provided by the ants' habitat as nest-defining cues. However, our results point to the flexibility of the ants' navigational system, which even makes use of cues that are probably most often sensed in a different context.     

Age‐related and light‐induced plasticity in opsin gene expression and in primary and secondary visual centers of the nectar‐feeding ant Camponotus rufipes

Camponotus rufipes workers are characterized by an age‐related polyethism. In the initial weeks of adult life, young workers perform tasks inside the nest before they switch to multimodal foraging tasks outside. We tested the hypothesis that this transition is accompanied by profound adaptations in the peripheral and central visual systems. Our results show that C. rufipes workers of all tested ages (between 1 and 42 days) express three genes encoding for ultraviolet (UV), blue (BL), and long‐wavelength (LW1) sensitive opsins in their retina, which are likely to provide the substrate for trichromatic color vision. Expression levels of all three opsin genes increased significantly within the first two weeks of adulthood and following light exposure. Interestingly, the volumes of all three optic neuropils (lamina, medulla, and lobula) showed corresponding volume increases. Tracing of connections to higher visual centers in the mushroom bodies (MBs) revealed only one optic pathway, the anterior superior optic tract, emerging from the medulla and sending segregated input to the MB‐calyx collar. The MB collar volumes and densities of synaptic complexes (microglomeruli, MGs) increased with age. Exposure to light for 4 days induced a decrease in MG densities followed by an increase after extended light exposure. This shows that plasticity in retinal opsin gene expression and structural neuroplasticity in primary and secondary visual centers comprise both “experience‐independent” and “experience‐dependent” elements. We conclude that both sources of plasticity in the visual system represent important components promoting optimal timing of the interior–forager transition and flexibility of age‐related division of labor. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1041–1057, 2016     

Behavioural and olfactory variations in the leek moth, Acrolepiopsis assectella, after several generations of rearing under diverse conditions

Females of the parasitic phorid Neodohrniphora sp. were collected in the field and released singly inside an observation chamber placed between a laboratory colony of Atta sexdens (L.) and its foraging arena. The number and speed of loaded and unloaded ants returning to the nest, the weight of foragers and their loads, the number of leaf fragments abandoned by ants, and the number of small workers 'hitchhiking' on leaf fragments were measured before phorids were released, while they were in the observation chamber, and after they were removed. Relatively few ants were attacked by Neodohrniphora sp., but the presence of flies prompted outbound ants to return to the nest and caused a significant reduction on the number and mass of foragers. Additionally, the weight of leaf fragments transported by ants was reduced and the number of abandoned fragments increased in response to Neodohrniphora sp. Presence of the parasitoid caused no significant changes in the number of hitchhiking ants. The regular ants' traffic was resumed after phorids were removed, but foraging activity remained below normal for up to three hours. In the field A. sexdens forages mostly at night, but colonies undergo periods of diurnal foraging during which ants are subject to parasitism from several species of phorid flies. Considering that daytime foraging may be necessary for nutritional or metabolical needs, phorids may have a significant impact on their hosts by altering their foraging behavior regardless of the numerical values of parasitism.     

Caste-specific postembryonic development of primary and secondary olfactory centers in the female honeybee brain

Eusocial insects are characterized by division of labor among a sterile worker caste and a reproductive queen. In the honeybee both female castes are determined postembryonically by environmental factors, and queens develop substantially faster than workers. Since olfaction plays a crucial role in organizing honeybee behavior and social interactions, we compared the development of primary and secondary olfactory centers in the brain. Age-synchronized queen and worker pupae were raised in incubators at 34.5 degrees C, and their external morphology was characterized for all pupal stages. The development of olfactory synaptic neuropil was analyzed using anti-synapsin immunocytochemistry, f-actin-phalloidin labeling and confocal microscopy. In the antennal lobes of queens olfactory glomeruli formed approximately 4 days earlier than in workers. The adult number of olfactory glomeruli was in a similar range, but the total glomerular volume was slightly smaller in queens. Olfactory and visual subdivisions (lip, collar) of the mushroom-body calyx formed early, whereas the basal ring separated late. Synaptic microglomeruli in the olfactory lip were established approximately 3-4 days earlier in queens compared to workers. We propose that developmental heterochrony results in fewer synapses in olfactory centers (smaller glomeruli, fewer microglomeruli) in queens, which may result in poorer performance on olfactory learning tasks compared to workers.     

Distance estimation in the third dimension in desert ants

Desert ants of the genus Cataglyphis perform large-scale foraging excursions from which they return to their nest by path integration. They do so by integrating courses steered and the distances travelled into a continually updated home vector. While it is known that the angular orientation is based on skylight cues, it still is largely enigmatic how the ants measure distances travelled. We extended the ants' task into the third dimension by training them to walk within an array of uphill and downhill channels, and later testing them on flat terrain, or vice versa. In these tests the ants indicated homing distances that did not correspond to the distances actually travelled, but to the ground distances; that is, to the sum of the horizontal projections of the uphill and downhill segments of the ants' paths. These results suggest a much more sophisticated mechanism of distance estimation than hitherto thought. The ants must be able to measure the slopes of undulating terrain and to integrate this information into their "odometer" for the distance estimation process.     

Kompass im Kopf

Ant compass – how desert ants learn to navigate Successful spatial orientation is a daily challenge for many animals. Cataglyphis desert ants are famous for their navigational performances. They return to the nest after extensive foraging trips without any problems. How do ants take their navigational systems into operation? After conducting different tasks in the dark nest for several weeks, they become foragers under bright sun light. This transition requires both a drastic switch in behavior and neuronal changes in the brain. Experienced foragers mainly rely on visual cues. They use a celestial compass and landmark panoramas. For that reason, naïve ants perform stereotype learning walks to calibrate their compass systems and acquire information about the nest's surroundings. During their learning walks, the ants frequently look back to the nest entrance to learn the homing direction. For aligning their gazes, they use the earth's magnetic field as a compass reference. This magnetic compass in Cataglyphis ants was previously unknown.     

Not everything that counts can be counted: ants use multiple metrics for a single nest trait

There are claims in the literature that certain insects can count. We question the generality of these claims and suggest that summation rather than counting (sensu stricto) is a more likely explanation. We show that Temnothorax albipennis ant colonies can discriminate between potential nest sites with different numbers of entrances. However, our experiments suggest that the ants use ambient light levels within the nest cavity to assess the abundance of nest entrances rather than counting per se. Intriguingly, Weber's Law cannot explain the ants' inaccuracy. The ants also use a second metric, independent of light, to assess and discriminate against wide entrances. Thus, these ants use at least two metrics to evaluate one nest trait: the configuration of the portals to their potential homes.     

Allocation of Colony‐Level Foraging Effort in Vespula germanica in Response to Food Resource Quantity,Quality, and Associated Olfactory Cues

In social insects, selection takes place primarily at the level of the colony. Therefore, unlike solitary insects, social species are expected to forage at rates that maximize colony fitness rather than individual fitness. Workers can increase the net benefit of foraging by responding to increased resource availability, by responding more strongly to higher‐quality resources, and by decreasing the uncertainty with which nestmates find resources. Unlike many ants and social bees, no social wasp is known to utilize a nest‐based recruitment signal to inform nestmates of food location. On the other hand, wasps do learn the odor of food brought to the nest and use this cue to locate the food source outside the nest. Here, we quantify the effects of three food‐associated variables on the allocation of foraging effort in the yellowjacket Vespula germanica. We used an experimental approach to assess whether resource quantity, quality, or associated olfactory information affect the probability that a forager will leave the nest on a foraging trip. We addressed these questions by inserting a known amount of sucrose solution directly into nests and recording foraging effort (departure rate) over the subsequent hour‐long observation period. No differences were found in foraging effort because of the presence/absence of olfactory cues, but there was strong evidence that foraging effort increased in response to resource influx and resource quality. Thus, while olfactory cues are learned in the nest, only resource quality and the cue of increased amount of food in the nest factor into a forager's decision of whether or not to depart on a foraging trip. However, as prior work has shown, once a wasp forager leaves the nest, it uses the learned olfactory cues to aid in finding resources.     

Path integration controls nest-plume following in desert ants

The desert ant Cataglyphis fortis is equipped with sophisticated navigational skills for returning to its nest after foraging. The ant's primary means for long-distance navigation is path integration, which provides a continuous readout of the ant's approximate distance and direction from the nest. The nest is pinpointed with the aid of visual and olfactory landmarks. Similar landmark cues help ants locate familiar food sites. Ants on their outward trip will position themselves so that they can move upwind using odor cues to find food. Here we show that homing ants also move upwind along nest-derived odor plumes to approach their nest. The ants only respond to odor plumes if the state of their path integrator tells them that they are near the nest. This influence of path integration is important because we could experimentally provoke ants to follow odor plumes from a foreign, conspecific nest and enter that nest. We identified CO(2) as one nest-plume component that can by itself induce plume following in homing ants. Taken together, the results suggest that path-integration information enables ants to avoid entering the wrong nest, where they would inevitably be killed by resident ants.     

Neuronal Plasticity in the Mushroom‐Body Calyx of Bumble Bee Workers During Early Adult Development

Division of labor among workers is a key feature of social insects and frequently characterized by an age‐related transition between tasks, which is accompanied by considerable structural changes in higher brain centers. Bumble bees (Bombus terrestris), in contrast, exhibit a size‐related rather than an age‐related task allocation, and thus workers may already start foraging at two days of age. We ask how this early behavioral maturation and distinct size variation are represented at the neuronal level and focused our analysis on the mushroom bodies (MBs), brain centers associated with sensory integration, learning and memory. To test for structural neuronal changes related to age, light exposure, and body size, whole‐mount brains of age‐marked workers were dissected for synapsin immunolabeling. MB calyx volumes, densities, and absolute numbers of olfactory and visual projection neuron (PN) boutons were determined by confocal laser scanning microscopy and three‐dimensional image analyses. Dark‐reared bumble bee workers showed an early age‐related volume increase in olfactory and visual calyx subcompartments together with a decrease in PN‐bouton density during the first three days of adult life. A 12:12  h light‐dark cycle did not affect structural organization of the MB calyces compared to dark‐reared individuals. MB calyx volumes and bouton numbers positively correlated with body size, whereas bouton density was lower in larger workers. We conclude that, in comparison to the closely related honey bees, neuronal maturation in bumble bees is completed at a much earlier stage, suggesting a strong correlation between neuronal maturation time and lifestyle in both species.     

Anatomical plasticity of synapses in the lamina of the optic lobe of the fly

Insects are frequently assumed to have hard-wired nervous systems that fail to demonstrate functional plasticity. We have produced changes in synaptic frequency, and analysed their developmental time course, dynamics and reversibility, in the lamina underlying the compound eye of the fly, by exposing young adults to different visual stimuli. The class of synapse examined feeds back from L2, one of the monopolar cells found in each lamina cartridge, to photoreceptor terminals; each site is a synaptic dyad marked by the presence of a few, round vesicles surrounding a T-shaped presynaptic ribbon and, in the photoreceptor, by a subsynaptic vacuole. In control adult flies reared in normal room lighting, the frequency of synaptic profiles scored in micrographs of single sections initially increased until one day post-eclosion (E + 1), but declined thereafter. Frequencies measured in left and right eyes of the same control animals were closely matched. Experimental flies were put for one to two days into an integrating sphere illuminated continuously with square-wave, 25 Hz green light. They had one eye occluded, so providing control comparisons between flicker-reared (FR) and occluded (dark-reared, DR) eyes within the same animal. The DR eyes invariably (n greater than 22) had higher frequencies of synaptic profiles than those seeing light, regardless of age or the period of light exposure, although the detailed relative effects of FR and DR depend upon the age of the animal. The evidence suggests that exposure to light actively depresses synaptic frequency and increases its variability. The greatest difference (30%) achieved was at two to four days after eclosion and there was no difference beyond six days, so demarcating a prospective sensitive period. Rearing in DC light was equally effective as FR, so visual contrasts per se are apparently inessential. Frequency values can change rapidly. During the first 24 h post-eclosion, DR resulted in new synapses adding to L2's complement of 25-35 at a maximum rate of 4 per 6 h, whereas light exposure caused a frequency decrease after as little as 6 h. Alternating 24 h periods of light and dark during the first two days produced reversible synaptic frequency changes. Individual synaptic contacts enlarge with age but not significantly with different visual experiences. The decrease in frequency of synaptic profiles with age thus actually underestimates the true decrease in synaptic number, whereas the altered synaptic frequencies seen after differential exposure represent true differences in synaptic number.(ABSTRACT TRUNCATED AT 400 WORDS)     

Orientation and Communication in the Neotropical Ant Odontomachus bauri Emery (Hymenoptera,Formicidae, Ponerinae)

The Neotropical species Odontomachus bauri employs canopy orientation during foraging and homing. An artificial canopy pattern above the ants is much more effective as an orientation cue than horizontal landmarks or chemical marks. However, both horizontal visual cues and chemical marks on the ground can serve in localizing the nest entrance. Successful O. bauri foragers recruit nestmates to leave the nest and search for food. However, the recruitment signals do not contain directional information. Antennation bouts and pheromones from the pygidial gland most likely serve as stimulating recruitment signals. Secretions from the mandibular and poison gland elicit alarm and attack behavior.     

Do Leaf Cutting Ants Cut Undetected? Testing the Effect of Ant-Induced Plant Defences on Foraging Decisions in Atta colombica

Leaf-cutting ants (LCAs) are polyphagous, yet highly selective herbivores. The factors that govern their selection of food plants, however, remain poorly understood. We hypothesized that the induction of anti-herbivore defences by attacked food plants, which are toxic to either ants or their mutualistic fungus, should significantly affect the ants' foraging behaviour. To test this "induced defence hypothesis," we used lima bean (Phaseolus lunatus), a plant that emits many volatile organic compounds (VOCs) upon herbivore attack with known anti-fungal or ant-repellent effects. Our results provide three important insights into the foraging ecology of LCAs. First, leaf-cutting by Atta ants can induce plant defences: Lima bean plants that were repeatedly exposed to foraging workers of Atta colombica over a period of three days emitted significantly more VOCs than undamaged control plants. Second, the level to which a plant has induced its anti-herbivore defences can affect the LCAs' foraging behaviour: In dual choice bioassays, foragers discriminated control plants from plants that have been damaged mechanically or by LCAs 24 h ago. In contrast, strong induction levels of plants after treatment with the plant hormone jasmonic acid or three days of LCA feeding strongly repelled LCA foragers relative to undamaged control plants. Third, the LCA-specific mode of damaging leaves allows them to remove larger quantities of leaf material before being recognized by the plant: While leaf loss of approximately 15% due to a chewing herbivore (coccinelid beetle) was sufficient to significantly increase VOC emission levels after 24 h, the removal of even 20% of a plant's leaf area within 20 min by LCAs did not affect its VOC emission rate after 24 h. Taken together, our results support the "induced defence hypothesis" and provide first empirical evidence that the foraging behaviour of LCAs is affected by the induction of plant defence responses.     

Visual lateralization and development of spatial and social spacing behaviour of chicks (Gallus gallus domesticus)

We investigated whether the development of spatial behaviour of the domestic chicken is influenced by light exposure of the embryo, as is known to be the case for some other lateralized visual functions. Ninety-six chicks were incubated in the dark or exposed to light during the final days of incubation. Half of the chicks in each group had the experience of moving behind opaque screens from 10 to 12 days of age. The other half were given transparent screens as a control. Chicks were tested in a detour test and a rotated floor test and their dispersal in groups was observed in larger pens. In the rotated floor test, chicks that had had experience with opaque screens used distal cues significantly more often than chicks that had experience with transparent screens (P = 0.042), regardless of whether they had been exposed to light before hatching or incubated in the dark. There were no significant differences between treatments in the detour test or in the dispersal behaviour. Hence, visual lateralization has no influence on the development of the spatial behaviour that we tested, whereas the occlusion experience is quite specific and results in shifted attention to distal spatial cues.     

Mimicry complex in two central European zodariid spiders (Araneae: Zodariidae): how Zodarion deceives ants

Ant-eating spiders, Zodarion germanicum and Z. rubidum , were found to resemble ants structurally (size, colour, setosity) and behaviourally (ant-like movement, antennal illusion). Zodarion germanicum mimics large dark ants, such as Formica cinerea , whereas Z. rubidum resembles red ants, e.g. Myrmica sabuleti . Thus, these spiders are generalized Batesian mimics. The two spiders use aggressive mimicry during prey capture. When a spider carries a captured ant it will try to pass by approaching ants using special deceiving behaviour, which is based on imitation of ants' nestmate recognition. The spider first taps the antennae of the curious ant with its front legs (transmitting a tactile cue), then exposes its prey (the ant corpse) which the ant antennates (thus the corpse transmits an olfactory cue). The distal part of the front legs of Zodarion are almost without macrosetae similar to the antennae of ants. Additionally, all the other legs are covered with flattened incised setae, which imitate the dense setosity of ants' limbs. These remarkable microstructural imitations are believed to improve imitation of tactile signals by spiders. Moreover, by tapping, zodariids can presumably recognize the approaching intruder and decide whether to undertake the risk of deception or to run away. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 75 , 517–532.