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Clerodendrumjistulosum Becc. is a true myrmecophyte as it offers nesting space for ants in hollow intemodes. In contrast to previous reports our investigations proved that these domatia open by themselves, thus providing cavities for a variety of different ant species. In Sarawak, Malaysia, we did not find an obligate relationship between C. jistulosum and a specific ant-partner. For comparison, studies on herbarium material of other Clerodendrum species were carried out a further species, C. deflexum from the Malay Peninsula and Sumatra presumably also is myrmecophytic.
In the humid tropics of SE Asia there are some 14 myrmecophytic species of the pioneer tree genus Macaranga (Euphorbiaceae). In Peninsular Malaysia a close association exists between the trees and the small, non-stinging myrmicine Crema togas ter borneensis. These ants feed mainly on food bodies provided by the plants and have their colonies inside the hollow intemodes. In a ten months field study we were able to demonstrate for four Macaranga species (M. triloba, M. hypoleuca, M. hosei, M. hulletti) that host plants also benefit considerably from ant-occupation. Ants do not contribute to the nutrient demands of their host plant, they do, however, protect it against herbivores and plant competition. Cleaning behaviour of the ants results in the removal of potential herbivores already in their earliest developmental stages. Strong aggressiveness and a mass recruiting system enable the ants to defend the host plant against many herbivorous insects. This results in a significant decrease in leaf damage due to herbivores on ant-occupied compared to ant-free myrmecophytes as well as compared to non-myrmecophytic Macaranga species. Most important is the ants' defense of the host plant against plant competitors, especially vines, which are abundant in the well-lit pioneer habitats where Macaranga grows. Ants bite off any foreign plant part coming into contact with their host plant. Both ant-free myrmecophytes and non-myrmecophytic Macaranga species had a significantly higher incidence of vine growth than specimens with active ant colonies. This may be a factor of considerable importance allowing Macaranga plants to grow at sites of strongest competition.
The woody climber Millettia niuewenhuisii (Fabaceae) and the shrub Myrmeconauclea strigosa (Rubiaceae) in Sabah, Borneo are associated with ants. The hollow stems of Millettia nieuwenhuisii are regularly inhabited by an aggressive Cladomyrma sp., which keeps pseudococcids inside the stem. On Myrmeconauclea strigosa the ants live in hollow internodal swellings near the end of the branches. In this plant many different ant species use the nesting space in an opportunistic manner.
Ficus obscura var. borneensis is a true myrmecophyte. It spontaneously forms cavities (domatia) in parts of its twigs which open by slits, These occur in the internodes and are usually not swollen. The domatia are inhabited by a variety of non-specific tree-living ants including Crematogaster spp., Cataulacus sp., Tetramorium sp., Cardio condyla sp. and Camponotus sp.. Additionally the plant providL a su~ar-containing secretion from extrafloral nectaries on the lower surfaces of the leaves. Examination of herbarium specimens of 37 other South-east Asian Ficus species did not reveal a single specimen with domatia.
1. Oviposition site selection is crucial for the reproductive success of herbivorous insects. According to the preference–performance hypothesis, females should oviposit on host plants that enhance the performance of their offspring. More specifically, the plant vigour hypothesis predicts that females should prefer large and vigorously growing host plants for oviposition and that larvae should perform best on these plants. 2. The present study examined whether females of the monophagous leaf beetle Cassida canaliculata Laich. (Coleoptera: Chrysomelidae) prefer to oviposit on large host plant individuals of the meadow clary and whether large host plants are of higher nutritional quality than small host plants. Subsequently, it was tested whether the female preference correlates with offspring performance and survival. 3. In the field, females preferred large host plant individuals for oviposition and host plant quality, i.e. leaf nitrogen content, was significantly higher in leaves of large than of small host plants. 4. In the laboratory, larval development time was shorter on leaves of large host plant individuals than on small host plant individuals, but this could not be shown in the field. 5. However, a predator-exclusion experiment in the field resulted in a higher survival of larvae on large host plants than on small host plants when all predators had free access to the plants. On caged host plants there was no difference in survival of larvae between plant size categories. 6. It is concluded that females of C. canaliculata select oviposition sites that enhance both performance and survival of their offspring, which meets the predictions of the plant vigour hypothesis.
Egg distribution in herbivorous beetles can be affected by bottom-up (host plant), and by top-down factors (parasitoids and predators), as well as by other habitat parameters. The importance of bottom-up and top-down effects may change with spatial scale. In this study, we investigated the influence of host plant factors and habitat structure on egg distribution in the leaf beetle Cassida canaliculata Laich. (Coleoptera: Chrysomelidae), a monophagous herbivore on Salvia pratensis L. (Lamiales: Lamiaceae), on four spatial scales: individual host plant, microhabitat, macrohabitat, and landscape. At the individual host plant scale we studied the correlation between egg clutch incidence and plant size and quality. On all other scales we analyzed the relationship between the egg clutch incidence of C. canaliculata and host plant percentage cover, host plant density, and the surrounding vegetation structure. Vegetation structure was examined as herbivores might escape egg parasitism by depositing their eggs on sites with vegetation factors unfavorable for host searching parasitoids. The probability that egg clutches of C. canaliculata were present increased with an increasing size, percentage cover, and density of the host plant on three of the four spatial scales: individual host plant, microhabitat, and macrohabitat. There was no correlation between vegetation structure and egg clutch occurrence or parasitism on any spatial scale. A high percentage of egg clutches (38–56%) was parasitized by Foersterella reptans Nees (Hymenoptera: Tetracampidae), the only egg parasitoid, but there was no relationship between egg parasitism and the spatial distribution of egg clutches of C. canaliculata on any of the spatial scales investigated. However, we also discuss results from a further study, which revealed top-down effects on the larval stage.
Fragmentation, deterioration, and loss of habitat patches threaten the survival of many insect species. Depending on their trophic level, species may be differently affected by these factors. However, studies investigating more than one trophic level on a landscape scale are still rare. In the present study we analyzed the effects of habitat size, isolation, and quality for the occurrence and population density of the endangered leaf beetle Cassida canaliculata Laich. (Coleoptera: Chrysomelidae) and its egg parasitoid, the hymenopteran wasp Foersterella reptans Nees (Hymenoptera: Tetracampidae). C. canaliculata is strictly monophagous on meadow sage (Salvia pratensis), while F. reptans can also parasitize other hosts. Both size and isolation of habitat patches strongly determined the occurrence of the beetle. However, population density increased to a much greater extent with increasing host plant density ( = habitat quality) than with habitat size. The occurrence probability of the egg parasitoid increased with increasing population density of C. canaliculata. In conclusion, although maintaining large, well-connected patches with high host plant density is surely the major conservation goal for the specialized herbivore C. canaliculata, also small patches with high host plant densities can support viable populations and should thus be conserved. The less specialized parasitoid F. reptans is more likely to be found on patches with high beetle density, while patch size and isolation seem to be less important.
We present the results of individual-based simulation experiments on the evolution of dispersal rates of organisms living in metapopulations. We find conflicting results regarding the relationship between local extinction rate and evolutionarily stable (ES) dispersal rate depending on which principal mechanism causes extinction: if extinction is caused by environmental catastrophes eradicating local populations, we observe a positive correlation between extinction and ES dispersal rate; if extinction is a consequence of stochastic local dynamics and environmental fluctuations, the correlation becomes ambiguous; and in cases where extinction is caused by dispersal mortality, a negative correlation between local extinction rate and ES dispersal rate emerges. We conclude that extinction rate, which both affects and is affected by dispersal rates, is not an ideal predictor for optimal dispersal rates.
The olfactory system of leafcutting ants: neuroanatomy and the correlation to social organization
(2009)
In leaf-cutting ants (genera Atta and Acromyrmex), the worker caste exhibits a pronounced size-polymorphism, and division of labor is largely dependent on worker size (alloethism). Behavioral studies have shown a rich diversity of olfactory-guided behaviors, and the olfactory system seems to be highly developed and very sensitive. To allow fine-tuned behavioral responses to different tasks, adaptations within the olfactory system of different sized workers are expected. In a recent study, two different phenotypes of the antennal lobe of Atta vollenweideri workers were found: MG- and RG-phenotype (with and without a macroglomerulus, MG). The existence of the macroglomerulus is correlated to the body size of workers, with small workers showing the RG-phenotype and large workers showing the MG-phenotype. In the MG, the information about the releaser component of the trail-pheromone is processed. In the first part of my PhD-project, I focus on quantifying behavioral differences between different sized workers in Atta vollenweideri. The study analyzes the trail following behavior; which can be generally performed by all workers. An artificial trail consisting of the releaser component of the trail-pheromone in decreasing concentration was used to test the trail-following performance of individual workers. The trail-following performance of the polymorphic workers is depended of the existence of the MG in the antennal lobe. Workers possessing the MG-phenotype were significantly better in following a decreasing trail then workers showing the RG-phenotype. In the second part I address the question if there are more structural differences, besides the MG, in the olfactory system of different sized workers. Therefore I analyze whether the glomerular numbers are related to worker size. The antennal lobes of small workers contain ~390 glomeruli (low-number; LN-phenotype), and in large workers I found a substantially higher number of ~440 glomeruli (high-number; HN-phenotype). All LN-phenotype workers and some of the small HN-phenotype workers do not possess an MG (LN-RG-phenotype and HN-RG-phenotype) at all, whereas the remaining majority of HN-phenotype workers do possess an MG (HN-MG-phenotype). Mass-stainings of antennal olfactory receptor neurons revealed that the sensory tracts divide the antennal lobe into six clusters of glomeruli (T1-T6). In the T4-cluster ~50 glomeruli are missing in the LN-phenotype workers. Selective staining of single sensilla and their associated receptor neurons showed that T4-glomeruli are innervated by receptor neurons from the main type of olfactory sensilla, the Sensilla trichodea curvata which are also projecting to glomeruli in all other clusters. The other type of olfactory sensilla, the Sensilla basiconica, exclusively innervates T6-glomeruli. Quantitative analyses revealed a correlation between the number of Sensilla basiconica and the volume of T6 glomeruli in different sized workers. The results of both behavioral and neuroanatomical studies in Atta vollenweideri suggest that developmental plasticity of antennal-lobe phenotypes promotes differences in olfactory-guided behavior which may underlie task specialization within ant colonies. The last part of my project focuses on the evolutionary origin of the macroglomerulus and the number of glomeruli in the antennal lobe. I compared the number, volumes and position of the glomeruli of the antennal lobe of 25 different species from all three major Attini groups (lower, higher and leaf-cutting Attini). The antennal lobes of all investigated Attini comprise a high number of glomeruli (257-630). The highest number was found in Apterostigma cf. mayri. This species is at a basal position within the Attini phylogeny, and a high number of glomeruli might have been advantageous in the evolution of the advanced olfactory systems of this Taxa. The macroglomerulus can be found in all investigated leaf-cutting Attini, but in none of the lower and higher Attini species. It is found only in large workers, and is located close to the entrance of the antennal nerve in all investigated species. The results indicate that the presence of a macroglomerulus in large workers of leaf-cutting Attini is a derived overexpression of a trait in the polymorphic leaf-cutting species. It presumably represents an olfactory adaptation to elaborate foraging and mass recruitment systems, and adds to the complexity of division of labor and social organization known for this group.