The paired exocrine metapleural glands present in the large majority of ant species produce compounds with antibiotic properties. In the leaf-cutting ant, Acromyrmex octospinosus, the secretion consists of more than 20 different compounds and it has generally been assumed that the glands serve as a general defence against various infectious microbes of fungal and bacterial origin. We present results illuminating the direct costs and benefits of these metapleural gland defences in A. octospinosus. We show that major workers of this leaf-cutting ant experience a significant reduction in their respiration rate when the metapleural glands are experimentally closed, indicating that metapleural gland secretion incurs a substantial cost and that the production of compounds from these glands is terminated when the ants are incapable of secreting them. In another set of experiments, we show that the ability to secrete antibiotic compounds from the metapleural glands is of significant importance when ants are exposed to a general but potentially virulent insect pathogen, Metarhizium anisopliae. Infection with this fungus is lethal within a few days when ants have their metapleural glands experimentally closed, but relatively harmless when the metapleural glands are functional. These findings support experimentally the view that the metapleural glands play an important hygienic role in leaf-cutting ants.

A prerequisite for honest handicaps is that there are signi¢cant condition-dependent costs in the expression of sexual traits. In the wolf spider Hygrolycosa rubrofasciata (Ohlert), sexual signalling (drumming) is costly in terms of increased mortality. Here we investigated whether this mortality may be caused by increased energy expenditure. During sexual signalling, metabolic rate was 22 times higher than at rest and four times higher than when males were actively moving. Metabolic rate per unit mass was positively related to absolute body mass during sexual signalling but not during other activities. This positive relationship is novel to any studies of metabolic rates. Indeed, it seems that the largest males can drum only 12 times per minute before reaching the maximum sustainable metabolic rate, whereas the smallest males may drum up to 39 times per minute. However, there is no relationship between body mass and drumming rate, indicating that larger males are able to compensate for the higher cost of drumming. There was a quadratic relationship between relative abdomen mass and overall body mass, which may provide a partial explanation for the increased energy expenditure of largest males while drumming. Altogether, our results indicate that sexual signalling is highly energetically demanding, which may be the main reason for the honesty of signalling in this species. In addition, the energetic costs are surprisingly strongly size dependent, which may compensate any disadvantage of small male size.

Summary1. Stable mutualistic interactions require the long-term alignment of fitness interests of participating species. This condition is fulfilled when the benefits of the relationship exceed the costs for all partners. 2. One apparent stabilizing factor in mutualisms is the vertical (parent to offspring) transmission of symbionts, as this tends to reduce the expression of virulent traits and reproductive conflicts. This study examines the cost and mode of transmission of a mutualistic Streptomyces bacterium that grows on the cuticle of leaf-cutting ants and produces antibiotics against a specialized fungal parasite of the ant fungus gardens. 3. It is shown that ant respiration rates are elevated by 10-20% when the bacterium is present on their cuticle. This increase is due to direct respiration of the bacterium and possible excess respiration by the ants. Although these two factors cannot be separated, it is clear that the total increase gives a reasonable quantification of the metabolic costs incurred by the Streptomyces symbiont. 4. Ants that actively maintain Streptomyces cultures on their cuticle tend to consume more of their mutualistic fungus garden than controls and this excess consumption increases with the amount of Streptomyces bacteria present. 5. Scanning electron microscopy showed that the mutualistic bacterium is not present on major workers immediately following eclosion, indicating that the bacterium is not transferred to callow workers until later. 6. The results of an experiment simulating within-colony transmission to callow workers suggest that the bacterium is predominantly transmitted from older to newly eclosed major workers, but that transmission may also occur via the fungus garden. The presence of Streptomyces bacteria in the fungus garden implies that rare events of horizontal transmission of the fungal cultivar of attine ants may also imply horizontal transmission of strains of the mutualistic bacterium.

Level set methods [Osher and Sethian. Fronts propagating with curvaturedependent speed: algorithms based on Hamilton-Jacobi formulations. J. Comput. Phys. 79 (1988) 12] have proved very successful for interface tracking in many different areas of computational science. However, current level set methods are limited by a poor balance between computational efficiency and storage requirements. Tree-based methods have relatively slow access times, whereas narrow band schemes lead to very large memory footprints for high resolution interfaces. In this paper we present a level set scheme for which both computational complexity and storage requirements scale with the size of the interface. Our novel level set data structure and algorithms are fast, cache efficient and allow for a very low memory footprint when representing high resolution level sets. We use a time-dependent and interface adapting grid dubbed the "Dynamic Tubular Grid" or DT-Grid. Additionally, it has been optimized for advanced finite difference schemes currently employed in accurate level set computations. As a key feature of the DT-Grid, the associated interface propagations are not limited to any computational box and can expand freely. We present several numerical evaluations, including a level set simulation on a grid with an effective resolution of 1024 3 .

It is well documented that ants can protect plants against insect herbivores, but the underlying mechanisms remain almost undocumented. We propose and test the pheromone avoidance hypothesis-an indirect mechanism where insect herbivores are repelled not only by ants but also by ant pheromones. Herbivores subjected to ant predation will experience a selective advantage if they evolve mechanisms enabling them to avoid feeding within ant territories. Such a mechanism could be based on the ability to detect and evade ant pheromones. Field observations and data from the literature showed that the ant Oecophylla smaragdina distributes persistent pheromones throughout its territory. In addition, a laboratory test showed that the beetle Rhyparida wallacei, which this ant preys on, was reluctant to feed on leaves sampled within ant territories compared with leaves sampled outside territories. Thus, this study provides an example of an ant-herbivore system conforming to the pheromone avoidance hypothesis.