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This paper introduces Antares, which is a bio-inspired algorithm for the construction of a decentralized and self-organized P2P information system in computational grids. This algorithm exploits the properties of ant systems, in which a number of entities/agents perform simple operations at the local level but together engender an advanced form of ldquoswarm intelligencerdquo at the global level. Here, the work of ant-inspired agents is tailored to the controlled replication and relocation of ldquodescriptors,rdquo that is, documents that contain metadata information about grid resources. Agents travel the grid through P2P interconnections, and replicate and spatially sort descriptors so as to accumulate those represented by identical or similar indexes into neighbor grid hosts. The resulting information system is here referred to as self-structured, because it exploits the self-organizing characteristics of ant-inspired agents, and the association of descriptors with hosts is not predetermined but adapts to the varying conditions of the grid. This self-structured organization combines the benefits of both unstructured and structured P2P information systems. Indeed, being basically unstructured, Antares is easy to maintain in a dynamic grid, in which joins and departs of hosts can be frequent events. On the other hand, the aggregation and spatial ordering of descriptors can improve the rapidity and effectiveness of discovery operations, which is a beneficial feature typical of structured systems. Performance analysis proves that ant operations allow the information system to be efficiently reorganized, thus improving the efficacy of both simple and range queries.