Viruses are known to be agents of important diseases of Insecta and Acarina, and many vertebrate and plant viruses have arthropods as propagative vectors. There is fossil evidence of arthropod pathogens for some micro-organisms, but not for viruses. Isolated virions would be hard to detect but, in fossil material, it could be easier to find traces of virus infection, mainly virus-induced cellular structures (VICS), easily recognisable by electron microscopy, such as virions encapsulated in protein occlusion bodies, aggregates of membrane-bounded virus particles and crystalline arrays of numerous virus particles. The following main taxa of viruses that multiply in arthropods are discussed both for some of their evolutionary aspects and for the VICS they cause in arthropods: A. dsDNA Poxviridae, Asfarviridae, Baculoviridae, Iridoviridae, Polydnaviridae and Ascoviridae, infecting mainly Lepidoptera, Hymenoptera, Coleoptera, Diptera and Acarina; B. ssDNA Parvoviridae, infecting mainly Diptera and Lepidoptera; C. dsRNA Reoviridae and Birnaviridae, infecting mainly Diptera, Hymenoptera and Acarina, and plant viruses also multiplying in Hemiptera; D. Amb.-ssRNA Bunyaviridae and Tenuivirus, that multiply in Diptera and Hemiptera (animal viruses) and in Thysanoptera and Hemiptera (plant viruses); E. -ssRNA Rhabdoviridae, multiplying in Diptera and Acarina (vertebrate viruses), and mainly in Hemiptera (plant viruses); F. +ssRNA Togaviridae, Nodaviridae, Tetraviridae, Flaviviridae and Picornaviridae, multiplying mainly in Lepidoptera, Hymenoptera, Diptera and Acarina (animal viruses), and in Hemiptera the plant Marafiviruses; G. RNA-RT Metaviridae and Pseudoviridae, multiplying in Diptera and Lepidoptera. Fossil arthropods included in amber or similar material derived from plant resins, that had the ultrastructure preserved, would be the best material for viral palaeontological studies. Volcanic ash and carbon deposits could also give good preservation.
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