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Simple Summary: Italy plays a central role in the research on Europe's biogeography because of its position in the middle of the Mediterranean, which is a global hotspot of diversity. This study investigated how present patterns of earwig species richness and composition in Italy are affected by climatic, geographical, and historical factors. Earwig richness does not decrease from the base to the tip of the Italian peninsula, which contrasts with the so-called 'peninsula effect'. However, richness does not increase southward either, suggesting that southern regions did not play a crucial role as a refuge during Pleistocene glaciations. Inter-regional similarities in species composition between regions is more influenced by their geographical proximity than climatic similarity, although richness is positively correlated with precipitation, in accordance with earwig preferences for humid conditions. The similarity in species composition with central European fauna decreases southward, indicating possible exchanges between central Europe and Italy. The majority of the earwigs of the Italian fauna are either widespread across Europe and the Palearctic, or confined to the main Italian mountain ranges, that is, the Alps and the Apennines. The isolation of ancient earwig populations on mountains resulted in the development of a high proportion of endemics, making the Italian earwig fauna one of the richest in Europe. Placed in the center of the Mediterranean biodiversity hotspot, Italy plays a central role for the study of Europe's biogeography. In this paper, the influence of climatic, spatial, and historical factors on current patterns of variation in earwig species richness and composition is investigated. The Italian earwig fauna is mainly composed of species which are either widely distributed in Europe and the Palearctic region or that are endemic to the Alps and the Apennines. Variation in species richness does not follow any obvious geographical patterns, but a positive influence of precipitation on richness is consistent with earwig preferences for humid climates. European mainland territories did not contribute substantially to the current biodiversity of Italian earwigs, which explains the lack of a distinct peninsula effect, although a southward decrease in similarity with the central European fauna was observed. However, southern areas did not exert a pivotal role during Pleistocene glaciations in determining current patterns of species richness. Variation in species composition among Italian regions can be mostly explained by geographical proximity, while climatic differences and historical (paleogeographical and paleoecological) events seem to have played a minor role. However, the isolation of ancient earwig stocks on Italian mountains led to the origin of a relatively large number of endemics, which makes the Italian earwig fauna one of the richest in Europe. [ABSTRACT FROM AUTHOR]