Spiders (Araneae) are the largest order of Arachnida and the sixth most speciose order of Animalia, comprising more than 50,000 extant species as well as over 1,400 species known from fossils. Despite this immense diversity that has been estimated to comprise 120,000–200,000 species, our knowledge of their systematics and distribution remains rather incipient. While attempts to evaluate the diversity and distribution patterns of spiders have been made for the Neotropical and a few other faunas, most other regions remain historically neglected.

The aim of this dissertation is to explore the systematics, diversity and distribution of spiders of Iran, a highly interesting region from a zoological and biogeographical point of view that unfortunately has been poorly investigated regarding its invertebrate fauna. For this purpose, I examined more than 9,000 specimens that were either collected during expeditions to numerous regions and ecosystems across the country or deposited in several natural history collections from around the world. As a result of these efforts, which were initiated in 2013 and mostly carried out in collaboration with researchers from various countries, a total of 11 genera and 147 species of Iranian spiders were described as new to science, and 419 taxa (i.e., 13 families, 87 genera and 319 species) were recorded in Iran for the first time. The total number of newly described and recorded species (i.e., 466 species) constitutes almost half (i.e., 49.83%) of the currently known Iranian species diversity of this group (i.e., 935 species). Amongst 147 species described, 137 are known only from Iran, representing 63.72% of all species currently considered endemic to this country (i.e., 215 species). These results were published in a total of 87 publications, 38 of which were published after the beginning of my doctoral studies at UTU in 2019. This dissertation, however, is primarily based on only nine taxonomic articles (i.e., I–IX), all published after 2019 and highlighting some of the more important findings.

Because of the newly obtained taxonomic information and an extensive database of all published records, it was possible to conduct a comprehensive review of spatial variation in the diversity patterns of spiders in Iran. This was the main objective of publication X, which also included an evaluation of the effect of sampling bias on the current understanding of the distribution of Iranian spiders. The analyses showed that the diversity of Iranian spiders remains inadequately studied and is heavily affected by the Linnean shortfall (i.e., gaps in taxonomic knowledge), despite a remarkable improvement in taxonomic research on this fauna since the beginning of the 21st century. There are 935 spider species in the 1,648,195 km2 of Iran. Comparing the number of species per area of 171 countries and other political regions indicated that Iran was in position 132, whereas many considerably smaller and less ecologically diverse countries were in lower positions. It was also found that this fauna clearly suffers from a severe Wallacean shortfall (i.e., lack of knowledge of species distributions), as approximately 85% of Iran lacks a single record of spiders. There is a highly uneven distribution of records throughout the country and its provinces and ecoregions, with most of the records situated near large cities. A high correlation was found between the number of records of spiders and the number of records of plants and other animals in Iran, indicating that the noted shortfalls are indeed corroborated by other taxa. Finally, it is suggested that to gain a more complete picture of the diversity of Iranian spiders, future collecting efforts should be primarily in the form of extensive systematic surveys instead of opportunistic sampling, and ideally targeting lesser sampled areas and ecoregions. Once a satisfactory amount of information regarding the taxonomy and distribution of species becomes available, it will be possible to properly assess the conservation status and risk factors that affect these species and to identify areas of higher conservation and management priority.