Calcareous fens are one of the most species-rich habitats of the temperate zone of the Northern Hemisphere. In spite of this species richness, however, calcareous fens are still rather poorly investigated. Consequently, the data of the fen-associated spider fauna are also largely lacking. The aim of the research was to study the spider fauna of the calcareous fens of Latvia and to draw conclusions about what kind of spider species and ecological groups typically inhabit calcareous fen habitats. Spiders were sampled in the summer months of 2010, 2011, and 2012 at nine different calcareous fens of the coastal lowland of Latvia. The spider collection was performed by pitfall traps and a sweep net. The examined material comprised 6631 adult spider individuals representing 21 families and 149 species. The main spider ecological groups that dominated in the studied calcareous fens were hygrophilous and photophilous species which largely reflect the main properties of our studied habitats, all of which were wet, open mire habitats. Nevertheless, the fen arachnofauna consisted also of spider groups which are less typical for moist, sun-exposed, and alkaline environments, like xerophilous, sciophilous, and sphagnophilous species, respectively. Finally, several spider species collected in this study have not been previously reported for the spider fauna of Latvia, and many more might still be undiscovered in these unique and poorly investigated habitats. Therefore, it is suggested that calcareous fens deserve special attention and they should definitely be investigated further.

During the last three centuries, people involved in natural history sciences have produced thousands of natural history collections worldwide. Specimens have been used for scientific proof of various discoveries and for gaining new knowledge in many disciplines of science not only in the area of biodiversity but also in taxonomy, species biology and ecology, parasitology, evolution, organisms' responses to climate change, nature conservation and many other biological sub-disciplines. Despite never-ending financial struggles, the world's largest collections as well as many regional midsize collections contain billions of specimens. This provides an enormous database for present and future studies. This article presents a critical description of the material of the Institute of Biology, University of Latvia collection (IBULC) containing roughly 59,000 identified arthropod individuals of about 3,700 species, including twenty-eight primary types as a part of around ninety type specimens, to illustrate the importance of midsize regional natural history collections in detail.