Despite more than 250 years of taxonomic research, we still have only a vague idea about the true size and composition of the faunas and floras of the planet. Many biodiversity inventories provide limited insight because they focus on a small taxonomic subsample or a tiny geographic area. Here, we report on the size and composition of the Swedish insect fauna, thought to represent roughly half of the diversity of multicellular life in one of the largest European countries. Our results are based on more than a decade of data from the Swedish Taxonomy Initiative and its massive inventory of the country’s insect fauna, the Swedish Malaise Trap Project The fauna is considered one of the best known in the world, but the initiative has nevertheless revealed a surprising amount of hidden diversity: more than 3,000 new species (301 new to science) have been documented so far. Here, we use three independent methods to analyze the true size and composition of the fauna at the family or subfamily level: (1) assessments by experts who have been working on the most poorly known groups in the fauna; (2) estimates based on the proportion of new species discovered in the Malaise trap inventory; and (3) extrapolations based on species abundance and incidence data from the inventory. For the last method, we develop a new estimator, the combined non-parametric estimator, which we show is less sensitive to poor coverage of the species pool than other popular estimators. The three methods converge on similar estimates of the size and composition of the fauna, suggesting that it comprises around 33,000 species. Of those, 8,600 (26%) were unknown at the start of the inventory and 5,000 (15%) still await discovery. We analyze the taxonomic and ecological composition of the estimated fauna, and show that most of the new species belong to Hymenoptera and Diptera groups that are decomposers or parasitoids. Thus, current knowledge of the Swedish insect fauna is strongly biased taxonomically and ecologically, and we show that similar but even stronger biases have distorted our understanding of the fauna in the past. We analyze latitudinal gradients in the size and composition of known European insect faunas and show that several of the patterns contradict the Swedish data, presumably due to similar knowledge biases. Addressing these biases is critical in understanding insect biomes and the ecosystem services they provide. Our results emphasize the need to broaden the taxonomic scope of current insect monitoring efforts, a task that is all the more urgent as recent studies indicate a possible worldwide decline in insect faunas.

The list of all known locality and host records from the literature on louse flies from Slovakia are summarized, with the addition of new collection data. New locality data are provided for five species. Three species are added to the Slovakian list:Icostaminor(Bigot in Thomson, 1858), which was erroneously cited for Moravia instead of Slovakia in the previous checklist, andOrnithophilametallica(Schiner, 1864) andOrnithomyachloropus(Bergroth, 1901), which were overlooked from the last checklist. As a result, the louse fly fauna of Slovakia increases to 19 species: 12 autochtonous species and seven rare, non-native species only occasionally imported to Slovakia or migrating to the country with their hosts. This is by far the largest regional fauna of Hippoboscidae in Central Europe, and matches the richest southern European faunas. In total, 78 host-parasite associations concerning 46 bird-host species from eight orders and nine species of mammals, including humans, have been found from a literature review in Slovakia. Two host-parasite associations are reported from Slovakia for the first time:Ornithomyaavicularia(Linnaeus, 1758) onPrunellamodularis(Linnaeus, 1758) (Aves: Prunellidae) andLipoptenafortisetosaMaa, 1965 onHomosapiensLinnaeus, 1758 (Mammalia: Hominidae).

The available keys for European Hippoboscidae are outdated and do not cover all species currently known from Europe. Therefore, identification keys to the eleven genera and 31 species of the European hippoboscids are provided here.Ornithomya comosa(Austen, 1930) (Diptera: Hippoboscidae) is recorded for the first time from the territory of Slovakia based on one female found on a sand martin,Riparia riparia(Linnaeus, 1758). The list of keds and louse flies recorded from the territory of Slovakia is increased to 20 species. New host records for Slovakia are presented.

Records of six species of the family Drosophilidae from the Czech Republic are presented. Drosophila tripunctata Loew, 1862, reported from the Old World long ago on the basis of a wrong determination, is newly confirmed from the region (Czech Republic: Bohemia); although this finding originates from a greenhouse, the record is important because this thermotolerant species has a potential to spread outdoors in Central Europe. Cacoxenus indagator Loew, 1858 is for the first time recorded from North Moravia (Czech Silesia) in the Czech Republic and Cacoxenus argyreator Frey, 1932 from Slovakia. Mycodrosophila poecilogastra (Loew, 1874) is new addition to the fauna of Bohemia, Drosophila (Sophophora) helvetica Burla, 1948 and Drosophila (Sophophora) suzukii (Matsumura, 1931) are new for Moravia.

Periscelis (Periscelis) fugax sp. nov. is described and illustrated based on series of specimens from Portugal and the Czech Republic. It is probably widespread in Europe but has not been previously recognized from the closely related and externally very similar P. (P.) winnertzii Egger, 1862. The new species differs from P. winnertzii not only in structures of the male and female terminalia (illustrated) but also in colour pattern of pedicel and mesonotum. Morphology of the male and female terminalia is studied in detail in both these species and terminology of their structures is discussed and redefined. A new modern key to Palaearctic species of Periscelis (s. str.) species is constructed.

The Eocene Baltic amber fossil flies of the genus Acartophthalmites Hennig, 1965 (Diptera: Acalyptratae) are revised. Seven species are recognized and described or redescribed. Five species, A. crassipes sp. nov., A. luridus sp. nov., A. rugosus sp. nov., A. tertiaria Hennig, 1965 (type species) and A. willii Pérez-de la Fuente, Hoffeins et Roháček, 2018 are retained in Acartophthalmites while Clusiomites gen. nov. is described for two other species, C. clusioides (Roháček, 2016) comb. nov. (type species) and C. ornatus sp. nov. Relationships of these fossil taxa are discussed and, because they cannot be confidently placed in any known family of Diptera, a new family, Clusiomitidae, is established for them. Clusiomitidae is recognized as a family of Opomyzoidea, probably most closely allied to Clusiidae. These results again confirmed that the diversity of acalyptrate flies was very high in the Mid-late Eocene amber forest.