Holarctic
Guides
Trochosa terricola
ground wolf spider
A medium-sized wolf spider (Lycosidae) widespread across Europe and extending into Asia and North America. Adults range from 7–14 mm, with females larger than males. The species occupies diverse terrestrial habitats including grassland, heathland, and woodland, showing particular affinity for drier conditions. It is primarily nocturnal and has been documented as prey for pompilid spider wasps.
Trogus
Trogus is a genus of parasitoid wasps in the family Ichneumonidae, subfamily Ichneumoninae. The genus contains twelve extant and one extinct species distributed across the Holarctic and Neotropic regions. All known species are specialized parasites of swallowtail butterflies (Papilionidae), attacking larval and pupal stages.
Trypeta
fruit flies
Trypeta is a genus of tephritid fruit flies in the family Tephritidae, established by Meigen in 1803. The genus contains approximately 90 described species distributed across the Holarctic region, with particular diversity in North America and Asia. New World species have been systematically revised, revealing a monophyletic group that diversified primarily in the southwestern United States and Mesoamerica. Species exhibit considerable morphological variation, particularly in wing pattern and body coloration.
Uenoidae
Stonecase Caddisflies
Uenoidae is a family of caddisflies (Trichoptera) commonly known as stonecase caddisflies. The family comprises approximately 7 genera and at least 80 described species, distributed across North America, Asia, and Europe. Larvae construct portable cases using mineral materials, hence the common name. The family was revised taxonomically in 1988 to include the subfamilies Uenoinae and Thremmatinae, incorporating genera formerly placed in separate families. Studies of Neophylax species demonstrate univoltine life cycles with temporal segregation between co-occurring species.
Underwoodia iuloides
Underwoodia iuloides is a millipede species in the family Caseyidae. It is one of three species in the genus Underwoodia, which exhibits a Holarctic distribution pattern. The species is characterized by specific configurations of the anterior gonopod colpocoxites, particularly branches a and c. Populations show a strongly female-biased sex ratio, suggesting parthenogenesis.
Veigaia
Veigaia is a genus of soil-dwelling mites in the family Veigaiidae (order Mesostigmata). The genus is one of the most ubiquitous in the Holarctic Region, with approximately 40 described species distributed across Europe, Asia, and North America. Detailed behavioral studies exist for Veigaia cerva, which exhibits a complex five-phase oviposition process involving rhythmic gnathosomal movements and precise egg positioning using chelicerae and appendages.
Veigaiidae
Veigaiidae is a family of free-living predatory mites in the order Mesostigmata. They inhabit soil and decaying organic matter, with some species specialized for rocky shorelines. The genus Veigaia is particularly widespread in the Holarctic Region.
Wagneria
Wagneria is a genus of tachinid flies (family Tachinidae) established by Robineau-Desvoidy in 1830. The genus contains approximately 18 described species distributed across the Holarctic region. Notably, Wagneria holds the record for having more junior homonyms than any other animal genus name, reflecting historical taxonomic complications. As members of Tachinidae, these flies are parasitoids, though specific host associations for most Wagneria species remain poorly documented.
Walckenaeria spiralis
Spiraling Sheet-web Weaver
Walckenaeria spiralis is a dwarf spider species in the family Linyphiidae, first described by Emerton in 1882. It belongs to a large genus of small sheet-web weaving spiders distributed across the Holarctic region. The species is known from scattered records across northern North America and the Russian Far North-East, suggesting a boreal distribution pattern.
Xanthochlorus
Xanthochlorus is a genus of long-legged flies (Dolichopodidae) established by Loew in 1857. Species in this genus are small to medium-sized predatory flies with characteristic yellow-green coloration. The genus occurs in the Holarctic region, with records from Scandinavia and North America. Like other dolichopodids, members are likely associated with moist habitats near water bodies.
Xantholininae
Xantholininae is a subfamily of rove beetles (Staphylinidae) comprising approximately 28 genera and over 100 species in North America alone, with additional diversity in the Palearctic and other regions. The group has been subject to extensive taxonomic revision due to historical confusion in genus-level classification, with multiple synonymizations required to stabilize nomenclature. Members are characterized by specific morphological features that distinguish them from other staphylinid subfamilies, though detailed biological studies remain limited compared to taxonomic treatments.
Xestia okakensis
Okak Cutworm, Okak Dart Moth
Xestia okakensis is a cutworm moth in the family Noctuidae. It was first described by Packard in 1867. The species has two recognized subspecies: X. o. okakensis and X. o. morandi. It occurs across northern regions of North America and Eurasia.
Xestia tecta
Xestia tecta is a noctuid moth with a disjunct Holarctic distribution, occurring in boreal Fennoscandia and northwestern Russia in Europe, and across northern Siberia, Alaska, and north-central North America. The species has a two-year life cycle, with adults active from July to August. Males are nocturnal while females are active in the afternoon.
Xestobiini
Xestobiini is a tribe of small beetles within the family Ptinidae (spider beetles and deathwatch beetles), subfamily Ernobiinae. Members of this tribe are wood-boring beetles that develop in dead or decaying wood. The tribe was established by Böving in 1927 and includes genera such as Xestobium. These beetles are characterized by their cylindrical body form and association with hardwood substrates.
Xiphydria
wood wasps, xiphydriid wood wasps
Xiphydria is a genus of wood wasps in the family Xiphydriidae, distributed across the Holarctic region including Europe, Asia, and North America. Adults are characterized by a distinctive elongated 'neck' formed by the long propleuron, antennae with approximately 20 segments, and mandibles with four teeth. Females oviposit into diseased or dead wood of broadleaf trees and vector symbiotic fungi that decompose wood for larval consumption. The genus is univoltine with one generation per year. While generally of minor economic importance, some species can damage oak and other hardwoods.
Xiphydriophagus meyerinckii
Xiphydriophagus meyerinckii is a species of chalcid wasp in the family Pteromalidae. The genus Xiphydriophagus is specialized as a parasitoid of wood-boring beetles in the family Xiphydriidae. The species has been recorded from Europe and eastern Canada. As a member of Pteromalidae, it likely develops as a solitary or gregarious parasitoid within host larvae, though specific biological details remain poorly documented.
Xyelinae
xyeline sawflies
Xyelinae is a subfamily of primitive sawflies within the family Xyelidae, representing one of the earliest-diverging lineages of Hymenoptera. Members are small to medium-sized insects with distinctive morphological features including broad heads and relatively short antennae. The subfamily includes extant genera such as Xyela and Pleroneura, with larvae that develop in plant tissues. Xyelinae sawflies are primarily associated with conifers, particularly pines, where they exhibit specialized host relationships.
Xylomoia
Xylomoia is a genus of moths in the family Noctuidae, established by Staudinger in 1892. The genus contains at least seven recognized species distributed across the Holarctic region, with the majority occurring in Europe and northern Asia. A 2024 integrative taxonomic review synonymized three species—X. strix, X. retinax, and X. stangelmaieri—as subspecies of X. strix based on morphological and genetic evidence. The genus belongs to the tribe Apameini within the subfamily Noctuinae.
Xylomya
wood soldier flies
Xylomya is a genus of soldier flies in the family Xylomyidae, commonly referred to as "wood soldier flies." The genus was established by Camillo Rondani in 1861. These flies are associated with forested environments and decaying wood habitats. Species in this genus are part of a small family of Diptera that is relatively poorly known compared to other soldier fly families.
Xylota
Leafwalkers and Forest Flies
Xylota is a Holarctic genus of hoverflies (family Syrphidae) comprising over 100 described species, with 12 species occurring in Europe. Adults are characterized by their elongate body form and association with woodland habitats. Unlike most syrphid flies, many Xylota species rarely visit flowers, instead feeding on pollen gathered from leaf surfaces. Larvae are saprophytic, developing in rotting wood of both broadleaved and coniferous trees.
Xysticus obscurus
Xysticus obscurus is a species of ground crab spider in the family Thomisidae, first described by Collett in 1877. It is a holarctic species with a broad distribution spanning North America, Europe, and across Russia from European Russia to Siberia. As a member of the genus Xysticus, it is an ambush predator that does not construct webs for prey capture. The species has been documented in Alberta, Canada, and is part of a genus known for ballooning behavior as a dispersal mechanism.
Ypsolopha
Ypsolopha is a genus of micromoths comprising over 120 described species, representing approximately 95% of the family Ypsolophidae's known world diversity. The genus is primarily Holarctic in distribution, with the majority of species recorded from temperate regions. Species exhibit considerable variation in external morphology, though male and female genitalia are remarkably homogeneous across the genus. Adults are predominantly nocturnal and display a distinctive resting posture with the head lowered and posterior body elevated.
Zabrini
Seed-eating Ground Beetles
Zabrini is a tribe of ground beetles in the family Carabidae, subfamily Pterostichinae, with more than 750 described species across three genera. The tribe is divided into two subtribes: Amarina (containing Amara and Pseudamara) and Zabrina (containing Zabrus). Species are found primarily in the Holarctic region, with exceptional diversity in China. The common name 'Seed-eating Ground Beetles' reflects granivory observed in some species, though feeding habits vary considerably across the tribe.
Zornella
Zornella is a genus of sheet-weaving spiders (family Linyphiidae) established by A. R. Jackson in 1932. The genus contains three described species with a disjunct distribution: two species occur in North America (USA and Canada), while one species ranges across northern Eurasia from northeastern Europe through Russia to Kazakhstan and Mongolia. As linyphiids, members construct horizontal sheet webs with a retreat.