Oniscidea Latreille, 1802

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Suggested Common Name: Woodlice and allies
Number of subordinate taxa: at least 3925 described species in 5 major clades, at least 300 described species in all 5 clades in our area.
Etymology: after Oniscus Linnaeus, 1758, see that account for more information. Common name refers to Crinocheta, see that account for more information.
Taxonomic History: Oniscidea Latreille, 1802
Size Range: up to +30mm
Description: Body generally ovate (rarely elongate), with specialized water transport system developed from scale patches along underside of appendages and coxal plates; sexual dimorphism outside basic features present, mainly involving pleopods 1-2. Antenna 1 heavily reduced, differentiation between flagellum and peduncle completely lost, total appendage 1-3-segmented, covered in high density of sensory setae; scale absent. Antenna 2 without scale. Mandible incisor present, multicuspidate; lacinia present, multicuspidate; spine row present, densely setose, on lobe;molar present or absent; palp absent. Maxilla 1 biramous; endopod with plumose robust setae (=penicils) set distomedially. Maxilla 2 reduced, uniramous, bilobed to secondarily entire, densely setose. Maxilliped epipod present; basis huge, subquadrangular dominating most of maxilliped size; endite small, quadrangular, lacking linking hooks; palp reduced, generally 5- or less segmented. Pereopods 2-7 coxae formed into coxal plates, plates fused to telson (except in Tylidae), indistinguishable (demarcated by a suture in Ligiidae and Stenoniscidae). Penes very short, tubercle-like (completely flush to sternite in Tylidae), very close-set to fused. Pleon all segments entirely free except in some highly aberrant genera. Pleonites 1-2 lateral margins not free, subequal in width. Pleopod 1 slightly modified, sexually dimorphic but present in both sexes, exopods relatively unmodified; ♂ pleopod endopod generally elongate-acute, forming stylet guide; ♀ pleopod endopod unmodified, short-acute. Pleopod 2 slightly modified, sexually dimorphic but present in both sexes, exopods relatively unmodified; ♂ pleopod endopod fused with appendix masculina, 2-segmented, geniculate (Ligiidae and Tylidae) or colinear (Holoverticata) elongate, stylet-like, forming inserting appendage; ♀ pleopod endopod unmodified, short-acute. Pleopod 3 endopod subtriangular, tip subacute. Telson small, flat, not forming a branchial chamber. Uropods set proximolaterally on telson, biramous or secondarily uniramous.
Type taxon: Oniscoidea Dana, 1852 [by extention Crinocheta Legrand, 1946]
Notes: Oniscidea is a highly successful suborder, containing almost 4000 described species and many more undescribed. It is the third most specious group of terrestrial arthropods behind Hexapoda and Arachnida. There appears to be multiple centers of diversity in Oniscidea, with major ones being the Mediterranean Basin, the Afrotropics, Indomalaya, Australasia and the Neotropics. The Nearctic has a relatively depauperate Oniscidean fauna, with very few species away from the coasts and almost all species present being introduced. Despite this, there appears to be traces of a Palearctic element (in the native Trichoniscids and Ligidiines), a Neotropical element (in the small radiation of Venezillo pillbugs in the desert southwest) and potentially a unique native element (in the "Porcellionides" virgatus complex) present in the Nearctic. The portion of the Neotropics present in the region covered is much more diverse but poorly studied, with glimpses of an amazing diversity visible in Delatorreiidae and other described taxa from Cuba and the small handful of oniscideans described from Central America.

Subordinate taxa: Buchnerillo, Crinocheta, Ligiidae, Synocheta, Tylidae

Key to basal families and major clades
1 a. Pleonites and telson margins folded underneath, forming thylacomeres and completely obscuring the uropods; pereonite coxal plates free from tergites, articulating; pale conglobulating isopods that form burrows in sandy beaches found mostly on tropical coastlines (to California and Florida in the north) --> Tylidae
b. Pleonites and telson margins not folded underneath, uropods visible terminally (hidden dorsally by telson in Stenoniscidae and Bisilvestriidae); pereonite coxal plates fused with tergites, usually indistinguishable (with suture in Stenoniscidae and Ligiidae); ecology and morphology various --> 2

2 (1) a. Flagellum long with +10 segments; uropod rami generally pin-shaped; ♂ pleopod 2 endopod segments geniculate; common on seashores (Ligia) or uncommon along inland streamsides (Ligidium) --> Ligiidae
b. Flagellum short or if long than less than 10 segmented and the uropod rami conical; ♂ pleopod 2 endopod segments colinear ; ecology and morphology various --> 3

3 (2) a. Flagellum wispy, narrow-conical, with difficult-to-distinguish segments, 4-6 segments visible under a microscope; body generally fragile with a thin cuticle, restricting species to wet environments --> Synocheta
b. Flagellum robust, generally with 1-4(-7) segments easily visible (hard-to-see in Syphacidae but then flagellum cylindrical); body usually robust (fragile in some families), allowing many species to take to much drier environments --> Crinocheta*

*The incertae sedis genus Buchnerillo is placed in the Crinocheta: Oniscoidea key to simplify the Oniscidea key.


Leistikow, A. (2000). The terrestrial isopod genus Ischioscia in Costa Rica: new species and records, and an analysis of its phylogeny (Crustacea, Isopoda, Oniscidea). Zoosystematics and Evolution, 76(1):19-49.

Stuttgart, V. F. E. (1996). Das pleonale Skelet-Musckel-System und die phylogenetisch-systematische Stellung der Familie Mesoniscidae (Isopoda: Oniscidea). Stuttgarter Beitr. Naturk. Series A. 538.

Tabacaru, I., & Giurginca, A. (2019). Contributions to the study of the Trichoniscidae (Isopoda, Oniscidea). I. Definition and taxonomic position. Travaux de l'Institut de Spéologie “Émile Racovitza", 58:47-63.

Taiti, S. & Ferrara, F. (2004). The terrestrial Isopoda (Crustacea: Oniscidea) of the Socotra Archipelago. Fauna of Arabia, 20:211-325.

Published: Jan 1, 2023
Updated: Nov 1, 2023