Trichoniscus pusillus
species of crustacean
Trichoniscus pusillus, sometimes called the common pygmy woodlouse, is one of the five most common species of woodlice in the British Isles. It is acknowledged to be the most abundant terrestrial isopod in Britain. It is found commonly across Europe north of the Alps, and has been introduced to Madeira, the Azores and North America.
Description
T. pusillus may be distinguished from other British woodlice chiefly by its small size, which reaches no more than 5 millimetres (0.2 in). Its body is elongate and quite rounded in cross section, and typically purplish. It may be separated from related species in North America by its eyes of three ocelli each, rather than the single ocellus in the eyes of its relative Hyloniscus riparius.
Reproduction
There are two distinct reproductive strategies within the species Trichoniscus pusillus. Many populations are, like most metazoans, bisexual and reproduce sexually; in other cases, females reproduce parthenogenetically, creating clones of themselves. The sexually reproducing form is diploid while the parthenogenetic form is triploid; since parthenogenesis always produces females, males are always diploid and can only be produced by sexual reproduction.
The frequency of males in the population decreases from south to north (a latitudinal cline) and in increasingly open habitats, with no males observed in most of Scotland and Scandinavia, but more than 15% males in the Iberian and Apennine Peninsulas.
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The date and method of observation matter as much as the stated conclusion, especially where classification or consensus has changed. The source revision retrieved here is dated Jan 13, 2026. The linked authority identifier is Q2392251. None of the 0 selected statements returned an explicit reference.
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This entry incorporates text from “Trichoniscus pusillus” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.