Mermessus trilobatus

(Emerton, 1882)

Trilobate Dwarf Weaver

Mermessus trilobatus is a dwarf spider in the Linyphiidae, native to North America and introduced to Europe and the Azores since the late 1970s. It has become the most spider in Europe, expanding across multiple countries within fifty years. Unlike most invasive spiders in Europe, it occurs primarily in agricultural and semi-natural open rather than buildings. First described by James Henry Emerton in 1882.

Pronunciation

How to pronounce Mermessus trilobatus: //mɜrˈmɛsəs traɪlɵˈbeɪtəs//

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Identification

Distinguished from similar dwarf spiders by its trilobate (three-lobed) abdominal marking, giving the its name. As a member of the Erigoninae, it is extremely small with a body length of approximately 2 mm. Identification to species level requires examination of external genitalia, particularly in mature .

Habitat

Open within agricultural landscapes, particularly hay meadows and grasslands maintained by mowing or grazing. Rare in completely undisturbed forests (2.4% of individuals) or highly disturbed crops with cultivated soil (1.3% of individuals). Shows low habitat specificity, having been found in every pitfall trap across diverse microhabitats in at least one study. Activity- linked to soil pH and temperature.

Distribution

Native to North America. Introduced to Europe, first detected in the late 1970s in the Upper Rhine valley near Karlsruhe, South-West Germany. Has undergone largely concentric range expansion across Europe, recorded in Austria, Belgium, Croatia, Czech Republic, France, Great Britain, Hungary, Italy, the Netherlands, Poland, Slovakia, Slovenia, Switzerland, and Ukraine. Also introduced to the Azores (São Jorge, São Miguel, Terceira).

Seasonality

Most found in summer.

Diet

Prey captured using horizontal sheet webs; specific prey items not documented.

Behavior

Obligatory builder of horizontal sheet webs for prey capture. High mobility facilitates transport via human activity. Poor competitor due to slightly smaller body size compared to native spiders in same ; webs may provide protection against . Unexpectedly sensitive to soil disturbance: experimental rotary tillage reduced densities by approximately 90%, contrary to predictions that invasion success was linked to .

Ecological Role

Among the most abundant spider in agricultural where it occurs. Potential biocontrol agent in agricultural . One of the few spiders that successfully invades semi-natural ecosystems rather than remaining confined to human buildings.

Human Relevance

Accidentally introduced to Europe through human activity, likely via agricultural transport. Rapid range expansion makes it a subject of invasion research. Not known to pose direct threats to human health or structures.

Similar Taxa

  • Oedothorax apicatusNative European dwarf spider found in similar grassland ; increases in abundance with soil disturbance, whereas M. trilobatus decreases sharply under the same conditions.
  • Other Erigoninae dwarf spidersSimilar extremely small body size (~2 mm) and sheet-web building ; require genital examination for definitive separation.

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Sources and further reading