The Micraria Mess

Micaria pulicaria is a common ant-mimic spider and fairly easy to recognize, although there are rarer similar species. At least, that was true until a new paper which was published (see below). In a nutshell, the authors performed DNA analysis of putative Micaria pulicaria specimens and found that they are a mix of cryptic species. We've been here before - Pardosa lugubris is now P. saltans, although in this case renaming was due to the fact that via behavioural observations researchers recognised that there were two distinct mating dances in the two species and then followed this up with morphological analysis which revealed some (subtle) physical differences. The new paper is DNA-led with morphological confirmation, but at this point it gets embarrassing. In the jargon, the cryptic species are known as "OTUs" (Operational Taxonomic units) - i.e. groupings with more similarities than differences. However, the morphological differences aren't exactly subtle:

OTU1 (= Micaria micans) has a dark longitudinal stripes on the dorsal of femora III and IV (sometimes also recognizable on the tibiae of these legs), caused by stripe-like arrangement of dark hairs and dark pigmentation of the cuticule (see the photo above).
OTU2&3 (= Micaria pulicaria) does not have the go-faster stripes.

You might have thought that this would have been easy to spot, but apparently not! Even more embarrassing was the fact that the species we know in the UK as Micaria pulicaria was originally named as Micaria micans, but this seems to have been lost along the way.


https://doi. org/10.1111/zsc.12404

It turns out that the true M. pulicaria appears to be confined to inaccessible northern or montane regions or otherwise specific locations. All day long UK arachnologists have been scrambling to look at their specimens and photographs, and it turns out most UK specimens are striped, hence Micaria micans rather than pulicaria.

We'd better get used to this - DNA is going to keep tripping us up for decades to come.

Read:

Muster C, Michalik P. (2019) Cryptic diversity in ant-mimic Micaria spiders (Araneae, Gnaphosidae) and a tribute to early naturalists. Zool Scr. 00: 1–13. https://onlinelibrary.wiley.com/doi/10.1111/zsc.12404
"Spiders of the genus Micaria are ground-living mimics of ants. Species delineation in these spiders is challenging, mainly because of exceptional high levels of intraspecific variation masking species boundaries. As implied by preliminary DNA barcode data from Central Europe, the Holarctic and very widely distributed glossy ant-spider M. pulicaria shows cryptic diversity. Here, we disentangle the hidden diversity by means of an integrative taxonomy approach, using mitochondrial DNA, morphometrics, traditional genitalic characters and ecology. Our data suggest the clear delineation of two distinct species, which supports the conception of 19th century taxonomists. These early naturalists distinguished M. pulicaria and a second closely related species based on morphology and natural history, which were synonymized in subsequent taxonomic studies. Therefore, we re-circumscribe M. pulicaria and revalidate the long forgotten M. micans. These two Micaria species co-occur sympatrically in vast areas of the western Palearctic, while the Nearctic region is populated by M. pulicaria alone. Male genitalic traits are more dissimilar in the area of sympatry than in allopatry, suggesting a decisive role of reproductive character displacement in species diversification. Our study emphasizes the value of the early taxonomic literature in integrative taxonomic studies, as it may contain crucial information on natural history that are not regularly recorded by modern taxonomists."


Update: Helpful advice from Richard Gallon, Cofnod:

• Micaria pulicaria sensu lato: all old records into until the specimens have been redetermined.
• Micaria pulicaria sensu stricto: records where the specimen has been redetermined in light of the new paper.
• Micaria micans: records where the specimen has been redetermined in light of the new paper.

It seems that specimens from warm dry sites (thermophilic) are Micaria micans. Genuine Micaria pulicaria s.s. appear to favour damper habitats. If the specimen has black dorsal stripes on femurs III & IV and is from a dry habitat then it is Micaria micans. If it lacks these dorsal stipes on femurs III & IV (particularly if it’s from a damp habitat), then it’s Micaria pulicaria sensu stricto (although be careful you are not dealing with old faded material). If you aren’t sure put it down as M. pulicaria sensu lato.

There are also characteristics on the epigyne and particularly the palp which help, but they are subtle! The colouration difference is a reliable and supported by DNA. It will take some time to work out the distribution of both species but the paper confirms that both species are present from the UK, including Wales and Scotland.

Update2 February 2020:
On 12.02.2020 the World Spider Catalogue formally recognised the split of M. pulicaria and M. micans. This change will be included in the next UK checklist in about 5 years time.
Danniella Sherwood (NHM) and Richard Gallon (World Museum Liverpool) have checked museum specimens and found the leg stripe character of UK specimens is consistent with the genital morphology of the corresponding taxa. Difficulty may arise with very faded specimens (e.g. historical) and immatures.