Another woodlouse - T. pusillus

[Gerel]

Unfortunately the telson is out of focus, but I think this is a Common Pygmy Woodlouse, Trichoniscus pusillus.



Steve

[Pudding4brains]

Hi Steve,

I'd say you're mostly right with that - even if very brownish coloured (usually more orangy-reddish)

However a problem with this "species" is that it used to have two subspecies Trichoniscus pusillus pusillus and Trichoniscus pusillus provisorius but the latter has been upgraded to species status fairly recently (past decades), meaning we would now officially need to choose between Trichoniscus pusillus and Trichoniscus provisorius - and that's a "no can do from photo"

The two can only be separated on the male genitalia and as T. pusillus is big on parthenogenesis there are very few males in the average population. By itself this can be a clue - if you find (m)any males it's probably T. provisorius (T. pus. will usually have less than 5% males or so).

Though it is assumed that the two species are mostly mutually exclusive in their habitats, possibilities of a mixed community should always be kept open for consideration. Of the two the parthenogenetic and triploid T. pusillus is the one best adapted to harsher environments.

In Britain historically research will have T. pusillus in the north and west and T. provisorius in the south-east. Where there is overlap T. pusillus will more likely inhabit harsher environments such as open grasslands and T. provisorius the milder habitats such as woods.

So, depending on where you found the animal and how strict you wish to be it might be a choice of:

• Trichoniscus cf. pusillus (north and west and/or harsh habitats)
• Trichoniscus cf. provisorius (south-east and forgiving habitats)
• Trichoniscus sp. very strict but correct - although it's clearly not T. pygmaeus
• Trichoniscus pusillus s.l. (=sensu lato) as most oldish literature will have both species as subspecies of T. pusillus anyway
• Trichoniscus pusillus/provisorius - not very scientifically correct probably, but to me nice and clear about what is meant

Take your pick

Just out of personal interest: Do you have a size estimate with this animal?

Cheers, Arp

[Gerel]

Thanks Arp, as usual you are a mine of information, I had no idea it was so complex .

I would estimate that this one was between 2 and 3 mm long. I photographed it in mid-Wales which, by most definitions (as north seems to start ridiculously far south in this country) would put it in the north and west of the UK. We are also circa 1000ft / 330m + above sea level but in woodland, would that count as a harsh environment ?. From your post I would be inclined to go for T. pusillus although I'm happy to settle for Trichoniscus sp.

Happy Christmas


Steve

[Pudding4brains]

Hi Steve,

Thanks for that size estimate - fits nicely with what I find here - I'm always a tad surprised to read in many British references that T. pusillus grows to be up to 5mm which imho is gigantic - 4mm I could sort-of-kind-a get my head around, but 5mm??

Yes, based on location/altitude I would estimate that the animal would almost certainly be T. pusillus. For correctness sake I would keep the "cf." in though. Hence: Trichoniscus cf. pusillus

Merry Christmas to you too

Cheers, Arp