Trichoniscus pygmaeus

CRUSTACEA > MALACOSTRACA > ISOPODA > Oniscidea > Synocheta > Trichonisidae

As its common name suggests, this species is smaller than the previously described form with individuals growing to no more than 2.5mm long. They are also much paler, feebly pigmented creamy-white to yellowish, often with a pale pink flush. The head is always a tad paler than the body, whereas in Trichoniscus pusillus the head colour is much the same as the body. But, other than size and colour, this species is actually just a smaller form with the same stepped body outline between the pereion and pleon, eyes comprised of three ommatidia, and tapered antennal flagella.

Separating juveniles of these two species can cause problems, but even immature Trichoniscus pusillus have some degree of reddish-brown pigmentation except when very young.

Trichoniscus pygmaeus can be found in a wide variety of habitats as long as the conditions are suitable, as it's very much a soil-dwelling woodlice that favours friable brown earth and calcareous or neutral sandy soils and clays in woodland and semi-natural grassland areas, but also in synanthropic sites such as verges, waste ground, old quarries and churchyards. And, in some areas, it also occurs at coastal sites where there are light free-draining soils such as at the base of sea cliffs. In all situations though it is probably going to be found either in the soil or under damp litter, or on the underside of large partially embedded stones or pieces of half-buried deadwood.

To date, my only encounter has been with a couple of six-segmented juveniles that were on a rather large damp rotting log that I was searching under. They were barely 2mm long and consequently the photos are poor, but the shape of the eye patch indicates three ommatidia rather than a single ocellus, and the body outline is clearly stepped, so both of these features rule out Haplopthalmus. And despite the fact that they're obviously very young individuals, there's no sign of any reddish-brown pigmentation - only the gut content showing - and, therefore, it's a pretty safe bet to assume they're Trichoniscus pygmaeus. Hopefully, I will be able to find an adult to photograph at some point, but for now I'm just pleased that I can at least represent the species with these photos.

New species added, Sept.26 (v.1)