ISOPODA | Juvenile woodlice

A few notes regarding the identification of immature woodlice

When new born baby woodlice emerge from their mother's brood pouch, they are known as mancae (singular: manca). Their exoskeletons are soft and fragile providing little protection from the elements or opportunist predators. They are also extremely small with a slightly foreshortened body comprising six rather than seven segments, and similarly, six rather than seven pairs of legs.

Unless you accidently uncover a brood site, it's unlikely you'll see them at this very early phase of their life cycle as they naturally tend to keep themselves hidden away. Personally, I've only managed to photograph them this young on one occasion. The couple of individuals I found were off-white and partially translucent as most woodlice are at this age, but the photos I took showed sufficient detail in terms of the body shape and eyes to determine they were Trichoniscus sp. and, due to the tiny size which was around 1.5-2mm long and the fact that there was no hint of any reddish-brown pigmentation, were almost certainly the Least Pygmy Woodlouse, Trichoniscus pygmaeus.

Six-legged trichoniscid mancae, almost certainly Trichoniscus pygmaeus - Least Pygmy Woodlouse

The 'manca stage' though doesn't last very young as within a day or two they will undergo their first half moult when

they gain the last body segment and final pair of legs. Once the full moult is completed, they will start to harden up. Appearance varies species to species but, in most cases, they will now look like pale miniature versions of the adults.

But the question is which species, because unless you've got really good eyesight or are looking at them through a hand lens or camera viewfinder, you'll probably not even going to be able to determine whether an individual is an immature larger species or one of the pale coloured pygmy woodlice. Sometimes it's obvious, such as with the Armadillidium genus where the body shape is distinctive, but generally tiny white woodlice can look very similar.

Juvenile pill woodice - most likely Armadillidium vulgare

Once they've gone through another couple of moults, or have grown to 4-5mm or so, they can usually be distinguished without too much problem although, saying that, caution is required with juvenile Oniscus asellus as they are less shiny than the adults and have a rough and bumpy rather than smooth texture and, as such, could be mistaken for immature Porcellio scaber. Yes, they could be correctly identified by the number of flagella segments, but with tiny specimens like these that level of detail may be difficult to capture in photos, which is why I wanted to include this page where a couple of examples of each species could be featured side by side.

Juvenile Oniscus asellus - Common Shiny Woodlouse (top) and Porcellio scaber - Rough Woodlouse (bottom)

Of course if you only photograph adult woodlice the problems with identification lessen, but from a personal point of view

I find it interesting to observe species at different stages of development and, whenever I can, I usually take a few photos for reference.

Woodlice don't live for very long in the wild, possibly up to 4 years in respect of Oniscus asselus if conditions are suitable, but with some other species such as Philoscia muscorum it could be as little as 1-2 years. I'm not sure whether the shorter-lived species take less time to mature or indeed how long any particular woodlice species spends in the juvenile stages of development, so I just photograph what I find and then try to fathom out what I can from the photos and the habitat and time of year when they were taken.

A further point to note that helps with identification is behaviour in terms of how the species reacts once disturbed. I've briefly made reference to the fact that some woodlice manage to disappear surprisingly quickly whilst others simply prefer to sit tight and hope that any potential threat passes by, but I thought it would be useful to list the more common species under their most appropriate category generally referred to as the 'runners', 'clingers', 'rollers' and 'crawlers'.

The 'runners' typically have a slender body with longer legs that enable them to run and find a hiding place, which is certainly their first reaction when uncovered and exposed to the light - the most notable example here is the Striped Woodlouse, Philoscia muscorum but, down at the coast, the Sea Slater, Ligia oceanica, would literally give it a run for its money! Similarly with the Plum Woodlouse, Porcellionides pruinosus that mainly lives in dung heaps and the like.

Philoscia muscorum (immature) - Striped Woodlouse

Porcellionides pruinosus - Plum Woodlouse

The 'clingers' have a much wider and more flattened body that allows them to clamp down to the substate until it's safe to move with the Shiny Woodlouse, Oniscus asselus, being the clear winner in this category. It's a particularly effective strategy as this species usually blends in with its habitat, which coupled with the habit of staying motionless really does make them difficult to spot at times. The Ant Woodlouse, Platyarthrus hoffmannseggii is another 'clinger' although from my limited experience of the species it doesn't react in the same way as the other species presumably as it's blind. I wouldn't say Porcellionides cingendus was a 'clinger' as such but, in respect of the couple of sightings I've had, they've tended to remain where found.

Oniscus asselus (immature) - Common Shiny Woodlouse

Porcellionides cingendus (immature)

The 'rollers' are the relatively slender species with a curved body that allows them to curl into a ball for protection as they have shorter legs that restricts them to walking pace when they need to move - the obvious inclusions here are members of the Armadillidium genus and species such as Cylisticus convexus.

Armadillidium vulgare (immature) - Common Pill Woodlouse

Armadillidium depressum (immature) - Southern Pill Woodlouse

Last, but not least, the 'crawlers' which tend to be the smaller pygmy species, although the Rough Woodlouse, Porcelio scaber is more of a 'crawler' than a 'runner' or 'clamper', but different individual act in different ways and I've seen them exhibiting all three of these tactics. I'd also note that Common Pygmy Woodlouse, Trichoniscus pusillus isn't necessarily

a 'crawler' at times as it can scamper away quite quickly and can infuriatingly difficult to photograph as it's so active.

Porcellio scaber (immature) - Rough Woodlouse

Page added, Oct.26 (v.1)

Trichoniscus pusillus (immature) - Common Pygmy Woodlouse