Porcellio Laevis Milk Back Isopods

In Britain this animal is in trouble. Not the morph - the species. Porcellio laevis lived in stables, cattle yards and dung heaps, and as working horses disappeared and cattle husbandry modernised it went with them. There are very few modern British records - the published review names two since 2000, against a species that has been here since the thirteenth century - there are sub-fossil remains from a medieval pit at Stonar in East Kent. The same animal is among the most heavily traded isopods on earth and a declining British native at once. We find that worth knowing while you keep a tub of them.

The animal

Smooth-backed - the fastest way to tell it from the rough-shelled P. scaber - and reaching 20mm. Males carry long spear-shaped uropods at the rear. The species group it belongs to is North African in origin, and it is now genuinely cosmopolitan - it has been synonym list reads like the same animal being named again on four continents, which is why the world checklist carries more than two dozen entries for it.

'Milk Back' describes creamy white markings over a smooth grey body. We should be straight that the trade does not agree on this: of ten independent sellers using the name, some describe a white stripe down the back, two describe a chocolate-brown animal, one says marbled, and several give no description at all. Ours look as our photographs show. Take the name as a rough guide to what arrives, not a specification.

Milk Back and Dairy Cow

We sell both, and no source we can find distinguishes them. Both are piebald grey-and-white P. laevis. One seller who lists both runs identical species text on each page and separates them by a single sentence. If you want the difference in our stock it is that Dairy Cow tends to irregular patches while these run more organised along the back - but that is our observation of our own colonies, not a described character, and no genetics has been published for either.

Keeping them, and a correction

This listing gave humidity as 60 to 80% with a dry side, and warned against letting the tub turn into a swamp. The gradient advice was right and stands. The band was too wet: four other sellers put this animal at 40 to 70% and one calls it semi-arid outright. We have brought the numbers down to fifty to seventy, still graded, still with a genuinely dry end. Temperature 18 to 26°C, no heat needed.

Substrate 5 to 7cm, deep leaf litter, bark to hide under, permanent calcium. Protein regularly - keepers describe this species as protein-hungry more often than almost any other, though we should say that is trade lore rather than a measured requirement.

No conglobation here. Nothing in this genus can close into a ball.

Two warnings we should not have cut

They climb, and they are accomplished escapers. A close-fitting lid is not optional.

And be careful what you put them in with. This is a fast, bold, protein-hungry animal, and an underfed colony will go looking for protein - which in a mixed vivarium can mean nipping soft-bodied tank mates or a shedding reptile. They can also outcompete slower species outright. Feed them well and think twice before adding them to anything with vulnerable neighbours. Never give them access to standing water either; woodlice drown in it.

How fast they breed

Unusually for this catalogue, there are real numbers. A published study found females first reproducing at around seven months, breeding up to seven times regardless of season, averaging just over three broods each, with an average of about sixty-seven young per brood at intervals of roughly eight weeks - our old page said 20 to 40, and that figure has gone. Another gives a lifespan of twelve to eighteen months across nine or ten moults. That is a fast, short-lived animal, and it explains why a starter culture becomes a full tub inside a year.

Posted early in the week on the pre-1pm service, in an air-holed tub bedded on moist moss inside a double-skinned box, with a fifth again over your stated quantity and a live arrival guarantee the whole way. The rest of the group sits under Porcellio.