The short version: Trachelipus is a genus of temperate woodlice that cannot roll into a ball and is almost completely overlooked by the hobby. We keep six species, and all six sit inside the range of an ordinary British room for most of the year, with no heat mat and no thermostat. That is rarer in this hobby than it sounds. This guide covers what the genus is, what is actually known about each of the six, and how to set them up.
What Trachelipus Is
The genus sits in the family Trachelipodidae, named by Strouhal in 1953. If you have an older book it will put these animals in the Porcellionidae alongside Porcellio and Porcellionides, and that is how most of the twentieth-century literature treats them. The arrangement used today splits that old Porcellionidae into three families, Porcellionidae, Trachelipodidae and Agnaridae, and it is what WoRMS, GBIF and the British Myriapod and Isopod Group all follow.
The family is not on firm ground. A molecular study of the Porcellionidae published in 2018 found Trachelipodidae lacking robust shared characters and called it a family in need of a sound revision. Only one Trachelipus was sampled in that work, so the position of the genus itself is barely tested. If you meet the family name treated as provisional somewhere, that is why.
Around fifty-seven to fifty-nine species are described worldwide, most of them well east and south of the area the European literature covers. Europe holds a fraction. When Christian Schmidt revised the European species in 1997 he recognised fifteen as valid, made nineteen names new synonyms and reduced nine more to nomina dubia.
That is the single most useful fact about the genus for a keeper, because it explains why names in this corner of the hobby are so unstable. Reliable identification turns on male characters, particularly the shape of the seventh pereopod and the first pleopod, which means a photograph on a listing page is not an identification.
The One You Might Find Yourself
Britain has a single member of the genus established here, Trachelipus rathkii, Rathke's Woodlouse, reaching about 15 mm. The British Myriapod and Isopod Group gives it a GB status of Least Concern and notes that it is probably under-recorded. Its range is a clumpy block across south-east England with scattered records west into Wales, and it has never been recorded in Ireland. It is a woodlouse of river floodplains, favouring ground with impeded drainage and tolerant of flooding, so wet meadows rather than dry walls. Records have crept outwards in recent years, reaching Yorkshire and Worcestershire, with a single specimen turning up near Edinburgh through the horticultural trade.
If you turn woodlice out of a damp riverside log pile in the south of England, this is the one to look at twice. It carries five pairs of pleopodal lungs, one on each of the five pairs of abdominal pleopods, against two pairs in Porcellio, and its antennal flagellum has two segments. Those are the characters that separate it from the Porcellio scaber it gets mistaken for. Our guide to woodlouse names and nicknames covers the rest of the British cast.
They Cannot Roll, and What They Do Instead Is Unclear
No Trachelipus conglobates. A 2022 review of antipredatory strategies in terrestrial isopods lists the families that can seal into a ball, and Trachelipodidae is not among them. That is evidence by omission rather than a positive statement about the genus, but it matches everything keepers see.
What they do instead is less settled than the hobby suggests, and that includes what we have written ourselves. You will read that the genus is built to cling, pressing flat into a crevice the way Oniscus asellus does. There is no published behavioural study of any Trachelipus to support it. One British field account of T. rathkii describes the animal as moving quite swiftly and as less likely to clamp down than Oniscus, and a Brazilian study of tonic immobility in woodlice tested three species, none of them in this genus. So: they cannot roll, they are quick, they want cover, and the rest is keepers' observation. Ours run for the nearest bark and go still underneath it.
The Six We Keep
Trachelipus caucasius
The largest we stock, and the one for a cold room. It comes from the eastern Black Sea coast, from around Sochi through western Georgia into north-eastern Turkey. The published material runs from a coastal site near Sochi and wet forest in Georgia up to Turkish records between 1000 and 2200 metres, some of it in spruce forest.
The largest animal measured in the most recent treatment of the eastern Black Sea species was 20.0 by 11.5 mm. That same description gives the body as blackish brown with yellowish rear edges to the side plates, which is worth saying because the grey-with-orange-edging wording used around the hobby, ours included, is not how the species is described in the literature. Its range suggests it will take cool conditions well, and our caucasius colonies do, sitting at 18 to 26 degrees and shrugging off the lower end. Nobody has measured cold tolerance in this genus, so that is experience rather than a figure.
Trachelipus trilobatus
The Trilobite, and the one that makes people stop at the shelf. It is endemic to Romania, and in the most recent survey it was found only around Baile Herculane in the Banat, at the southern edge of the Carpathians, in humid spots on steep limestone slopes in forested gorges.
Measured specimens run to 18 by 11 mm in males and 19 by 12 mm in females, and the eyes carry 22 to 23 facets. We have carried 24 mm on the trilobatus listing and in its care panel; no measured series supports that, so it is coming down to 19 mm. One more correction while we are here. The three lobes of the name are on the head, not the tail: the pleotelson tapers to a sharp point, and it is the cephalic lobes, a well-developed pentagonal median lobe between two lateral ones, that the description dwells on.
Trachelipus difficilis
Radu described this one in 1950 as Tracheoniscus difficilis, which is why the authority sits in brackets now. Its range is Carpathian: Romania, Hungary, Slovakia and Poland. It is also the species with two characters depending on which country's literature you read. Slovak workers treat it as a relict confined to undisturbed natural sites, living on rock and on the walls just inside cave entrances. Romanian workers describe an ordinary forest species found from the mountains down to the plains. Both are probably right, with a relict rock-dwelling population at the northern edge of a range where the animal is unremarkable in the core.
On size, we have carried 20 mm, taken from a photograph caption in a Slovak cave survey. A measured series gives males 8.5 by 5 mm to 14 by 6 mm and females 9 by 5 mm to 14 by 7 mm, so the panel is moving to 14 mm: a caption is one animal and probably an estimate, and a series beats it. Our difficilis run cool at 15 to 22 degrees, and a British summer heatwave worries them more than February does.
Trachelipus mostarensis
The ghost. Verhoeff named it in 1901 from Herzegovinian material, and the world's occurrence databases hold ten records, every one a preserved museum specimen, several of them Verhoeff's own, spread across Frankfurt, Berlin, New York, Cambridge, London, Washington, Leiden and Helsinki. There are no field observations at all.
It is not among the fifteen species Schmidt accepted in 1997, but it has not been formally sunk either: the World Register of Marine Species still lists it as accepted. No published size for it exists: the original is a four-page note of 1901 in the Berliner Entomologische Zeitschrift, and across those four pages Verhoeff gives no measurement of any kind. The 12 to 15 mm quoted around the trade traces to vendor listings and nothing else. The animal on the shelf is an unremarkable temperate woodlouse to keep, which is the strange part.
Trachelipus ratzeburgii
A central European forest species, not British, whatever the odd stray record suggests. Germany and Austria account for most of the records, with Italy, France, Czechia, Switzerland, Hungary, Poland and Slovenia behind them, and a northern outpost in southern Norway where it is listed as Near Threatened on the 2021 national red list.
Romanian measured series run to about 12 mm while continental species profiles quote up to 16 mm, so 12 to 16 mm is the fair upper spread rather than a single figure. The hind tips of the coxal plates are yellow-reddish with yellow spots at their bases, which is the field character. This ratzeburgii line is a true woodland animal, found under wood, bark and litter and rarely out in the open.
Trachelipus cavaticus
A cave endemic known only from Crete, described in 2004 from the cave of Kournas in Chania. The holotype is a male of 13.0 by 8.0 mm collected in March 1987, and the largest specimens measured were 15.0 by 9.3 mm.
Its only Cretan congener in that account is T. kytherensis, a name now treated as a synonym of T. aegaeus, and the differences the authors recorded are the interesting part: about twelve eye facets against about twenty, wider and flatter side plates, longer and frailer legs, all read as adaptation to cave life, with the differences constant and no intermediates between them. One correction to our own page, which says the two were collected together at Kournas. The kytherensis material came from the lake of Kournas, not the cave. Same name on the map, different site. Our Cretan cave line wants 18 to 24 degrees and the highest humidity of the six, and the survey that described the species brought Crete's terrestrial isopod list to fifty-five.
Setting Them Up
A five- to ten-litre tub with a clip-lock lid and cross-ventilation on opposite walls suits a starter group of any of the six. Build about five centimetres of a living base from flake soil, well-rotted hardwood and organic topsoil, and leave coco coir out of it: it holds water and feeds nothing, which is the wrong trade for an animal that eats its own substrate.
Top that with a generous layer of oak and magnolia leaf litter, then cork bark laid flat and stacked in layers. For difficilis, add flat stones and slate offcuts, because a rock-dweller will take the underside of a stone over a burrow every time. Keep a calcium source in permanently, cuttlebone or crushed eggshell, in more than one place, and feed the standard detritivore spread: litter and white-rotted wood always available, soft vegetables a couple of times a week, a little protein now and then.
Humidity runs as a gradient rather than a number. One damp corner under sphagnum and litter, the far end allowed to breathe and dry between mistings. Across the genus that lands between 55 and 75 per cent depending on the species, with cavaticus the outlier at the wet end.
Temperature is the easy part and the reason to keep these animals at all. difficilis wants 15 to 22 degrees, trilobatus takes anything from 15 to 26, cavaticus and ratzeburgii sit at 18 to 24, caucasius at 18 to 26, and mostarensis prefers 20 to 25. An ordinary British room covers all of that for most of the year. A radiator wall and a sunny windowsill are the two things to avoid.
What Is Known About Their Tolerances
Very little, and it is better to say so than to quote numbers with nothing behind them. There is one real desiccation experiment involving the genus. Nine woodlouse species were held at roughly 30, 75 and 100 per cent relative humidity; T. rathkii came out middling at the driest setting and better than most at 75 per cent, losing significantly less water than Cylisticus convexus, Orthometopon planum and Porcellionides pruinosus, with a quarter of animals lost under extreme drying. The study's broader conclusion was that cuticle thickness and external shape predicted water loss better than how closely related the species were.
Beyond that, nothing. No supercooling point, no critical thermal maximum, no thermal preference has been measured for any species in this genus, and no published optimum humidity exists. Every temperature and humidity band you read for these animals, ours included, is a keeper's working figure. The same goes for the genus's reputation as a forgiving one: no scientific source rates any Trachelipus for ease of captive husbandry.
Breeding
Brood size is published for exactly one species. In an urban Russian study of T. rathkii, females carried between 7 and 106 eggs with an average of 35.9, and fecundity rose steeply with body size. That fifteen-fold spread is the practical lesson for anyone starting a colony: the size of your founding females matters more than the count.
Gravid females were found there from June to August. In a Maryland population of the same species, breeding ran from late April or early May into September, and size-corrected brood size fell as the season went on, so later broods are smaller even from equally large mothers.
Age at maturity and lifespan have not been published for any Trachelipus. What we can say from our own shelves is that a group of ten or more in a settled tub produces visible young within a few months and then keeps going quietly, and that the woodlouse life cycle runs the same way here as in any other terrestrial isopod: eggs carried in the female's brood pouch, mancae released as miniature adults, growth by moulting in two halves.
Frequently Asked Questions
Can Trachelipus roll into a ball?
No. Conglobation is restricted to a handful of woodlouse families, including the Armadillidiidae and Armadillidae, and Trachelipodidae is not one of them. A threatened Trachelipus stays flat and heads for cover.
Are Trachelipus isopods good for beginners?
In our experience yes. They are hardy, quick to breed, undemanding, and they need no heating in a normal British room, which is the thing that trips up beginners with tropical species. No study has ever rated a Trachelipus for ease of keeping, so that is keepers' experience rather than a published assessment.
What is the British Trachelipus species?
Trachelipus rathkii, Rathke's Woodlouse, up to 15 mm, established across south-east England and spreading, absent from Ireland, and a characteristic animal of riverside meadows and ground with impeded drainage. It is the only member of the genus on the British checklist.
How big do Trachelipus get?
It depends which one. Measured series give caucasius to 20 mm, trilobatus to 19 mm, cavaticus to 15 mm, difficilis to 14 mm and ratzeburgii to about 12 mm, with continental profiles quoting up to 16 mm for the last. For mostarensis there is no published figure at all: the 1901 description gives no measurement.
Do Trachelipus need a heat mat?
No. Every species we keep sits within ordinary British room temperature for most of the year, and difficilis in particular is more at risk from a summer heatwave than from a cold winter. Keep the tub off a radiator wall and out of direct sun.
Why are Trachelipus names so confusing?
Because the genus has been revised hard. The 1997 revision of the European species accepted fifteen names, synonymised nineteen and left nine as doubtful, and identification turns on male characters that no photograph will show. Several names circulating in the hobby have no modern taxonomic footing at all.
Where to Start
If you want size and a species that takes a cool room, start with caucasius. If you want the animal people ask about, start with the Trilobite. If you like a species with a story, difficilis on a pile of slate is the one. A group of ten or more, a five- to ten-litre tub with cross-ventilation, five centimetres of living substrate, deep litter, stacked bark and a permanent calcium source, and then leave them alone. Our livestock travels in double-walled boxes with sphagnum moss and ventilated deli tubs under a live arrival guarantee, with any losses claimable within six hours of delivery.
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