Adult females of a millipede newly described from the Kazakh Altai have almost no second pair of legs, the subadult females still have them, and in both of the only two adult females collected the flattened leg bases block the genital opening

Characters new to the whole order

The paper describing a new millipede from the Kazakh Altai names a combination of four features it calls novel for the entire order Polydesmida. There are side keels notched into small lobes, male sperm-transfer organs whose shaft splits into three branches, a reservoir at the end of the sperm groove set into the base of a short process called the solenomere, and, in adult females, a second pair of legs reduced almost to nothing.

It is the last of the four that does the most work. In both of the only two adult females ever collected, the flattened bases those legs should have grown from lie across the opening the animal mates through. The subadult females in the same sample still have the legs, complete and ordinary.

The animal is Triramidesmus kazakhstanus, a new species and a new genus at once, described in ZooKeys on 4 September 2026. The describers are Pavel Nefediev of Altai State University in Barnaul, Sergei Golovatch of the Severtsov Institute of Ecology and Evolution in Moscow, and Julia Nefedieva of Tomsk State University.

Adults of both sexes carry twenty body rings. The holotype male measures 11.4 millimetres long and the other adult males 11.4 to 12.1, while the one adult female the description measures is 11.1. Each is a shade over a millimetre across the widest part of a midbody ring, pinkish beige in alcohol with paler legs.

The genus name is not about the missing legs at all. Triramous means three-branched, and it points at the male’s gonopods, the modified legs that deliver sperm, whose main shaft carries the three-way split.

The keels are the character a reader could see. They run down the rings behind the collum, moderately developed and set high, just below a faintly convex back, with their edges notched into four or five small bristled lobes. On ten of the rings the second-to-last lobe swells to carry a teardrop-shaped opening, an ozopore, the term for the outlet of a millipede’s defensive gland. Those ten are rings 5, 7, 9, 10, 12, 13 and 15 to 18, a pore formula the paper calls normal.

All of it came from a single day’s collecting. Yu.V. Dyachkov took the material from litter and soil on 16 August 2024, about ten kilometres southeast of the village of Verkhnie Tainty in the Kalbinskiy range of East Kazakhstan Region, at 1,120 metres, under poplar, birch and willow with tall grass. The specimen label gives the wrong village, Nizhnie, and the paper corrects it in print.

The subadult still has the legs

Among the paratypes are immature animals at what myriapodologists call stadium VII, still carrying nineteen rings instead of twenty, between 7.9 and 8.6 millimetres long. The subadult females have a normal walking leg pair in position two.

That comparison is what fixes the timing. Because the young females have the legs and the adults do not, the paper concludes that the reduction happens at the final moult, from stadium VII into adulthood, and its verb for that conclusion is “indicates”.

The adult condition does not look like injury. What remains of the leg pair is a set of minute telopodite remnants sitting in the middle of two flattened, roughly square coxae, the basal segments the legs would have grown from. “Apparently, this reduction is the norm for this species, being neither a teratological nor an aberration,” the paper says, and the evidence offered for it is both known adult females.

The male series is described rather than dated. Subadult males at stadium VII carry either a pair of tiny rounded gonopodal buds or a pair of visibly unfinished gonopods in place of the eighth leg pair. In the adult male those are the finished three-branched organs the genus is named for, and they retract into a gap made by the sternite of the sixth leg pair, which is reduced, hollowed out and hairless. The moult-timing conclusion is drawn from the female series only.

Anatomy of a blocked aperture

In Triramidesmus kazakhstanus the two flattened coxae cover the vulvar aperture entirely. The paper describes the bean-shaped vulvae as fully blocked in position by those coxae. Behind the aperture runs a well-developed transverse ridge, straight and roughly rectangular, with no median outgrowth.

The organs underneath are anything but simplified. The front face of the bursa carries two rows of relatively large projections of the spermatheca, flanked by ten pairs of thread-like fringes, some of them twisted at the tip. Sac-shaped or spiral structures sit at the bottom of the bursa’s gutter.

The males, meanwhile, carry very short, membranous, cylindrical gonapophyses on their own second-pair coxae. The paper describes both conditions without drawing a line between them.

Its reasoning about mating runs from the plumbing outward. Since a permanently sealed aperture would leave the animal unable to breed at all, and since both adult females have blocked openings and vulvae the paper calls definitely functional, the authors argue that both females must already have mated. From there they offer a mechanism, carefully: it is possible, they write, that the male closes the female’s aperture with her flattened coxae during copulation.

How strongly that points at sexual competition depends on which page you read. The abstract says the situation suggests sexual competition and the discussion says it strongly suggests it, two strengths printed a few pages apart with no word about the difference. The abstract also narrows the finding itself, putting the blockage presumably only in females that have already mated.

On the strength of two adult females

Two adult females is the entire adult female record for the species, and they came from one patch of forest floor on one day. So far the species is known only from that type locality. The paper reports no observed mating and no male seen closing anything; the sequence it proposes is reconstructed from anatomy in alcohol.

The reduction appears to be unique within Polydesmida, the order this millipede belongs to, and the paper’s remarks put the boundary there deliberately. It records the reduction of female leg pair two to tiny vestiges as well known in several other orders of millipedes, naming families and genera within the Chordeumatida, within Julida, and in part of the Callipodida.

The comparison the paper reaches for sits inside Polydesmida too. In three species of Tasmaniosoma, of the polydesmidan family Dalodesmidae, females are usually found without their second legs. Nefediev and his co-authors read those as seemingly removed by males during copulation, broken off and then, in their own scare quotes, “healed”.

That mechanism is not a finding of the paper they cite. The paper they cite for the missing legs reports only that the legs are missing: usually so in two of the species, where it defers the observation to a manuscript in preparation, and across all three known specimens of the third, on its own authority. What the Kazakh animal has is a leg pair that is reduced to remnants at the final moult rather than torn off.

The idea that missing legs make copulation easier for the male is attributed in the paper to a personal communication from Robert Mesibov, who described the Tasmanian species. The sentence carrying it across to the Kazakh animal opens with “if so”.

The family label turns on a single chamber

The describers set out a second question, which is where the genus belongs. Superficially, they write, three of its features would put it in Trichopolydesmidae sensu lato instead of Polydesmidae: a short seminal groove, a short and simple solenomere, and no hairy pad capping that groove. As a close match on that reading, the genus would seem to sit near the Himalayan Hingstonia, whose thirteen described species have gonopod branches numbering two to four and a solenomere that is likewise short, simple and spine-like.

What decides it for the authors is one structure. A prominent, elongated accessory seminal chamber, sitting at the base of and partly inside the short solenomere, is typical only of Polydesmidae, and its presence here leaves them in no doubt. The sperm groove stays entirely basal to all three branches, never running out along one of them, and ends in that chamber.

The same character ties the new genus to Schizoturanius tabescens, which the paper treats as a disjunct outlier of a genus otherwise widespread in southwestern Siberia. The two animals resemble each other in body size and in the notched keels, and part company on the gonopods, three-branched with a short groove here and two-branched with a long one there. Genus and species are each registered as their own nomenclatural act, which fixes both names whatever the family question does next.

A type series lopsided by sex

The ratio in the collection is the loose end the paper leaves open. Its own summary is forty-odd males and only two females, from the same litter on the same day, which seems to the authors “quite strange and interesting”. Their response is to decline to explain it: in order not to breed speculation, they write, additional research is necessary.

Itemising the list of specimens gives the shape of it. Forty-one adult males, two adult females, and seven subadults, three of them male and four female, fifty animals in all.

Around ten millipede species had been recorded from the Kazakh Altai before this one, two of them still awaiting description and one an arrival with human traffic. On that base, a single day’s sifting produced a genus nobody had named, which is a fair measure of how thin the survey is. A third adult female would settle more than another forty males could, and a mating watched instead of reconstructed would settle most of it. What exists is fifty specimens in 75 per cent alcohol in Moscow and Barnaul, two of them adult females.