A previously unknown and very bizarre species of millipede has been discovered in the depths of a California cave. It has the usual hundreds of legs common to the genus. Less usually, it has enigmatic mouthparts whose function nobody can figure out, and four penises.

Technically, what the animal has are four gonopods, which are specialized legs that act as male genitals, but that is a small distinction.

The discovery in Sequoia National Park was published in Zookeys by the team exploring the cave.

Brushing aside the usual complement of cave spiders and pseudoflies, the team realized they'd found something weird in the threadlike beast and sent a specimen to diplopodologists (experts who study millipedes), says the report.

There are over 7,500 known species of millipede, a harmless invertebrate that prefers damp environments and usually lives on decaying plant matter, though certain species will eat fresh plants and some will eat meat. Most sport the usual complement of penises, though some species have two.

Some millipede species don't penetrate the female when mating: they leave their sperm in external spermatophores that the females pick up. The rest do mate and it seems the millipede may even have quite the dramatic love life, with wooing and foreplay, followed by copulation in the millipede equivalent of the missionary position.

The new species, a round, not flattened type of millipede, has been named Illacme tobini after cave biologist Ben Tobin of the National Park Service. Its whitish color is quite usual for underground beings that never see the light of day.

Millipedes grow by molting and adding more body segments, each of which will have one or two sets of legs: a specimen of Illacme tobini that the team of Paul Marek, Jean Krejca and William Shear examined had 414 legs.

Open gallery view The new species of the extremely leggy multiply-gonopodded millipede from a Sequoia National Park cave. Credit: Paul Marek, Virginia Tech

That sounds impressive until you realize that the leggiest sort of millipede was counted to have 750 legs and the usual range for most of the species is 34 to 400 legs. (None have the thousand legs that the name 'millipede' would seem to suggest.)

The enigmatic millipede was sent to diplopodologists Bill Shear and Paul Marek, who realized its relationship to the generic millipede, Illacme plenipes.

Another enigmatic feature of Illacme tobini is that the segments are covered in silk-secreting hairs, says the team. The beast also has paired nozzles on each body segment that squirt a chemical, which the scientists assume is defensive in nature. What the substance is remains to be analyzed.

Finding unknown species, or variants, in isolated caves has happened before. In 2015, scientists were astonished to find myriad species of bizarre animals living in the thoroughly toxic environment of a cave in Romania, near the Black Sea. The cave and its sulphidic underground lake had been isolated for 5.5 million years, they estimate, and was crawling with unknown types of centipedes, insects and what not. So far 33 new species were named, and counting, from that cave system alone.

In Israel, in 2006, quarriers broke into a previously unknown cave near Ramle, in central Israel, which also proved to have a prehistoric ecosystem dating back millions of years. Among the unique animals found there were new species of crustaceans and white invertebrates similar to scorpions, with no eyes. Scientists suspect that the animal life in the Ramle cave, dubbed the Ayalon Cave, was trapped there millions of years ago and continued to evolve in isolation in the cave, which has its own underground lake.