For decades, scientists had a solid grasp of how most animals reproduce—but the question of how millipedes mate remained a stubborn mystery. With over 13,000 known species, each sporting a tangle of legs that obscures whatever is happening underneath, researchers simply could not see how these ancient arthropods transferred sperm from male to female. That changed in 2020, when a team at the Field Museum of Natural History in Chicago combined ultraviolet light, micro-CT scanning, and some unusually cooperative millipedes to finally reveal the mechanics of millipede mating. Here is everything we now know about how millipedes mate, reproduce, and bring the next generation of mulch-munchers into the world—a topic that puzzled entomologists for the better part of a century.
The Anatomy of Millipede Mating: Gonopods, Vulvae, and That Blue Ejaculate
To understand how millipedes mate, you first need to know what they are working with—and the answer is surprisingly elaborate for a creature most people barely notice on the forest floor.
Male millipedes possess a pair (or sometimes two pairs) of highly modified walking legs called gonopods. These specialized appendages—the name literally means “sex legs”—are located on the seventh body segment and serve as the sperm-transfer organ. Unlike a mammal’s penis, gonopods do not produce sperm directly. Instead, the male’s testes are positioned near his second pair of legs, on the third body segment. He must first release a packet of sperm (a spermatophore) from his genital openings, or gonopores, and then contort his body forward to scoop the sperm onto his gonopods, which sit several segments further back.
The sperm itself is reportedly a blue-ish liquid—a detail that surprised even the researchers who first observed it. “It’s a blue-ish liquid,” confirmed Petra Sierwald, associate curator at the Field Museum and co-author of the landmark 2020 study, with charming understatement.
Female millipedes have two small openings called vulvae, positioned one on each side between their second pair of legs. Behind these openings lie internal storage chambers called spermathecae, where sperm can be kept viable for months—a reproductive strategy that lets the female wait until environmental conditions are ideal before fertilizing her eggs.
Under magnification, the tip of each male gonopod reveals tiny, claw-like structures that hook behind corresponding ridges on the female’s vulva. This forms what scientists describe as a lock-and-key mechanism—the male and female anatomy fitting together with species-level precision, ensuring that only members of the same species can successfully mate.
How Scientists Finally Solved the Mystery: UV Light and Micro-CT Imaging
For roughly 80 years, entomologists could describe millipede genitalia in broad terms but had no way to watch the parts interact during actual copulation. Traditional dissection destroyed the spatial relationships between tissues, and conventional microscopy could not penetrate the tangle of legs and segments that millipedes bring to the act.
Millipedes are not the only creature whose reproductive secrets took science decades to crack—researchers also only recently solved the mystery of how eels reproduce. The millipede breakthrough came from an unlikely combination of technologies, detailed in a 2020 paper published in the journal Arthropod Structure and Development. Researchers at the Field Museum and the University of California, Davis, turned to two complementary imaging methods:
- Ultraviolet (UV) fluorescence photography: Millipede genital tissues glow under UV light, making the different structures—chitinous sclerites, membranes, and muscles—visually distinct. Stephanie Ware at the Field Museum took dozens of photographs at slightly varying focal distances and then used focus-stacking software to combine them into perfectly sharp, three-dimensional images.
- Micro-CT (micro-computed tomography) scanning: Xavier Zahnle at UC Davis placed intact, mating-pair specimens into a micro-CT scanner, which captured thousands of X-ray slices without damaging the animals. Specialized software then reconstructed the slices into detailed 3D models, revealing every muscle, gland, sperm conduit, and chitinous plate in its natural position.
The combined imaging answered a question that had vexed researchers for generations: which part of the male gonopod actually enters the female, and where exactly does it go? The 3D models showed the tiny fleshy lobe at the gonopod tip sliding into the vulva, with the claws engaging the surrounding ridges—a lock turning in a key.
Perhaps the most surprising finding concerned the gooey secretion that seals the female’s vulvae shut after mating. Researchers had long assumed the male produced this sealant to prevent rivals from mating with the female. The CT scans told a different story: glands inside the female’s own vulvae appear to produce most, if not all, of the secretion. “I always thought it came from the male, because I thought the male wanted to seal off the female so that she couldn’t mate again,” Sierwald said. “But now, having seen the glands inside the female’s vulvae through the CT-scanning, I think most of that secretion comes from the female.” Whether she seals herself to protect her vulvae or to preserve the sperm remains an open question.
The Step-by-Step Millipede Mating Process
While the specifics vary between species, the general sequence of millipede mating follows several distinct stages that researchers have now documented in detail.
1. Courtship and Positioning
In many species, the male initiates mating by walking along the female’s back, stimulating her with rhythmic leg movements. He may also use his antennae to tap or stroke her—a behavior known as antennating. If the female is receptive, she raises her front segments, allowing the male to entwine his body around hers. Depending on the species, the pair may position themselves face-to-face, in parallel, or with the male coiled around the female.
Not all encounters go smoothly. In some millipede groups, researchers have documented precopulatory struggles in which the female coils tightly (her standard antipredator response) while the male attempts to uncoil her through persistent coercion.
2. Sperm Loading
Before copulation can begin, the male must transfer sperm from his gonopores (near the second pair of legs) to his gonopods (on the seventh body segment). He accomplishes this by curling his body forward—a contortion that brings the two distant body segments close enough for the spermatophore to be deposited onto the gonopods. Once loaded, the gonopods are ready for insertion.
3. Copulation and the Lock-and-Key Mechanism
With his gonopods charged with the blue-ish ejaculate, the male positions his seventh body segment against the female’s second segment, aligning his gonopods with her vulvae. The tiny claws on the gonopod tips hook into the ridges surrounding each vulva, locking the pair together. Sperm transfer then occurs through a microscopic seminal canal running through the core of each gonopod—a channel whose topology had remained unresolved for nearly 80 years until the 2020 study mapped it in 3D.
Copulation duration varies dramatically. The Australian millipede Cladethosoma clarum wraps things up in about 4.7 minutes. At the other extreme, the South African forest millipede Centrobolus inscriptus can remain locked together for an average of 170 minutes—nearly three hours.
4. Post-Mating: Sealing and Storage
After the pair separates, the female’s vulvae become sealed with a viscous secretion. As the CT scans revealed, this sealant likely comes from glands within the female herself. Trapped inside her spermathecae, the sperm can remain viable for weeks or even months—throughout an entire breeding season in some species. When conditions are right, she lays her eggs, and they become coated with the stored sperm on their way out of her body, completing external fertilization.
Millipede vs. Centipede Mating: What’s the Difference?
Millipedes are ancient animals—the oldest known bug fossil is a 425-million-year-old millipede ancestor discovered in Scotland. Millipedes and centipedes may look similar at a glance—both are long, segmented, and generously legged—but their approaches to reproduction could hardly be more different. Here is how they compare:
| Feature | Millipedes | Centipedes |
|---|---|---|
| Sperm transfer method | Direct copulation via gonopods (modified legs) | Indirect: male deposits a spermatophore on the ground or a web; female picks it up |
| Male reproductive organs | Gonopods on 7th body segment; testes near 2nd pair of legs | No specialized transfer limbs; male spins a small silk web to hold the spermatophore |
| Courtship behavior | Male walks along female’s back, antennates, rhythmic leg tapping | Male performs a courtship dance; may tap female with antennae; deposits spermatophore and guides female over it |
| Mating position | Face-to-face, in parallel, or male coiled around female | No physical coupling; female walks over the spermatophore and collects it |
| Egg fertilization | External—eggs coated with stored sperm upon laying | Internal—sperm stored and eggs fertilized inside the female |
| Egg-laying behavior | Female builds a nest in soil or decaying matter; may guard eggs | Female lays eggs individually or in clusters in soil; some species guard their young |
| Parthenogenesis | Documented in several species (e.g., Poratia genus) | Rare but reported in a few species |
The fundamental difference comes down to direct versus indirect sperm transfer. Centipedes, despite their more predatory and aggressive lifestyle, have a more hands-off mating strategy—the male leaves a packet of sperm and hopes the female finds it. Millipedes, the gentle detritivores of the arthropod world, engage in prolonged physical copulation with species-specific anatomical hardware.
Not All Millipedes Mate the Same Way: Diversity Across 16 Orders
With 16 recognized orders comprising over 13,000 described species—and scientists estimating at least that many remain undiscovered—millipede mating strategies are far from uniform. Different lineages have evolved dramatically different approaches, and the variation tells a story of millions of years of evolutionary experimentation.
Bristle Millipedes (Order Polyxenida): The Web-Spinners
The most primitive living millipedes, bristle millipedes do not use gonopods at all. Instead, males spin delicate silk-like webs using specialized glands, deposit spermatophores onto these webs, and then leave. Females later walk over the webs and collect the sperm packets on their own. This indirect method is thought to represent the ancestral condition for millipedes—and it is strikingly similar to how many centipedes and some primitive insects still mate today.
Pill Millipedes (Superorder Pentazonia): The Rear-Grabbers
Pill millipedes—the ones that roll into perfect spheres like armadillos—lack true gonopods entirely. Instead, males possess enlarged appendages at the rear of their bodies called telopods. These function less as sperm-transfer organs and more as grasping devices, allowing the male to hold the female firmly during mating. Sperm transfer in this group is less well understood, but the telopods appear to play a role in positioning rather than insertion.
The Vast Majority (Subterclass Helminthomorpha): Gonopod Specialists
The overwhelming majority of millipede species—including the familiar cylindrical types found in gardens and forests worldwide—belong to the Helminthomorpha. These species all use gonopods on the seventh body segment for sperm transfer, but the form those gonopods take is astonishingly varied:
- Callipodida species use a single pair of gonopods (leg pair 8), with leg pair 9 reduced.
- Chordeumatida species use two pairs of gonopods, with both leg pairs 8 and 9 modified.
- Polydesmida (the largest order, including Pseudopolydesmus) have gonopods that are often elaborately sculpted with spines, scoops, and hooks—structures that may function in sperm competition, rival displacement, or female stimulation.
Sperm Competition: Scooping Out the Rivals
In some South African millipedes of the genus Centrobolus, the gonopods have evolved into literal scooping tools. Research published in New Scientist revealed that males of Orphorporus pyrhocephalus possess gonopods with spoon-like features and spiny regions designed to remove sperm deposited by previous males before depositing their own. The displaced sperm collects on the spines of the gonopod while the male’s fresh ejaculate takes its place in the female’s spermathecae.
Radiolabel tracer studies on Centrobolus species have confirmed that males engage in self-sperm displacement—each mating partially replaces the sperm from previous partners. Females, for their part, appear to exercise cryptic choice by differentially retaining or dumping sperm from different males over the 24 hours following mating.
Parthenogenesis: When Females Go It Alone
A small number of millipede species have dispensed with males entirely. Species in the genus Poratia, for example, reproduce through thelytokous parthenogenesis—females produce diploid daughters without fertilization. Intriguingly, research published in African Entomology found that these parthenogenetic populations are free of Wolbachia, the bacterial symbiont that induces parthenogenesis in many other arthropods, suggesting millipedes evolved this reproductive strategy independently.
From Eggs to Hatchlings: The Millipede Life Cycle
Once mating is complete and sperm is safely stored, the female millipede turns her attention to egg-laying.
She burrows into soil or decaying organic matter and constructs a small nest chamber, often using her own fecal material and soil particles to form the walls. Depending on the species, a single female can lay anywhere from a few dozen to as many as 2,000 eggs in a single clutch.
As the eggs pass out of her body, they are coated with sperm from the spermathecae, achieving external fertilization. The female may guard the nest—keepers of captive giant African millipedes (Archispirostreptus gigas) report females remaining with their eggs until hatching—though this level of parental care is not universal across all species.
When the eggs hatch, the young millipedes emerge with only three pairs of legs—a far cry from the dozens they will eventually possess. With each successive molt, they add new body segments and leg pairs, gradually growing into their adult form. This process can take months to years, depending on the species. Giant African millipedes, for example, live 7 to 10 years in captivity, making them one of the longer-lived invertebrate pets. For those interested in keeping these creatures, millipedes are valuable additions to naturalistic terrariums, where they serve as living decomposers.
In temperate regions, millipede mating typically occurs during warmer, wetter months, though in tropical species, reproduction can take place year-round. Some species time their reproductive cycle to coincide with seasonal leaf litter accumulation, ensuring an abundant food supply for the next generation of these essential decomposers.
Frequently Asked Questions
How can you tell if millipedes are mating?
Mating millipedes are typically seen entwined together, often with the male walking along the female’s back or coiled around her body. If you look closely, you may see the pair positioned so that the male’s seventh body segment aligns with the female’s second segment—the point where gonopods meet vulvae. The pair may remain locked together for anywhere from a few minutes to several hours, depending on the species.
What does millipede mating look like?
It resembles two millipedes intertwined, sometimes face-to-face, sometimes lying parallel, and sometimes with the male coiled around the female in a spiral. Under UV light—though you are unlikely to have one handy in the garden—their genital tissues glow, revealing the precise point of connection between the male’s gonopods and the female’s vulvae.
Do millipedes squirt?
Not in the way some animals do. Male millipedes release a blue-ish ejaculate from their gonopores near the second pair of legs, then transfer it to their gonopods on the seventh body segment. The sperm is then inserted directly into the female’s vulvae in a controlled transfer rather than being squirted or sprayed. This question likely arises from the observation that the ejaculate is a visible blue liquid, which is unusual among arthropods.
Do millipedes have genders?
Yes, the vast majority of millipede species have distinct males and females. Males can be identified by the presence of gonopods—modified legs on the seventh body segment that look different from normal walking legs. In some species, males also have a gap or saddle-shaped depression where the gonopods are located. Females lack gonopods and have vulvae on the underside between their second pair of legs. A small number of species are parthenogenetic, meaning populations consist entirely of females that reproduce without males.
How long do millipedes mate for?
It depends heavily on the species. Some, like the Australian Cladethosoma clarum, complete copulation in under 5 minutes. Others, like the South African Centrobolus inscriptus, average around 170 minutes (nearly 3 hours) per mating session. The duration appears linked to mating strategy: species with prolonged copulation often involve sperm competition and complex post-copulatory behaviors.
How often do millipedes reproduce?
Most millipede species reproduce once per year, typically during a defined breeding season triggered by temperature and humidity cues. In tropical environments, some species may breed multiple times per year. Females of many species can store sperm for months, meaning a single mating can fertilize multiple clutches of eggs laid over an extended period.
Do millipedes lay eggs in houses?
Millipedes rarely lay eggs inside homes. They require moist soil, decaying leaf litter, or rotting wood to construct their nests—conditions rarely found in the dry indoor environment of a typical house. If millipedes are entering your home, they are usually seeking shelter from extreme weather or accidentally wandering in, not looking to breed. Addressing moisture issues, sealing cracks, and removing leaf litter from around the foundation are effective ways to prevent millipede intrusions.
Conclusion
Millipede mating, hidden for decades beneath a forest of legs and a shroud of decaying leaves, turns out to be one of the more fascinating reproductive systems in the animal kingdom. From the male’s awkward contortion to load his distant gonopods with blue ejaculate, to the female’s internal sperm-storage chambers that let her choose precisely when to fertilize her eggs, to the elaborate lock-and-key genitalia that differ so dramatically between species that taxonomists use them to tell millipedes apart—every step of the process reflects millions of years of evolutionary fine-tuning.
The 2020 imaging breakthrough solved the central mechanical mystery, but as Sierwald herself notes, there are 16 orders of millipedes, and “for most of them, we have only faint ideas what the vulvae look like.” The story of how millipedes mate is, in many ways, just getting started.
