In mature millipedes, the easiest sexing clue is usually on the underside: adult males often develop modified reproductive legs called gonopods near the front third of the body, while females keep regular walking legs in that area. Juveniles are unreliable to sex because those features are not fully developed.

The quickest practical approach is to check the underside of an adult and look for gonopods rather than relying on size, colour or body shape alone. If you need to see the animal more clearly, a gentle, low-stress view from below is best; our Millipede Behaviour, Biology & Handling Guide covers careful handling.

Where to look on the underside

Gonopods are specialised reproductive appendages, not ordinary walking legs. In many species they replace one or more pairs of legs near the front third of the body, although the exact position and appearance can vary between groups.

On a female, that same area usually looks more uniform because the normal walking legs are retained. From above, the difference can be hard to judge, so an underside view is often much more useful than a top-down glance.

In some millipedes the gonopods are obvious; in others they are tucked beneath the body and may only be seen clearly with good lighting and a careful look.

What to look for

Before trying to sex a millipede, make sure it is mature. In juveniles, the reproductive structures have not developed enough for reliable identification, so even if the animal has grown well, the sex may still be unclear.

• Modified legs near the front of the body - adult males in many groups have gonopods in place of a pair of walking legs.
• Regular walking legs in the same area - females usually show a more even leg pattern underneath.
• Underside view - this is usually the clearest angle for sexing.

Common points of confusion

The most common mistakes are using size alone, confusing front legs with gonopods, and relying on colour or body shape. None of those features is enough on its own.

Some male gonopods are short, folded or hook-like, so they can be easy to miss at first glance. A clear underside photo taken in good light can help more than trying to decide from memory or from one quick look.

If you want to understand how the reproductive structures fit into breeding behaviour, see How Do Millipedes Mate? and How Do Millipedes Reproduce?.