14 years ago
Their genitalia are on their 3rd body segment, and the males have their 1st pair of legs on their 7th body segment modified into a special pair of clasping organs, they often carry these held up close to the body and this allows you to tell what sex your Millipede is.
To sex a millipede, the best way I have found is to place them in a clear glass bowl, curved preferably. This will let you get a very good view of their undersides, as at some point they will try to climb up out of the bowl, causing their body to curve backwards. This stretches the gap between each pair of legs, making their gender easier to identify.
Another, but far less certain, method is to hold them by their rear ends. whilst some will not, most millipedes held like this will stretch out straight, trying to find a surface to get hold of. Generally, this will give you a good view of their undersides; however, they tend to swivel around and twist a bit, usually away from your view!
Now, what you're actually looking for whilst you attempt the above. Male millipedes have, on their seventh segment, a modified pair of legs; these form the gonopods, part of their reproductive system. In some males, all you will see is a gap on this segment (hidden gonopods), with no legs of any sort, whereas in others, you will see a pair of shiny, sometimes off-coloured stumps (visible gonopods). Both serve the same purpose.
On female millipedes, same as with humans, everything is held neatly inside the body, so all you will see is a uniform line of pairs of legs; the two pairs are on the seventh segment as usual, unmodified.
Young millipedes may be difficult, but not impossible to determine gender.