Millipede sex was already pretty weird before the extra genitals
Mammals’ internal and external sexual gear are located in similar spots, and connected with a series of tubes. In Julid millipedes, both male and female millipede gonads (ovaries and testes) are internal, on their third ring. That’s a bit like having your ovaries on your sternum.
Female external genitals are also found on the 3rd ring, but are housed in a sealed box. The male has to push open the lid of her box to deposit his sperm. He does this with special sexual tackle called gonopods (“sex legs”). These gonopods replace his 8th and 9th pairs of legs. They aren’t penises; they are sort of a sperm-carrying strap on.
The testes are not connected to these sexy legs; and to transfer the sperm, millipedes have to…. well, go fuck themselves. Males contort and curl around to insert their gonopods into a tiny pore between their second pair of legs. The gonopods are hollow, and suction up the sperm. Now the male is ready for love.
Normal male Ommatoiulus moreleti, the species in question, have 4 sex legs. Like everything else on a millipede, they are in pairs. But even for millipedes, 16 sexual appendages is rather a lot.
Silencing of the Genes
A special class of genes have “control” functions; they are commonly known as Hox genes. They are the genes that help instruct a developing embryo about segment identity: “mouth goes here, anus goes there.” Hox genes work with other regulatory genes to tell developing cells which of all their potential DNA choices to activate. If it all works correctly, you don’t end up with hairy eyeballs.
Our well-endowed millipede is a “homeotic mutant.” Somehow, the instructions to make genitals did not get turned off, and the millipede just kept replacing its legs with sexual appendages. Sixteen of them. Did I mention there were 16 of them?
It’s Genitals all the Way Down
To add even more complexity into the story, when millipedes hatch out of their eggs, they do not have functional gonads or sexual appendages. They have their first three pairs of legs, and no more. As millipedes grow and shed their exoskeletons, they add “diplosegments” with two sets of legs on each ring. Eventually, around the 4th or 5th time they shed, their genitals appear.
Even stranger? Male millipedes of some species are able to reverse puberty. Adult males with functional gonopods can actually revert back to their pre-sexual appendage stage with a molt.
Programming for the location and construction of future genitals is set early in embryo development. In the diagram below, the authors suggested a mechanism for how extra genitals might arise in a young millipede. The black circles represent cell groups destined for future genital building; the grey circles are regular segments.
The monster’s extra genitals were displaced by exactly 16 units, which could be produced by 4 cycles of splitting in an embryonic primary segment:
There is a long history of mutants in millipedes; we just haven’t had genetic tools to document what’s going on genetically until fairly recently. Researchers are just beginning to pick apart millipede genetics; for most millipedes, we don’t know much about their genomes. Some very exciting discoveries await.
Drago et al. 2008. Non-systemic metamorphosis in male millipede appendages: long delayed, reversible effect of an early localized positional marker? Frontiers in Zoology 2008, 5:5. doi:10.1186/1742-9994-5-5
Akkari et al. 2014. Segmentation of the millipede trunk as suggested by a homeotic mutant with six extra pairs of gonopods. Frontiers in Zoology 2014, 11:6. doi:10.1186/1742-9994-11-6