Why do different species of millipedes have different coloured glowing genitalia?

An image to illustrate millipedes which have glowing genitalia
P. canadensis millipede glowing under blacklight; under normal light, they're brown. © Stephanie Ware, Field Museum

Researchers have discovered that almost identical millipedes species have different coloured glowing genitalia that can be seen under ultraviolet light.

The new study has investigated why different species have glowing genitalia in different colours and tries to explain the reproductive habits of male millipedes.

Millipedes

Some interesting facts about millipedes include:

  • They are arthropods, relatives of centipedes and distant cousins of spiders and insects;
  • While they have a lot of legs, they have nowhere near the thousand legs that is implied by their name. Some of the millipedes in this study have 70 legs;
  • Male millipedes’ sperm comes out from an opening behind their second pair of legs; and
  • Male millipedes have a seventh pair of legs called gonopods which are adapted to transfer sperm to a female millipede’s vulvae.

Petra Sierwald, an associate curator at the Field and the lead author of a paper describing the findings in the Zoological Journal of the Linnean Society, said: “This paper is reviewing and mopping up and synthesizing what we know about these species of North American millipedes.”

Why does a millipede need glowing genitals?

Sierwald explains: “We think, he dips his gonopods into his ejaculate, which is kind of a blue-ish liquid, and then he goes in search of a female, walking around with his sperm on his gonopods. The males’ gonopods have all these special features, little knobs and things. Some of them look like the bristles on a toothbrush. And the differences between the different gonopods are even more vivid under UV light.”

She added: “Millipedes don’t see well at all, they don’t see images. I don’t know if they see any color. I think the fluorescence is an artifact or byproduct of the cuticle, the covering of the gonopod. Maybe it makes the cuticle stronger, but that is something we don’t really understand.”

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