A Calm Millipede's UV Fluorescence

Let's talk about millipedes! You may have read my entry on a millipede that fluoresces under ultraviolet light I found this past fall that grabbed my attention. I've since done some more research, and after working on a poster a few weeks ago for the Ohio Natural History Conference, I have more information and motivation to summarize what I've learned.

How did I find out about these UV fluorescing millipedes in the first place? I was on a night hike with some friends last fall and had the foresight to bring a UV flashlight along, just in case there was anything neat to look at under UV. It was a bit chilly, and we weren't finding much.

Then I turned the flashlight on.

As we passed by areas with fallen leaves, I started catching glimpses of blue-green light: millipedes were fluorescing while milling about in the leaf litter. If you haven't seen this, you owe it to yourself to grab a UV flashlight and check it out. As it turns out, the millipedes I was seeing were one species: Semionellus placidus. This millipede is in the family Xystodesmidae and is about an inch and a quarter long. Many Xystodesmid millipedes are known to fluoresce under UV, and one genus contains the only bioluminescent millipedes known to exist (note that bioluminescence, when an organism produces its own light, is different from UV fluorescence).

I was intrigued by this millipede, but didn't know anything about it at that point--not even its species. I keyed it out to family with the help of an unpublished key by Bill Shear, and posted it to BugGuide, where Rowland Shelley helped me with the species. It's in the tribe Chonaphini, which is mostly restricted to the Pacific Northwest. Breaking the mold, Semionellus placidus ranges from "Minnesota and Michigan east to New York, south in the mountains through Maryland and Virginia to Fort Benning, Georgia" (Chamberlin & Hoffman). It is also described as sporadic in its range, rather than blanketing it uniformly.

 S. placidus under low UV light.

S. placidus under normal light.

I finally had my ID for this species! I started checking the literature to find out more and discovered....there's not much published on it. Oh. To make sure I wasn't missing any hidden information anywhere, I did a thorough job of Googling my enigmatic millipede, which turned up some old publications that are a little difficult to interpret. It's that 100 year gap in English, you know? My research is also made trickier by the synonyms this species has. Here's the list:

• Chonaphe michigana 
• Leptodesmus borealis 
• Leptodesmus placidus 
• Polydesmus floridus 
• Polydesmus placidus 
• Trichomorpha placida

Millipedes don't have the same PR and research dollars behind them as a group like the butterflies (big insects, pretty colors and all that jazz), so that leaves me in the position of doing the research myself. Of course, that's exciting! I have a figure of the male gonopod hanging up on a board in the lab (see below), with the main things known about the millipede written beside it in dry erase marker as motivation for delving deeper.

Figure 1 from "Some Records and Descriptions of Diplopods Chiefly in the Collection of the Academy" by Ralph V. Chamberlin. (Currently hanging up in my lab.)

Let's take a moment to talk about the above picture, Figure 1. You'll notice that number 3 is what we're interested in: that's the gonopod of Semionellus placidus. The gonopods are a pair of modified legs the male millipede uses during mating for sperm transfer to the female. These structures can be pretty elaborate and sometimes don't look like anything that ever could have been a leg. To really get a feel for what the gonopods look like, check out the three pictures below.

Millipede flipped on its back, camera looking down at the gonopods.

 Same as before, but the millipede has been turned slightly to get a side view. The gonopod on the right was overlayed from a picture with better focus.

Side view.

I've successfully kept some of these millipedes I collected from the field station alive since October, and a few females have laid some batches of eggs in their container. The eggs fluoresce under UV as well, which is pretty neat. They're laid in a soil cavity, which the mother excavated at the bottom of her plastic container about 1.5 inches down. UV fluorescence in a life stage other than the adult hasn't been reported before, so that observation might be important in figuring out why/how the millipedes fluoresce.

Next steps:
It's time to do some research! With capstone looming ever closer on the horizon, I must devote my research time to finishing that, but I'm hoping to hit the field station during the summer with a UV light to see if this millipede is active yet. I've encountered it during September and October, but so far that's it. I'm also going to work on some type of publication based on what's known, and I'm waiting for another reference to come in. If my plans work out and I'm available during the summer, I hope to try to observe the assassin bug Rhiginia cruciata's relationship with this or any other millipede to quantify its feeding habits. (Remember: R. cruciata is an assassin bug in a subfamily known to feed on millipedes.)

A dish of millipedes under UV light. Most of the ones fluorescing in blue are Semionellus placidus, while the two fluorescing red are Pseudopolydesmus serratus. Red fluorescence under UV hasn't been reported before in arthropods, to my knowledge.

References:

Chamberlin, RV. 1920. A new leptodesmoid diplopod from Louisiana. Proc. Biol. Soc. Washington, 33: 97-100. Link.
Chamberlin, RV. 1947. Some Records and Descriptions of Diplopods Chiefly in the Collection of the Academy. Proceedings of the Academy of Natural Sciences of Philadelphia, 99: 21-58.
Chamberlin, R.V. & Hoffman, R.L. 1958. Checklist of the millipeds of North America. Bulletin
of the US National Museum, 212: 1–236.
Wood, HC. 1864. Descriptions of new species of North American Polydesmidae. Proc. Acad. Nat. Sci. Philad, 16: 6-10. Link.