Oxidus

While this genus only has eleven species, one of its members is responsible for arguably the most sightings of millipedes anywhere in the world, the ubiquitous pest species O. gracilis. Oxidus are native only in Asia, with many species calling Japan home, though O. gracilis has spread where ever organic matter has been transported by humans. One species, Oxidus sontus, is supposedly from California but the only record of the species is from 1910 in its original description; while research is ongoing by The MED Team, it is here speculated that O. sontus might actually be O. gracilis that was accidentally described again, as the presence of a native Oxidus is circumspect for the area.

Oxidus gracilis, the only species kept commonly in captivity, is the epitome of the proverbial pest species, even more so than other widespread adventives such as Asiomorpha coarctata; one contributor kept a few adults found in a garden on compost soil in a 2.5 ounce deli cup with successful breeding; in laboratories specimens have even been raised on 1% agar with a diet composed strictly of Ficus sycamore leaves (2). Aside from not poisoning specimens, letting them dry out, or keeping them on gravel this species is about as no-frills as one can get when it comes to caring for millipedes. Specimens can commonly be found in and under potted plants at large home supply stores or nurseries; a guide to differentiating this species from the similar-looking A. coarctata can be found below.

O. gracilis is usually a brownish coloration with tannish edges on the rectangular segments, with the paranota rounded off somewhat. Legs and antenna are a light brown to tan. This species generally does not grow over 0.9"/2.2 cm.

Wild observation of Oxidus gracilis, by Connor Smotzer

At a park in Pennington, New Jersey under a cut log of Quercus rubrum there were 2-5 individual polydesmids. I was not sure of the ID as O. gracilis until I compared photographs of a species I had photographed the week prior. The conditions were following heavy rain from Hurricane Ida, the day/night before, about 70F°/21.1C°, mid day between 12:00-1:00 PM. The soil was pretty well saturated, but it was not noticeably humid.

The individuals were pretty small, around one, at most, one and a half inches in length, and a darker brown coloration and smaller in length and width than Asiomorpha coarctata. The individuals were not very responsive to my activity uncovering them and photographing them, unlike a decent percentage of Asiomorpha coarctata which fled at first disturbance. My interaction with this species was minimal, and I was unable to observe any population data over the course of the rest of the day, and I could not find any other Oxidus gracilis after the first encounter.

Photo compliments of Connor Smotzer.

Comparison between Asiomorpha coarctata and Oxidus gracilis, by Connor Smotzer

For me at first glance the differences between these two species as far as identification with the naked eye is concerned are quite cryptic, and I can see why they are easily confused. But upon reviewing macro photography of the two there is a clear difference in the morphology of the paranota, or the extending lateral metatergite margins, in particular the size and shape, between the two species. With the specimens I photographed differences were visible in nearly all segments after the first two but other observations say it is clearest after segments 8-9. On Asiomorpha coarctata the metatergite paranota are wider, winged, and pointed posteriorly, some extending past the segments themselves. On Oxidus gracilis the paranota are smaller, rectangular in general shape, and rounded both anteriorly and posteriorly. Note the differences in the below photo.

Sierwald, P.; Spelda, J. (2021). MilliBase. Oxidus Cook, 1911. Accessed at: https://millibase.org/aphia.php?p=taxdetails&id=892210 on 2021-12-24
(2021). Breeding Oxidus gracilis for research. Accessed at: https://arachnoboards.com/threads/breeding-oxidus-gracilis-for-research.349312/#post-3224041 on 2021-12-24

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