During a brief visit to south Florida last weekend, I found interesting tropical millipedes. The biggest specimens were about 50 mm in length.
At night, especially after a splash of warm Floridian rain, these millipedes were abundant on soil and stones in the humid, lushly aromatic darkness. Using the headlamp that I seldom travel without, I found them all over a luxuriant garden in Boca Raton.
Although I cannot safely conjecture on the exact species, these millipedes are in the family Rhinocricidae (Diplopoda: Spirobolida) and belong to either of two similar Caribbean genera, Anadenobolus and Eurhinocricus. Two or more of these species, at least one from each genus, were recently introduced from the Caribbean to the very southern tip of Florida and have been swiftly expanding their range in the region. First recorded in the Florida Keys and Miami-Dade County several years ago, rhinocricid millipedes have pushed their way northward through Broward County and, clearly, have now reached Boca Raton in Palm Beach County. When I visited the same locality three years ago, I did not find any of these millipedes. This time, however, they were extraordinarily numerous, providing some idea of how rapidly these millipedes are colonizing the area.
Of course, invasive millipedes pose no problem for anybody. In fact, you really can't have too many millipedes in your ecosystem, where (like earthworms) they feed on decaying vegetation at ground level and are therefore important for nutrient cycles. As they feed, millipedes fragment plant detritus and open it up for the microbial decomposition that recycles nutrients from vegetation back into the soil. In subtropical and tropical forests, where earthworms are not plentiful, millipedes take their place as the dominant soil-forming decomposers.
Unlike many spirobolid millipedes, which tend to be dark, these rhinocricid species are notable for their yellow and black banding.
This vivid coloration is almost certainly an instance of aposematism: memorable bright colors that often evolve in chemically defended species because they warn predators that the potential prey is unpalatable. This cautionary visual signal can deter a predator attack pre-emptively, before injuries can be inflicted. (Stinging wasps famously bear similar yellow and black types of aposematic coloration.) In millipedes, chemical defenses consist of bad-tasting glandular secretions, typically benzoquinone compounds, that are released through tiny lateral pores when the millipede is disturbed. Benzoquinone secretions have been experimentally shown to repel many predatory arthropods, along with mammals and birds.
Obviously, however, the value of aposematic coloration, a strictly visual cue, depends entirely on light. The presence of aposematism is therefore puzzling in nocturnal animals such as millipedes—which, moreover, tend to occur in dark microhabitats under soil and leaf-litter. Indeed, despite their chemical defenses, many millipedes are not brightly colored. Why are Caribbean rhinocricid millipedes so vividly aposematic? The simple answer may be that they are not entirely nocturnal. During the early mornings of my stay in Florida, I noticed that some of the millipedes continued to move about on the surface for a few hours after dawn. The extension of foraging time into daylight hours may be beneficial to the millipedes, especially in the favorable damp conditions of the morning, before the tropical sun has burned off much of the epigean moisture. This behavior would explain the evolution of a reflexive aposematic signal to deter diurnal predators, especially birds.