Among the arthropods that strongly favor moist conditions is a group of small, dark to nearly black, worm-like organisms that occasionally occur in fair numbers on the foundation of houses, on the side of stone walls, or in a masonry fire pit in front of a lean-to. These are the millipedes, a group of small, yet important, arthropods that play a vital role in our soil ecology.
Millipedes are characterized by a short, string-like body that contains numerous sets of tiny, bristle-like legs. As a rule, the first three body segments have only one set of legs, yet every segment thereafter supports two pairs. In order for a millipede to grow, it must first shed or molt its thin, external body covering, or exoskeleton. When this occurs, it typically develops another body segment and gains two additional sets of legs. This is why some millipedes have only a half dozen segments while others of the same species contain more. Since a millipede can molt several times a month when conditions are optimal, its age cannot be determined by simply counting the number of body segments as one would do with the rings of a tree. However, the longevity of an individual is indicative of the number of segments, or its body length.
It is commonly reported that the millipede derives its name from the prefix “milli”, implying that this organism has a thousand legs. There are no millipedes anywhere in the world that can develop that great a number of appendages, regardless of how favorable conditions for it have been. Also, the prefix “milli” actually means one-thousandth, as the prefix “kilo” is used to denote a thousand; so this organism should really be named the kilopede.
The response of the millipede to danger is another means that can help identify this bug. Rather than run, or attempt to seek cover, a millipede instinctively curls-up into a ball. This reduces the amount of surface area exposed to a predator, like a bird or shrew, and when seen against a background of soil, a rolled-up millipede appears like any other rounded object on the forest floor, such as a small stone, a fragment of bark or a piece of twig.
Like several other bugs, the millipede lacks a waxy cuticle, which makes this slender entity prone to dehydration. When the relative humidity drops and the breeze increases enough to circulate air across the forest floor, the millipede is forced to retreat into a nook or cranny that protects it from losing moisture or move to a spot where dampness still prevails. The soil against a basement foundation, crevices within a pile of boulders, cracks in a chunk of wood, or the space beneath a piece of decaying bark on a fallen tree all serve as moisture rich locations for the millipedes in an area to wait when the sky is clear, the sun is shining, and the humidity is low.
As the dew begins to form in the evening, the millipede leaves its shelter for any place close by that can provide it with food. The millipede is an opportunistic scavenger, consuming whatever suitable organic matter is available. Because its mouth is small and lacks the ability to chew hard items, the millipede targets matter that has already become softened with the absorption of water, has experienced some decay, and is fragile enough to come apart with the slightest disturbance. Tiny chunks of leaves that have been on the forest floor for several seasons, flakes of old bark on dead trees, plant stems, fallen seeds and berries, and the remains of dead invertebrates are all ingested. The millipede also consumes fungal threads that form in the soil and become abundant during periods when the soil becomes damp. This is another reason why the millipede thrives during those occasions when rainy weather prevails. By devouring the assortment of decomposing matter around it, the millipede performs the same general function as the earthworm by facilitating the process of decay and converting organic material on the ground into living, millipede tissues which eventually serve as food to predatory creatures.
Because this ground dweller is able to mate, lay eggs and bring many additional millipedes into the world in the span of only a few weeks, its population can quickly swell when prolonged periods of cool, damp weather settle over the region. Considering the amount of moisture that our region has received the past month, it is not unusual to spot these thin, black, caterpillar-like organisms clinging to a cement wall in a shady spot. Millipedes are not at all harmful to any living creature.
Photo: The common millipede Boraria stricta. Photo by J. Anderson.