Nematodes (phylum Nematoda), or “roundworms,” are among
the most ubiquitous organisms on Earth and occupy nearly every biological niche (Maggenti, 1981). Nematodes inhabiting the intestines of
millipedes pose no risk to humans. Most nematodes are beneficial to
humans and other organisms. They promote nutrient recycling and
are important for the overall health of the environment. Given the ecological and environmental importance of nematodes, what exactly are
they? Nematodes are bilaterally symmetrical, have a complete digestive tract (mouth, intestinal tract, and anus), contain only longitudinal
muscles, and have a cuticle consisting primarily of collagen. Except for
their lack of respiratory and circulatory systems, nematodes have the
same organ systems as vertebrates. The presence of a vulva and eggs
readily identifies females; males are generally smaller and usually have
spicules (mating organs). Males are often rare or absent in millipede
intestines; therefore, for this inquiry, they can be ignored. Figure 1
shows the generalized morphology of a commensal nematode typically found in North American millipedes.
Key to Females of Common
Nematodes Found in Millipede
A user-friendly key to identify common millipede intestinal nematodes is as follows, and a video demonstration can be seen at https://
1. First annule cap-like, much longer than next annule; basal
bulb without grinding valve; amphid apertures on small horns
protruding from anterior end (Figure 2A) . . . Coronostoma
1′. First annule variable but not cap-like, width usually similar
to next annule; basal bulb with grinding valve; amphid apertures not on horns . . . 2
2. Esophagus with two parts: thick or thin procorpus and subspherical basal bulb with a grinding valve, the two parts not
separated by slender isthmus (Figure 2B–E) . . . 3
2′. Esophagus with three parts: enlarged procorpus, slender
isthmus, and basal bulb (Figure 2F, G) . . . 6
3. Esophagus with thick, muscular procorpus and basal bulb
(Figure 2B) . . . Rhigonema
3′. Esophagus with long, slender procorpus and basal bulb . . . 4
4. Anterior end of female highly ornate with cuticular collar
spines, head with deep grooves lined with rows of scale-like
structures (Figure 2C) . . . Heth
4′. Anterior end of females without spiny ornamentation . . . 5
5. Head end tapering, rounded or flat anteriorly (Figure 2D) . . .
5′. Head end mushroom-like, first annule wider than second
annule (Figure 2E) . . . Stauratostoma
6. Isthmus longer than basal bulb (Figure 2F) . . . Aoruroides
6′. Isthmus shorter than basal bulb (Figure 2G) . . . Aorurus
• Define commensalism.
• Explain why millipedes are good organisms to study.
• Identify taxonomic groups of millipedes and nematodes.
• Prepare a data sheet (e.g., in Microsoft Excel) showing total
nematode loads, taxonomic grouping (species), and the frequency of species.
Figure 1. Nematode morphology.