American giant millipede vs Greenhouse millipede

These two are easy to mix up. Here's how to tell an American giant millipede from a greenhouse millipede, and whether either one bites or stings.

≠The key differences

Narceus is much larger (up to about 10 cm long), cylindrical and robust; O. gracilis is flattened and only about 2 cm long.

	American giant millipede	Greenhouse millipede
To humans	Irritating secretion	Non-toxic
Size	5-10 cm (50-100 mm) long	About 15-23 mm long
Colours	Dark brown, Reddish brown, Cream, Black	Light brown, Dark brown, Yellowish tan
Family	Spirobolidae	Paradoxosomatidae
Habitat	Moist terrestrial habitats: leaf litter, rotting logs, under stones, compost piles, and garden mulch; also inside basements and crawl spaces when moist	Moist leaf litter, compost, garden mulch, greenhouses, potted plant soil, under logs and stones
Range	Eastern and central North America, from the northeastern United States through the southeastern states; reports extend into parts of the Midwest and southern Canada	Cosmopolitan (introduced globally from East Asia); common in temperate and tropical greenhouses, nurseries and urban gardens worldwide
Active	Mostly nocturnal and active in warm, wet seasons; activity peaks in spring through fall in temperate regions	Active year-round in heated greenhouses and indoors; outdoors most active in wet seasons and at night or in cool, damp weather
Pest?	No	Yes

Large cylindrical body with many segments, each segment bearing two pairs of legs
Often 5-10 cm long and coils into a tight spiral when disturbed
Alternating darker dorsal coloration with lighter margins or subtle banding on segments
Short bead-like antennae and numerous short legs visible from the sides

Small flattened body 8-20 mm long with many short legs and obvious segmental ridges (paranota) along sides
Uniform pale to dark brown colour with faint segmental banding; lacks the large size and bright rings of some native millipedes
When disturbed it curls into a loose coil rather than forming a tight ball
Commonly found in damp potting soil, under bark, in compost or in greenhouses

Narceus is much larger (up to about 10 cm long), cylindrical and robust; O. gracilis is flattened and only about 2 cm long.

American giant millipede: Irritating secretion. Greenhouse millipede: Non-toxic. Not venomous or aggressive. Does not bite or sting.

Identification from photos is never certain: don't handle insects you can't identify, and get medical advice for bites, stings or allergic reactions.