I have two millipedes now and they look like this

 

Long story short, I was at the Los Angeles County Arboretum looking for accidentally imported Marchantiophyta in the greenhouse and impulsively caught these pyrgodesmids(?) instead (I could not find any marchantiophytes). Or maybe they're Haplodesmidae.

I'm not really even interested in millipede husbandry any more! How does one get overexcited enough to impulse-acquire things one doesn't even slightly want? What's wrong with me?

Anyways they've been doing well in potting mix but seem annoyingly specific about what potting mix they consider acceptable. Even microwaving the one they like (and then  reinoculating the clean substrate with their presumably favored microflora) seems to induce tantrums I can only seem to stop by putting them back in the unsanitized dirt.

Update: Derek Hennen says it looks haplodesmid.