There are two described Demodex species that are parasites of humans: D. folliculorum and D. brevis. They are characteristically differentiated by their location on the host and the length of their body. Demodex folliculorum lives in hair follicles and has a long opisthosoma (abdomen/posterior end), whereas D. brevis lives in sebaceous glands attached to hair follicles and has a shorter opisthosoma. Molecular analysis (e.g. using 16S rDNA) can also be used to differentiate the two species, and generally correlate with the morphology-based identifications.
Here is a general overview of the Demodex body segments:
Now here's a poem by Blaine:
So many people think they live immaculate lives
but among their hair follicles another creature thrives
you can shower and scrub to your heart's content
but all that effort won't make a dent
because the mite Demodex folliculorum continues to survive