SOIL GUARDIANS
Stratiolaelaps scimitus 
(formally Hypoaspis miles)
 
This mite is fantastic against many pest species that affect crops, houseplants, and terrariums. Observed as effective against snake mites, spider mites, flour mites, fungus gnat, shore fly, black vine weevil, rose midge, thrips, root aphids, and many other soil dwelling pests. 
When there's no suitable arthropods for them to eat they can survive on algae and pollen, and even go weeks without any food at all! 
A great beneficial invert to have in the terrarium.