large black bug with brown legs and very long feelers on either side of the body. flat bodied. crawls sideways and very fast. Wickenburg, AZ
Your photo is of a whipscorpion. Unlike scorpions, the tail cannot sting, but is used as a sensory organ.
These arachnids are carnivorous, nocturnal hunters feeding mostly on insects and millipedes, but sometimes on worms and slugs. They are beneficial because they eat roaches and crickets. They love dark damp places.