This is a piece written (by a pupil of Claude Debussy’s) for woodwind, percussion, bass vocal and two theremin cellos – an instrument similar to a theremin, and invented by Leo Teremin, but built like a cello with a neck and bow. Most of the theremin cellos ever made were built for performances of this piece as far as I’ve read (there were only 10), and later in life Varese rescored it for another theremin-like instrument, the ondes martenot.
I first heard about the piece (and the theremin cello) in this podcast episode about Frank Zappa, who counted Varese as one of his primary influences.