Horseshoe bats (genus Rhinolophus) emit sounds from their nostrils, then listen to the frequency of the sound reflected from their prey to determine the prey's speed. (The "horseshoe" that gives the bat its name is a depression around the nostrils that acts like a focusing mirror, so that the bat emits sound in a narrow beam like a flash-light.) A Rhinolophus flying at speed v bat emits sound of frequency f bat ; the sound it hears reflected from an insect flying toward it has a higher frequency f refl .