In most vertebrates the visual system plays a dominant role in orientation, allowing animals to orient themselves with respect to their environment and to assess the size, shape, distance, nature and movements of objects. During the past 20 years, largely through the efforts of Donald R. Griffin and co-workers, it has become increasingly apparent that the auditory system of bats is capable of performing these same functions. Indeed, laboratory experiments have shown that bats can perform remarkable feats without the aid of vision. Flying at speeds of 3-4 m/sec they can detect 1 mm diameter wires at a distance of 2 m, fly unhindered through an array of 0-2 mm diameter wires, and capture mosquitoes and fruit flies at rates of up to two/sec (Grinnell & Griffin, 1958;Griffin, Webster &;Michael, 1960). This is accomplished by echo-location, that is, the emission of ultrasonic pulses and the subsequent perception and analysis of echoes. Ordinarily the emitted pulses are frequency modulated with the fundamental frequency sweeping one octave or less downward during each cry. The frequency range usually lies between 150 and 12 kc/sec and the peak-topeak sound pressures, when measured at distances of 3-10 cm from a bat's mouth, average from 70-80 db (re-0-0002 dyne/cm2) in so-called 'whispering' bats, to values well above 100 db (re-0-0002 dyne/cm2) in loud bats (Griffin & Novick, 1955;Novick, 1958Novick, , 1962Novick, , 1963b. These and other pulse characteristics vary, not only from species to species, but within the same species depending upon the activity of the animal. Indeed, on the basis of changes in the pulse characteristics, different phases of echo-location can be recognized. In the case of Myotis hunting fruit flies, echo-location can be divided into a search phase, an approach phase and a terminal phase (Griffin et al. 1960;Griffin, 1962).During the search phase each emitted pulse has a duration of 2-3 msec, and the pulse repetition rate is about 10-20/sec. The emitted pulses appear to have peak-to-peak sound pressures of 109 db (at a distance of 10 cm from the bat's mouth) and the fundamental frequency of each pulse sweeps
