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Articles cies below 10 kHz or above 200 kHz. Higher frequencies improve resolution, but they attenuate at a greater rate (Surlykke 1988) and the detection distance is reduced accordingly. The source level is the sound pressure level (SPL relative to 20 µPa), in decibels (dB), measured 10 cm in front of the bat’s mouth. A bat using a source level of 110 dB at 20 kHz could detect the echo from an object the size of a moth at more than 5 m. Detection would occur at no more than 2.4 m if the bat used 100 kHz (Surlykke 1988). From the insects’ perspective, bats advertise their presence with the ultrasonic pulses used to stroboscopically probe the environment. Thus, insects are forewarned if they can hear ultrasound. This coincidentally exerts considerable selection pressure against those insects that either cannot hear or do not react (Miller 1982). Thus, the stage was set in the Eocene for an evolutionary escalation …