The other solution to this problem was the development of a system of biological sonar called echolocation as illustrated in the figure below:
The bat emits a loud sound (represented by the orange lines) that travels out into the environment until it hits something (such as the insect in this picture). When the sound hits, some is reflected back as an echo (the gray lines in the figure). The bat can hear the echo and use that to tell a lot about the obstacle, such as how far away it is and some fine details such as its size and shape. This allows many of these bats to capture flying insects by hunting them down like a radar-guided missile. These types of bats (the Microchiroptera) tend to have large ears and small eyes. While vision is not as important to these bats as it is to the Megachiroptera, these bats can see, so the saying "blind as a bat" is a myth!
Luckily for us, the sounds these bats make are generally not within the range of human hearing. Humans can typically hear sounds up to about 20,000 Hertz, (Hz) while most bat echolocation sounds are 30,000 Hz or more, and some bats go as high as 200,000 Hz. Bats are not the only animals to use such ultrasound, as it is found in many rodents in addition to some birds and frogs. However, bats seem to be the masters of using their sonar system to navigate and capture prey.