It seems like there are insects that have their ears on their legs. Granted, they are not close relatives.
Insects vary. Some detect acoustic waves from their knees or sides, others from the chest or abdomen (the two most common), and yet others by antennae. Some can hear ultrasonic waves; others hear subsonic waves; yet others can barely hear at all. The detection mechanism can be either vibrating membranes or undulating hairs.
For some odd reason, the designer didn't simply reuse the best blueprint for insect hearing, but rather seems to have changed his mind a few dozen times. Maybe he just doesn't like bugs..