Although katydids do not have external ears like humans, they do they do have tympanal organs located just below the “knee” on the insects’ front legs.  If you look closely at a katydid, you will notice there is a tiny oval hole on the upper part of each front leg.   These tympanum organs allow the insect to detect sounds.  I guess you can say the katydid does indeed have its ears on its knees.