What are the circles/holes ahead of the wing of an MD-82?
The three large circles indicated in the question are static port plates. These measure the static pressure of the surrounding air, which is used to measure airspeed and altitude.


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Thanks to a link from Jonathan Walters, here is a closeup of one:



The four smaller areas in a row along the centerline are the radar altimeter transmitters/receivers. The radar altimeter uses radio waves to determine the aircraft's altitude above the ground. This diagram for the A345 has them further aft, they're in similar configurations on other aircraft.


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