Most insects have 4 wings except true flies (Order Diptera = Two wings) that have only two wings. True flies include house flies, mosquitoes, horse flies, gnats, midges, crane flies and many many other flies. The visible two wings of true flies are the front pair of wings; their rear pair of wings becomes what we called halteres, the knobbed organs which act as gyroscopic stabilizers during flight.
---Chen Young