True flies are insects of the order Diptera. "Diptera" is from the Greek di = two, and ptera = wings. Insects of this order fly using only the two forewings. The two hindwings are reduced to clublike sense organs called halteres. Diptera is a large order containing an estimated 1,000,000 species including horseflies,[a] craneflies, hoverflies and others, although only about 150,000 species have been described. It is one of the major insect orders with ecological and human importance.