Butterfly shaped corolla is the characteristic feature of the family

Corolla is made of petals. It is brightly colored part of the flower to attract insects for pollination. Corolla is often differently colored, differently shaped part of the different types of flower. 
Mimosaceae family flower has very small corolla with 5 petals. It is not the attractive part of the flower in this case.
Caesalpinaceae family flower shows corolla with 5 petals in overlapping imbricate aestivation. It does not form butterfly shaped corolla. 
Orchidaceae family flowers show 3 dissimilar petals, of which lower petal is called as lip petal. Lip petal is modified variously.
Papilionaceae flower shows papilionaceous corolla. It contains 5, free petals as odd, largest, posterior petal called as standard or vexillum, two lateral petals called as wing and two innermost, anterior ones are united to form a bat shaped structure called as keel. This form a butterfly shaped corolla.