Elongated, curving form of the female body morphing into the dragonfly tail.
Curved surface of the dragonfly wings.
WHY
Curving form ensures that the brooch curves to the contours of a woman's chest, making it more comfortable to wear as it shapes to the body. Also makes it more aesthetically pleasing, as it wouldn't jut out at awkward angles.
Fact that dragonfly wings are hinged would allow them to be moved to fit well with the position of the body.