Behind the thorax of this crane fly are two knobs on stalks. They are the halteres.

Halteres are small knobbed structures modified from the hind wings in flies. They vibrate during flight, and help the insect fly better.

The halteres evolved from wings. Ancestral flying insects had two pairs of wings, like dragonflies, hymenoptera and lepidoptera still do. Flies have only the front pair of wings.