Flies have a mobile head with large compound eyes on the sides of the head, and three small ocelli on the top. For visual course control, flies’ optic flow field is analysed by a set of motion-sensitive neurons. A subset of these neurons is thought to be involved in using the optic flow to estimate the parameters of self-motion, such as yaw, roll, and sideward translation. Other neurons are thought to be involved in analysing the content of the visual scene itself, such as separating figures from the ground using motion parallax. The H1 neuron is responsible for detecting horizontal motion across the entire visual field of the fly, allowing the fly to generate and guide stabilizing motor corrections mid-flight with respect to yaw. The antennae take a variety of forms, but are often short, which reduces drag while flying.