then moved their arms backward so that the angle created between their fingertips and spine was 90 degrees, this would be a 90-degree offset. The halteres of flies work the same way. They are positioned behind their bodies, forming a 90-degree angle between the haltere bulbs and the center of their thorax. It is necessary for the halteres to be positioned like this in order to detect the three axes of motion. Those axes are yaw, pitch and roll, as illustrated in the above figure (Directions of rotation). The mechanoreceptors at the base of the halteres are only able to measure force in two directions (horizontal and vertical), so a single haltere is only able to measure rotations along two of the three axes. Because the halteres are set at different angles (90-degree offset), they also beat along two separate horizontal and vertical axes. This gives them the ability to acquire information from two non-parallel planes and allows sensation of rotation in all three directions. However, flies are most sensitive to pitch. Neurophysiology When halteres are experimentally induced to flap, volleys of action potentials within the haltere nerve occur in synchrony with the haltere-beat frequency. When flies are then rotated, these volleys break down, likely in response to different groups of sensilla being activated to inform the fly of its recently changed body position. Haltere afferents have also been shown to terminate in the mesothoracic neuropil where flight muscle neurons are located. Haltere afferent activity responding to rotations and wing steering behavior converge in this processing region. The haltere nerve Sensory inputs from five sensory fields located at the base of the haltere all converge onto one nerve, the haltere nerve. How these sensory fields are organized at the level of the central nervous system is currently unknown. It has been determined that those five sensory fields all project to the thorax in a "region-specific" way and afferents originating from the forewing were also shown to converge in the same regions. Not every specific target for the haltere afferents have been determined, but a few connections between motor neurons known to be involved in wing steering control and particular haltere sensory fields have been identified, particularly one synapse between the haltere nerve and a wing steering motor neuron known as mnb1. Muscles Flies use indirect flight muscles to accomplish wing movement, and the beating haltere movements are driven by the