Why does a blowfly maggot swing its head while moving away from light?

When a blowfly maggot moves away from a light source, its head-swinging behavior is a form of stereotyped response known as klinotaxis.1

Because the maggot possesses only a single intensity receptor located on its head, it cannot make simultaneous comparisons of light intensity on both sides of its body. Instead, it achieves orientation by swinging its head alternately to the left and right as it crawls. By comparing the light intensity between these successive positions, the maggot is able to determine the direction of the light and adjust its path to move away from it. This trial-and-error mechanism allows the organism to orient itself effectively despite its limited sensory apparatus.1