Technical Abstract: 5-HT immunoreactive neurons were detected in the CNS of the stable fly. The finding of strong innervations of the cibarial pump muscles and the foregut by 5-HT IR neurons in the feeding-related systems suggests that 5-HT may play a crucial role in the control of the feeding behavior in both the larval and adult stages of the stable fly. Pharmacological and electrophysiological experiments are currently underway to further elucidate the functional roles of 5-HT and its receptor in stable fly feeding behavior. Most current insecticides act on a limited number of targets in the insect nervous system, but few specifically target the 5-HT receptors. Given the importance of 5-HT and 5-HT receptors in insect nervous system function and behaviors, 5-HT receptors may serve as a new molecular target for developing novel insecticides.