Immunohistological localization of serotonin in the CNS and feeding system of the stable fly Stomoxys calcitrans L. (Diptera: Muscidae)

Serotonin, or 5-hydroxytryptamine (5-HT), plays critical roles as a neurotransmitter and neuromodulator that control or modulate many behaviors in insects, such as feeding. Neurons immunoreactive (IR) to 5-HT were detected in the central nervous system (CNS) of the larval and adult stages of the stable fly, Stomoxys calcitrans, using an immunohistological technique. The location and pattern of the 5-HT IR neurons are described and compared for these two different developmental stages. Anatomical features of the fly feeding system were analyzed in third instar larvae and adult flies using a combination of histological and immunohistological techniques. In third instar larvae, the cibarial dilator muscles were observed within the cibarial pump skeleton and innervated by 5-HT IR neurons in nerves arising from the brain. There were four pairs of nerves arising from the frontal surface of the larval brain that innervate the cibarial pump muscles, pharynx, and muscles controlling the mouth hooks. A strong serotoninergic innervation of the anterior stomatogastric system was observed, which suggests 5-HT may play a role in the coordination of different phases of food ingestion by larvae. Similarly, many 5-HT IR neurons were found in both the brain and the thoracico-abdominal ganglia in the adult, some of which innervate the cibarial pump dilator muscles and the stomatogastric muscles. This is tnhe first report describing neuromuscular structures of the stable fly feeding system. The results reported here suggest 5-HT may play a critical role in feeding behaviors of stable fly larvae and adults.