Innervation and structure of mouth part sensilla in females of the black fly Simulium venustum (Diptera: Simuliidae).

Abstract

Associated with the mouth parts of female Simulium venustum are 13 morphologic types of sensilla: four on the labium, seven on the labrum, one in the cibarium, and one on the genal process to which the mandibles articulate. Seven types are probably sensitive only to mechanicla cues and three only to chemical ones, whereas the other three probably function in both modes. These sensilla likely monitor feeding-associated chemical features of blood, sugar, and water and mechanical cues generated by the physical acts of ingestion. Each S. venustum female has approximately 450 chemosensitive and 230 mechanosensitive neurons in the mouthpart-associated sensilla. Both the total number of chemosensory neurons and the ratio of chemosensory to mechanosensory neurons in S. venustum are intermediate between those for blow flies, which feed on a wide variety of foodstuffs, and tsetse flies, exclusive blood-feeders. These differences may be related to whether determination of acceptability of a potential food source occurs at the site of feeding and is dependent upon simultaneous sensitivity to many chemical cues, as in blow flies, or is the result of a complex stimulus chain composed of all host-location steps and culminating with the detection of but a few phagostimulants in the food itself, as in blood-feeders.