Despite being vectors of a variety of pathogenic agents, horseflies have received relatively little attention when compared with other blood-feeding flies. The foregut of a horsefly is composed of—but not limited to— the labrum (through which blood flows into the fly), the epipharynx (the transitional zone between labrum and cibarium), and the cibarium (the pump which draws the blood up through the labrum and into the remaining foregut structures). Found within these three foregut structures are aggregations of sensilla that are of two general types: setiform and basicone. Setiform sensilla are known to be mechanoreceptors. Basiconic sensilla can be either chemoreceptors or mechanoreceptors depending on the presence of a pore at the terminal end of the sensory structure. In this study, SEM is used to determine the likely function of the basiconic sensilla (i.e. porous or aporous) within the foregut of the horsefly, Tabanus atratus.

Sensilla location

The two types of sensory structures are positioned in five distinct areas within the three regions of the foregut. 

Foregut

The foregut of the horsefly is positioned as illustrated within the head of the horsefly. It begins with the labrum, one of the six piercing stylets that enter the skin when a horsefly “bites”. The foregut continues into the epipharynx, a transitional zone between the labrum and the remainder of the foregut. Following the epipharyngeal region is the cibarium, the third structure of the foregut, which pumps the blood up through the labrum and into the remainder of the fly gut. 

The Labrum

SEM image of the left half of the labrum of the horsefly, Tabanus atratus.Setiform and basiconic sensilla apparent within the wall of the vestibule and the food canal.