Regurgitation by the Face Fly, Musca autumnalis DeGeer

Face flies normally began regurgitating within 10 minutes after feeding, and continued for periods of up to 240 minutes. Almost 100 percent of flies fed a concentrated (4.8%) solution of trypticase-soy broth were observed regurgitating. The deposition of droplets onto a substrate was almost never observed, indicating that the regurgitation is not analogous to vomiting. The droplet exuded contained from five to ten percent of the food consumed, with the crop the source of the fluid. The crop is also the most likely destination of the regurgitated materiel. The purpose of this phenomenon is still not clear. However, the fluid in the regurgitation droplet was almost constantly circulating, possibly increasing the concentration of the liquid through the evaporation of excess water. This theory is supported by results from tests on the osmolality of the regurgitated droplets. The osmolality of the regurgitated droplets was more than two times greater than that of the ingested food, indicating some change had taken place.

The effect of various factors on regurgitation was also determined. The concentration and amount and type of food all significantly affected regurgitation. Solid foods did not result in regurgitation, and large amounts and high concentrations of liquids increased the frequency and duration of the process. Low relative humidity levels significantly increased the occurrence of regurgitation. Significant differences were found when interactions between these factors were examined.

Since regurgitation droplets were rarely deposited onto a substrate, various factors were examined which would result in this deposition. Increasing the number of flies within a given area increased the number of drops deposited, however, the actual number of drops deposited per fly did not increase. More important was the effect of adding unfed flies to flies already regurgitating. Flies which were regurgitating normally remained motionless, with no interactions occurring between regurgitation flies. Unfed flies actively searched for a meal, and during the search often disturbed regurgitating flies. This resulted in the deposition of droplets onto the substrate. The presence of a cow also increased this deposition of droplets. This was due to activity of the cow, and may also have been a result of the microclimate produced by the cow. These results prove that face flies frequently regurgitate and may in fact be ideal vectors of pathogens.