Effect of fractionated blood components on trypsin activity in the stable fly, Stomoxys calcitrans (L.) (Diptera: Muscidae)

Female stable flies, Stomoxys calcitrans (L.), were fed either whole blood, serum, plasma, or intact or lysed red blood cells and tryptic activity and protein content in the midgut were determined at various intervals after feeding. In flies fed either whole blood, serum, or plasma, posterior midgut trypsin activity increased more than 10-fold, reaching maximum activity 12–18 h after feeding. Protein content of the posterior midgut correlated significantly (p < 0.001) with tryptic activity. Ingestion of either intact or lysed red blood cells resulted in only a 2-fold increase in trypsin activity, and protein content of the posterior midgut did not correlate with trypsin activity. These results indicate that the principal stimulus, or secretagogue, for trypsin production is contained in the noncellular components of the ingested blood meal.