Ultrastructural and biochemical aspects of digestion in the imagoes of the flyRhynchosciara americana

The midgut of adultRhynchosciara americana Wiedemann (Diptera: Sciaridae) displays, in contrast to the midguts of other adult Diptera, two caeca connected to a ventriculus. All midgut cells exhibit long apical microvilli, and narrow and ramified basal channels with openings to the underlying space. These morphological features are thought to be involved in the absorption of nutrients from food. Enzymatic assays inR. americana adults revealed that amylase occurs in salivary glands and midgut, whereas aminopeptidase, α-glucosidases and trypsin occur only in the midgut, mainly in the ventriculus. There is a soluble (Mr 105000) and a membrane-bound aminopeptidase (solubilized form, Mr 110000). Soluble α-glucosidase inactivates easily and could not be characterized, whereas membrane-bound α-glucosidases were resolved after solubilization into three molecular species (Mr 186000, 105000 and 84000) with different substrate specificities. The activities of trypsin (pH optimum 9.0), which was inhibited completely by soybean trypsin inhibitor, and of amylase (pH optimum 5.5), were not sufficiently high to be further characterized. The data support the assertion thatR. americana adults are able, to a limited extent, to digest and absorb starch and proteins, in addition to nectar sugars. The results, supported by published data, suggest that there is an inverse correlation between the digestive enzyme activities and midgut absorptive surface in insects which has nectar as a major food.