The effect of sugars on ingestion and diet destination in Culiseta inornata

Abstract

Effects of a series of sugars on ingestion and diet destination in female Culiseta inornata were determined. The percentage of responding insects that ingested large (> 1 μl) amounts was used as the index of potency for phagostimulation (the “gorging response”). The percentage of insects that ingested large amounts and directed the diet to the crop exclusively was used as the index of potency for the “crop response”. For both the gorging response and the crop response, the potency of the sugars ranked: sucrose, 50 50α - glucose fructose , maltose, fructose, α-glucose, isomaltose, cellobiose, lactose, β-glucose, gentiobiose. Relative potencies fit a model involving a single receptor with pyranose and furanose sites. Characteristics of the receptor are suggested by the following: (1) disaccharides containing α-glucosidyl linkages are more potent than those with β-linkages, (2) disaccharides with C 1 to C 4 glucosidyl linkages are more potent than those with C 1 to C 6 linkages, and (3) sugars with exposed axial hydroxyl groups on the C 1 carbon are more effective than those with equatorial hydroxyl groups. Two response systems are suggested, one controlling the amount ingested, the other controlling diet destination. At 0.5 M, sucrose, maltose, fructose, α-glucose, and isomaltose induce the opening of the crop valve and the closing of the midgut valve, which results in ingestion into the crop exclusively in the majority of insects. Cellobiose, β-glucose, lactose and gentiobiose induce closing of the midgut valve in very few insects. This lack of effect on the system controlling the midgut valve is associated with those molecules having either β-glucosidyl linkages or, in the case of β-glucose, a β configuration of the C 1 hydroxyl group. The higher specificity to sucrose of the crop response compared to the gorging response suggests two different chemoreceptor cell-types providing input.