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<p>Mosquitoes pose significant threats to human health because they act as vectors for disease causing viruses and protozoans. Indeed, <em>Aedes aegypti</em> is known as the Yellow Fever Mosquito because of its role as a vector for viral infections that kill thousands of people each year. A more thorough understanding of mosquito physiology will aid development of novel control strategies. Previous work on ion transport across the midgut has been focused primarily on larval <em>A. aegypti</em>, while research on the midgut of the adult stage is less complete. The posterior midgut of the adult female is of particular interest because it is used for the storage and digestion of the blood meal which is required for the production of eggs. This study used an array of electrophysiological methodologies such as the Scanning Ion Electrode Technique (SIET) in order to elucidate the patterns and mechanisms of Na⁺, H⁺ and K⁺ transport across the posterior midgut at intervals during postprandial diuresis and digestion of the blood meal. Measurements of transepithelial potential indicated that the lumen was at its most negative (-13.2 mV) three hours after the blood meal and then gradually became less negative during the time course of digestion. Na⁺ was absorbed (from lumen to bath) at all intervals after the blood meal (6 min, 30 min, 2h, 24 h); calculations of the electrochemical potential indicated that absorption required active transport. H⁺ absorption at all times (6 min – 48 h) after the blood meal was also active (<em>i.e.</em> against the electrochemical gradient for H⁺) and was greatly reduced by inhibition of carbonic anhydrase. K⁺ transport across the midgut exhibited two distinct phases. During diuresis, luminal concentrations of K⁺ were in the range 24 – 28 mM and secretion into the midgut was opposed by the electrochemical gradient, indicating active transport. After diuresis, during blood meal digestion, concentrations of K⁺ in the midgut contents were high (95 – 134 mM) and absorption of K⁺ was favoured by the electrochemical gradient. K⁺ absorption was sensitive to the channel blocker Ba²⁺ during this period.</p> / Master of Science (MSc)