<p> The blood and sugars, on which adult black-flies fed, stimulated the buccal sensilla so that blood. went directly to the midgut and sugars to the crop. Agglutinin and anticoagulant were found in the salivary glands of female simuliids. </p> <p> The sequence of peritropbic membrane formation and disappearance are discussed in relation to blood digestion. </p> <p> Digestive enzymes, such as that of trypsin and invertase activity occurred in the midgut of both sexes of several black-fly species unfed or fed on sucrose, but an increase in enzyme Activity was stimulated in females by a blood meal. A weak amylase-like activity was found in the midgut, but the activity was not stimulated by blood- feeding. A greater amylase activity was found in the residual fluid (haemolymph). Trypsin activity in females, but not in males, increased also after feeding blood-sucrose mixtures. Blackfly invertase promoted oligosaccharide synthesis. Trypsin, amylase and invertase showed pH optima of 8.4, 6.5 and 6.2 respectively. </p> / Thesis / Doctor of Philosophy (PhD)