Blood sucking insects have defence strategies against infection. In the stable fly, Stomoxys calcitrans, one of the defensive mechanisms is a lectin which is blood meal stimulated. While attempting to clone and sequence the lectin from the reservoir zone of the midgut tissue of S. calcitrans, a trypsin was obtained. The trypsin is blood meal induced, and is midgut tissue specific. The cDNA of the mature protein has 260 amino acid residues and has a structural similarity with a trypsin - like proteinase Try29F of the fruit fly Drosophila melanogaster (57% sequence identity); with trypsin 7 precursor of African malaria mosquito Anopheles gambiae and trypsin 3Al precursor of yellow fever mosquito Aedes aegypti (both 55%). The protein has a relative molecular weight of 28 kDa. It has both the serine and the histidine active site signatures; with its catalytic domain between 60 to 240 amino acids. It has an N-glycosylation site (NES) at position 163 to 166 with a significant probability of occurrence (P = 0.005). The cloned trypsin was extracted from the midgut of 8 - 9 day old flies which have been starved for 24 hours prior to dissection. The extract from which the trypsin was purified, was capable of agglutinating rabbit red blood cells an activity that was strongly inhibited by N-acetyl-D-glucosamine. The molecule was purified by affinity chromatography designed to purify lectins and the subsequent isolation of lectin using SDS-PAGE. The trypsin molecule suggests the presence of a trypsin - lectin dimer in S. calcitrans midgut, and in the light of work on tsetse flies, this possibly helps to explain why Stomoxys is not a cyclical vector of trypanosomiasis despite being sympatric with Glossina. The study has also shown that parasites have adaptive evolutionary changes which help them to evade the defence mechanisms of vectors. The study presents three groups of newly isolated trypanosomes of T godfreyi types similar to the known T godfreyi by 96 & 95.6% sequence identity; the T simiae Tsavo by 95% and T vivax type by 86.2%. This work has also shown that different tsetse species respond differently to infection even if they share the same habitat and hosts. The patterns of infection in four different species investigated, ( Glossina swynnertoni Austen, G. morsitans morsitans Westwood, G. brevipalpis Newstead and G. pallidipes Austen) varies from species to species and from one place to another and also are conserved within species.