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Trypanosomiasis caused by Trypanosoma cruzi and transmitted by triatomine bugs is widespread in South and Central America, human Infections, chronic or acute, being known as Chagas' disease. Since blood forms of the trypanosome in man and animals are commonly too few to be found in blood films, conclusive evidence of infection is still by xenodiagnosis. This requires vector bugs to be engorged on a suspected infected vertebrate and the bug faeces examined a month or so later by which time the trypanosomes should have greatly multiplied in the insect gut. It is known that there is variation between triatomine species and between individuals within species in ability to develop T. cruzi infections. The present research investigated the genetic basis of this variable susceptibility in one species, Rhodnius prolixus, by selection for susceptible and refractory bug populations and revealed evidence of polygenic control of susceptibility to infection with T. cruzi; this contrasts with the major gene mechanisms reported for other pathogens in other insect vectors. Male bugs showed significantly higher levels of infection than females and may be more efficient for xenodiagnosis. The radioresistance of triatomine bugs may be related to their chromosome morphology. Male R. prolixus given a sub-sterilising irradiation dose were less sterile than their progeny, this delay resulting from the diffuse structure of the centromeres of triatomine chromosome. These results are discussed in relation 10 possible control of triatomines by genetic manipulation. Investigation of spermatogenesis by autoradiographic techniques male R. prolixus revealed that starvation induced diapause inhibits spermiogenesis. A blood-borne factor produced when the diapausing bug is fed may directly affect the rate of mulosis. The karyotype is described for several triatomine species. A method was developed for distinguishing between individual mitotic chromosomes within the complement of Triatomi infostan., and K. prolixus. the chromosome markers being valuable for further genetic studies of triatomine bugs.