The cuticular surface of the rectum of Triatoma infestans and its colonization by a Trypanosoma cruzi strain originating from the same locality as the bugs were studied by scanning electron microscopy at different weeks post infectionem. On the basis of the cuticular folding, the rectum can be subdivided into five regions. The rectal gland has the finest structure and the region anterior to the anus the deepest folds. Try. cruzi always prefers to colonize the rectal gland, while the other regions are colonized in varying densities. Most of the flagellates are epimastigotes (long and short), except in the region at the entrance of the midgut, where trypomastigotes predominate.

Transovarial transmission was not detectable among Blastocrithidia triatomae-infected Triatoma infestans. Rather, B. triatomae was transmitted directly between triatomines by cannibalism and coprophagy. Cannibalism conditions that excluded coprophagy always resulted in an infection of Dipetalogaster maxima. The efficiency of transmission was not influenced by the blood source--mice or chickens--fed to the infected donor bugs although chicken blood lyses the epimastigotes of the stomach population. Triatoma infestans was infected by coprophagy only if fed, not if unfed. Blastocrithidia triatomae in dry feces was taken up only if the feces were redissolved in fresh feces. Infections also appeared in groups of bugs fed on chickens previously used for feeding infected bugs.

The colonization of the different regions of the rectum of Triatoma infestans by Trypanosoma cruzi was studied in unfed larvae, during and after feeding of the bug by scanning electron microscopy. The rectal pads always possessed the highest population densities, but in some bugs the main rectal sac and the region around the anus were also covered by a "carpet" of flagellates. Such high densities were never observed at the midgut/rectal junction. A slight decrease in this region might be caused by blood ingestion and the resulting excretion of urine. However, the flagellates in this region cannot be responsible for a phenomenon described by other authors, namely that the percentage of metacyclics is low in the first drop of faeces and increases in the following drops of deposited urine. Our observations indicate that metacyclics lying on the "carpet" of flagellates in bugs before and right after the start of feeding might later become loosened. In all dissections, numerous metacyclics were attached to the rectal wall. Further studies are necessary to clarify the exact origin of metacyclics in the urine.

ABSTRACT. The morphology of Blastocrithidia triatomae, when colonizing the rectum of Triatoma infestans, was studied by scanning electron microscopy. The posterior end of the epimastigote's body is tapered and often rolled up. The “straphangers” (cyst stages) are connected with the flagellum by “cytoplasmic bridges”; after “straphanger” detachment these bridges persist as remnants. A comparison of colonization densities of B. triatomae and Trypanosoma cruzi shows that both prefer to colonize the rectal gland, but minor density differences are apparent for B. triatomae in other rectal regions. The colonization density in established infections of B. triatomae is always greater than that of T. cruzi, especially near the midgut exit.