SUMMARYBlastocrithidia familiaris were found to be parasitic in the midgut, ileum and rectum of Lygaeus pandurus. The host—parasite relationship is described. Attachment of parasites in the midgut and ileum occurs by interdigitation of expanded flagella over and between the microvilli. No attachment to microvilli was observed where extracellular membranes form well-organized layers which lie parallel to the gut wall and completely separate the microvilli border from the lumen. The extracellular membranes originate from delamination of the outer unit membrane of microvilli which consists of a double plasma membrane. The function of the extracellular membrane layers and their possible role as a peritrophic membrane in preventing parasite attachment is discussed in relation to previous studies on midgut cells in Hemiptera with a similar apical coat on midgut microvilli. In the rectum, parasites attach to the cuticle of the gland cells and not to the rest of the rectal wall. Attachment to the cuticle occurs by the formation of hemidesmosomes. A comparison of the relationship of B. familiaris and its host to previous ultrastructural studies of associations between kinetoplastid flagellates and their respective hosts is discussed.

SUMMARYA study of Leptomonas lygaei in Lygaeus pandurus is described. Flagellates with promastigote configuration were found in the midgut, ileum and rectum. Cysts and encysting stages (straphangers) were found in the rectum either attached to the flagella of promastigotes or free in the lumen. In the posterior midgut, the crypts were frequently filled with flagellates, but no attachment to host epithelium was observed. A close association between the flagellates and extracellular membrane layers was observed. In the rectum the flagellates frequently attached by hemidesmosomes to the cuticle of the gland cells and less frequently to the rest of the rectal wall. Interflagellar desmosomes between the expanded sheaths of flagella of adjacent parasites were also observed. Straphanger cysts attached to parental flagella by the formation of zonular desmosomes. Differences were apparent between the organelles of the encysting stages and the parental flagellates. In mature cysts, the cellular organelles were unrecognizable. A comparison between the cysts and flagellates of L. lygaei and Blastocrithidia familiaris is made.

An investigation of transmission and ecology of the monogenetic trypanosomatids, Blastocrithidia gerridis and Crithidia flexonema, in Gerris is described. Motile free-living flagellates of both species were found in the faeces of Gerris and in the water on which the bugs inhabited. Transmission of both trypanosomatid species occurred from naturally infected wild-caught bugs to flagellate-free laboratory-bred bugs via water. Crithidia flexonema was also transmitted to laboratory-bred bugs after being isolated in culture. Observations of experimentally infected bugs indicate that C. flexonema flagellates are imbibed and pass through the fore- and midgut to the hindgut where they become attached and multiply. There was no evidence to suggest transovarial transmission. In a 3-yr investigation into the prevalence of trypanosomatids in a natural population of adult Gerris odontogaster, it was found that the infection rate varied between 19% and 100%. There was no significant difference in infection rates between females and males. The infection rate peaked for each year in late spring or early summer. The significance of these results is discussed in relation to the ecology and behaviour of Gerris. The results indicate that the infections are maintained in hibernating bugs over winter.

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Blastocrithidia gerridis the type species of the genus Blastocrithidia is parasitic in the gut of bugs of the genus Gerris. The ultrastructure and host–parasite relationship of the epimastigote stages of B. gerridis in the ventriculus of Gerris odontogaster are described. The ultrastructure is typical of other kinetoplastid flagellates but the elongated epimastigotes in the lumen of the ventriculus possess armlike protrusions of the plasma membrane. Intense infections are found in the ventriculus and masses of parallel orientated flagellates are attached to each other by their flagella and to the ventriculus epithelium by interdigitation with microvilli. Parasites and flagella are also wedged in folds of epithelium. Where the epithelium has lost microvilli junctional complexes between flagella and epithelial cell plasma membranes occur but no junctional complexes are observed between flagella and microvilli. A comparison is made between these mechanisms and those described previously for other trypanosomatid–insect or leech associations. The possibility of mixed infections of kinetoplastid flagellates in Gerris is discussed which reinforces the need for a study of the origin or source of promastigote and amastigote organisms found in Gerris species.

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