| Summary: | Bacterial symbionts in arthropods are common, vary in their effects, and can dramatically influence the outcome of biological control efforts. <i>Macrolophus pygmaeus</i> (Heteroptera: Miridae), a key component of biological control programs, is mainly predaceous but may also display phytophagy. <i>M. pygmaeus</i> hosts symbiotic <i>Wolbachia</i>, which induce cytoplasmic incompatibility, and two <i>Rickettsia</i> species, <i>R. bellii</i> and <i>R. limoniae</i>, which are found in all individuals tested. To test possible involvement of the two <i>Rickettsia</i> species in the feeding habits of <i>M. pygmaeus</i>, we first showed that the microbiome of the insect is dominated by these three symbionts, and later described the distribution pattern of the two <i>Rickettsia</i> species in its digestive system. Although both <i>Rickettsia</i> species were located in certain gut bacteriocyes, in caeca and in Malpighian tubules of both sexes, each species has a unique cellular occupancy pattern and specific distribution along digestive system compartments. Infrequently, both species were found in a cell. In females, both <i>Rickettsia</i> species were detected in the germarium, the apical end of the ovarioles within the ovaries, but not in oocytes. Although the cause for these <i>Rickettsia</i> distribution patterns is yet unknown, it is likely linked to host nutrition while feeding on prey or plants.
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