Microorganisms associated with insects can be maternally transmitted, socially transferred, environmentally reacquired, transiently ingested, or developmentally maintained in specialized host tissues. These biologically different partnerships are often grouped under the term insect microbiome, obscuring differences in persistence, functional dependence, causal support, and ecological context. This evidence-mapping review organizes recent work around three linked questions: how microbial partners are transmitted or reacquired during development, which nutritional, immune, and developmental functions have direct experimental support, and when those functions change with diet, environment, host stage, or taxonomic setting. Strong microbial dependence occurs in particular systems, including specialized nutritional symbioses and experimentally tractable gnotobiotic models, but important counterexamples exist. Environmental acquisition may be behaviorally targeted or host-filtered, microbial effects may depend on nutrient limitation or strain identity, and community change does not necessarily imply altered host fitness. We therefore propose separating microbial occurrence, persistence, transmission, functional association, perturbation response, rescue or sufficiency, mechanistic resolution, and ecological consequence as distinct evidential stages. This structure identifies where evidence is comparatively strong, where causal interpretation remains unresolved, and where laboratory mechanisms require field validation.
